Endurance running hypothesis

From Wikipedia, the free encyclopedia

 

The endurance running hypothesis is the hypothesis that the evolution of certain human characteristics can be explained as adaptations to long-distance running. The hypothesis suggests that endurance running played an important role for early hominins in obtaining food. Researchers have proposed that endurance running began as an adaptation for scavenging and later for persistence hunting.

Anatomical and physiological adaptations

Running vs. walking

Much research has been geared towards the mechanics of how bipedal walking has evolved in the genus Homo. However, little research has been conducted to examine how the specific adaptations for running emerged, and how they influenced human evolution. 

The bit of research that has focused on human running provides much evidence for bodily function and structures that improve running only, and are not used in walking. This suggests that running was an adaptation, not that it came about as a byproduct of walking. 

Running and walking incorporated different biomechanisms. Walking requires an "inverted pendulum" where the body's center of mass is shifted over the extended leg, to exchange potential and kinetic energy with each step. Running involves a "mass spring" mechanism to exchange potential and kinetic energy, with the use of tendons and ligaments. Tendons and ligaments are elastic tissues that store energy. They are stretched and then release energy as they recoil. This mass spring mechanism becomes less energetically costly at faster speeds and is therefore more efficient than the inverted pendulum of walking mechanics when traveling at greater speeds. Tendons and ligaments, however, do not provide these benefits in walking. 

Although the mass spring mechanism can be more energetically favorable at higher speeds, it also results in an increase in ground reaction forces and is less stable because there is more movement and pitching of the limbs and core of the body. Ground forces and body pitching movement is less of an issue in the walking gait, where the position of the body's center of mass varies less, making walking an inherently more stable gait. In response to the destabilization of the running gait, the human body appears to have evolved adaptations to increase stabilization, as well as for the mass-spring mechanism in general. These adaptations, described below, are all evidence for selection for endurance running.

Skeletal evidence

Many researchers compare the skeletal structures of early hominins such as Australopithecus to those of Homo in order to identify structural differences that may be significant to endurance running. 

Nuchal ligament: Because the head is decoupled from the shoulders, early Homo needed a way to stabilize the head. The nuchal ligament is an important evolved feature in head stabilization. It starts at the midline of the occiput and connects to the upper trapezius. This ligament is also important in terms of archaeological findings, because it leaves a small indentation and ridge in the skull, allowing researchers to see if various species had a nuchal ligament. The ability to see traces of ligaments in archaeological findings is rare because they degrade quickly and often leave no trace. In the case of the nuchal ligament, a trace of its existence is left with the presence of the skull ridge. Because neither Australopithecus nor Pan had the skull ridge, it has been concluded that this feature is unique to Homo. Because the nuchal ligament is only activated while running, the amount of running can be inferred from the rugosity of the muscle insertions. In the case of Homo Erectus and Neanderthals, very strong nuchal ligament markings are present, but are less marked in modern humans, indicating a decrease in running behavior.

Nuchal ligament of Homo sapiens

 

Shoulder and head stabilization: The human skeleton is different from early hominins as there is less of a connection between the pectoral girdle parts of the shoulders and upper back and head, which would be advantageous for climbing but would hinder the movements of the upper body needed to counter leg movement and therefore stabilize the body and head when running. This stabilization is unnecessary in walking.

Limb length and mass: Homo has longer legs relative to body mass, which helps to decrease the energetic costs of running, as time in contact with the ground increases. There is also a decrease in mass of distal parts of limbs of humans, which is known to decrease metabolic costs in endurance running, but has little effect on walking. Additionally, the mass of the upper body limbs in Homo has decreased considerably, relative to total body mass, which is important to reduce the effort of stabilizing the arms in running.

Joint surface: Humans have evolved to absorb great shock and force on the skeletal structure while running. The impact force on the body can reach up to 3–4 times body weight in endurance running, putting the skeletal structure under great stress. To reduce this stress humans have increased joint surfaces relative to body mass to spread force over larger surface areas, particularly in the lower body. This adaptation, which allows humans to absorb great shock and force applied to the skeleton, is not seen in australopithecine skeletal structures.

Plantar arch: The plantar arch in the human foot has an elastic spring function that generates energy for running but not walking. Fossils of the australopithecine foot show only partial arch, suggesting less of a spring capacity. For the plantar arch spring mechanism to function fully, there must also be restricted rotation in the hind and front parts of the foot. This restriction comes from projected toe bone and compacted mid-foot joint structures in humans, which does not become present until Homo habilis.

Calcaneal tuber and Achilles tendon: Studies have explored the calcaneal tuber, the posterior half of the calcaneus bone, as a correlate for Achilles tendon length and have found correlation between calcaneal tuber length and Achilles tendon length. Because shorter calcaneal tuber length leads to greater Achilles stretch, more kinetic energy is converted to elastic energy, translating into better overall running economy. Comparisons between Neanderthals and modern humans reveal that this adaptation was absent in Neanderthals, leading researchers to conclude that endurance running capabilities may have been enhanced in anatomically modern humans.

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  Calcaneus
  

Shorter toes: Human toes are straight and extremely short in relation to body size compared to other animals. In running, the toes support 50 to 75% of body mass in humans. Impulse and mechanical work increase in humans as toe length increases, showing that it is energetically favorable to have shorter toes. The costs of shorter toes are decreased gripping capabilities and power output. However, the efficiency benefits seem to outweigh these costs, as the toes of A. afarensis remains were shorter than great apes, but 40% longer than modern humans, meaning that there is a trend toward shorter toes as the primate species moves away from tree-dwelling. This 40% increase in toe length would theoretically induce a flexor impulse 2.5 times that of modern humans, which would require twice as much mechanical work to stabilize.

Stabilization

Semicircular canal: The semicircular canal, a series of three interconnected tubes within each ear, is important for sensing angular rotations of the head and thus plays a crucial role in maintaining balance and sensing and coordinating movement. Comparative studies have shown that animals with larger semicircular canals are able to sense a greater range of head movements and therefore have greater speed and agility. Evolutionarily, greatly reduced semicircular canal diameters are evident in Neanderthals but expanded in modern humans, suggesting that this adaptation was selected for in response to increased endurance running.

Vestibulo-ocular reflexes (VORs): VORs are enabled by muscles in the eye, which sense angular accelerations of the head and adjust eye movements to stabilize these images. This was an important adaptation for running because it allowed Homo to see more clearly during the rough pitching motion that occurs during running.

Gluteals: The gluteus maximus in Homo erectus is significantly larger than that of Australopithecus. It is suited to stabilize the trunk while running, but gluteals of that size and strength are not necessary for walking. 

Iliac spine: Homo has expanded areas on the sacrum and posterior iliac spine for greater muscle attachment. These areas are used to stabilize the trunk and reduce the body's forward pitch caused by running strides.

Increased efficiency

Thermoregulation

In addition to advances in skeletal structure and stabilization, adaptations that led to increased efficiency in dissipation of heat were instrumental in the evolution of endurance running in Homo. The duration for which an animal can run is determined by its capacity to release more heat than is produced to avoid lethal temperatures. 

The majority of mammals, including humans, rely on evaporative cooling to maintain body temperature. Most medium-to-large mammals rely on panting, while humans rely on sweating, to dissipate heat. Advantages to panting include cooler skin surface, little salt loss, and heat loss by forced convection instead of reliance on wind or other means of convection. On the other hand, sweating is advantageous in that evaporation occurs over a much larger surface area (the skin), and it is independent of respiration, thus is a much more flexible mode of cooling during intense activity such as running. Because human sweat glands are under a higher level of neuronal control than those of other species, they allow for the excretion of more sweat per unit surface area than any other species. Heat dissipation of later hominins was also enhanced by the reduction in body hair. By ridding themselves of an insulating fur coat, running humans are better able to dissipate the heat generated by exercise. 

In addition to improved thermoregulation, hominins have evolved an enhanced method of respiration consistent with the demands of running. Due to their orientation, respiration in quadrupedal mammals is affected by skeletal and muscular stresses generated through the motion of running. The bones and muscles of the chest cavity are not only responsible for shock absorption, but are also subjected to continuous compression and expansion during the running cycle. Because of this movement, quadrupeds are restricted to one breath per locomotor cycle, and thus must coordinate their running gait and respiration rate. This tight coordination then translates into another restriction: a specific running speed that is most energetically favorable. The upright orientation of bipedal hominins, however, frees them from this respiration-gait restriction. Because their chest cavities are not directly compressed or involved in the motion of running, hominins are able to vary their breathing patterns with gait. This flexibility in respiration rate and running gait contributes to hominins having a broader range of energetically favorable running speeds.

Storage and utilization of energy

During periods of prolonged exercise, animals are dependent on a combination of two sources of fuel: glycogen stored in the muscles and liver, and fat. Because glycogen is more easily oxidized than fat, it is depleted first. However, over longer periods of time, energy demands require that fat stores be utilized as fuel. This is true for all mammals, but hominins, and later modern humans, have an advantage of being able to alter their diet to meet these prolonged energy demands. 

In addition to flexibility in the utilization of energy, hominins have evolved larger thyroid and adrenal glands which enable them to utilize the energy in carbohydrates and fatty acids more readily and efficiently. These organs are responsible for releasing hormones including epinephrine, norepinephrine, adrenocorticotropic hormone (ACTH), glucagon, and thyroxine. Larger glands allows for greater production of these key hormones and ultimately, maximized utilization of stored fuel.

Taken together, the flexibility in diet and the enhanced usage of fuel heightens the previously mentioned finding that, unlike quadrupeds, hominins do not have a single energetically optimal running speed. For quadrupeds, increasing running speed means increasing the demand for oxygen and fuel. Due to skeletal structure and bipedalism, hominins are free to run energetically over a broader range of speeds and gaits, while maintaining a constant energy consumption rate of approximately 4.1 MJ per 15 km. Thus their utilization of energy is greatly enhanced.

Endurance running and scavenging

All of the aforementioned adaptations enabled Homo to scavenge for food more effectively. Endurance running could have been used as a means of gaining access to distant carcasses or food stores faster than other scavengers and/or carnivores. Scavenging may have taken one or both of two forms: opportunistic scavenging and strategic scavenging.

Early Homo almost certainly scavenged opportunistically. Scavenging is considered opportunistic when one "come[s] across carcasses in the course of [their] daily foraging activities".

Strategic scavenging involves a planned search for carcasses. This style of scavenging would have benefitted from endurance running much more than opportunistic scavenging. Strategic scavenging would have involved the use of long range cues, such as birds circling overhead. Endurance running would have been advantageous in this setting because it allowed hominins to reach the carcass more quickly. Selection pressures would have been very high for strategic scavenging, because hominins were diurnal, while their major competitors (hyenas, lions, etc.) were not. Thus, they would have had to make sure to capitalize on daytime carcasses. Selection pressure also came from the weakness of Homo. Because they were very weak, they were unlikely to drive off any large competition at the carcass. This fact led to an even higher need for a way to reach the carcass before these competitors.

Endurance running and persistence hunting

Persistence hunting is "a form of pursuit hunting in which [the hunter uses] endurance running during the midday heat to drive [prey] into hyperthermia and exhaustion so they can easily be killed". Many question persistence hunting's plausibility when bow and arrow and other technologies were so much more efficient. However, in the Early Stone Age (ESA), spears were only sharpened wood, and hominins had not begun using tools. The lack of spearheads or bows meant they could only hunt from very close range—between 6 and 10 meters. Hominins thus must have developed a way to stab prey from close range without causing serious bodily harm to themselves. Persistence hunting makes killing an animal easier by first bringing it to exhaustion, so that it can no longer retaliate violently.

Persistence hunters work by hunting in the middle of the day, when it is hottest. Hunters choose a single target prey and chase it at a speed between its trot and gallop, which is extremely inefficient for the animal. The hunter then continues pursuing over a period of hours, during which he may lose sight of the animal. In this case, the hunter must use tracks and an understanding of the animal to continue the chase. The prey eventually overheats and becomes unable to continue fleeing. Homo, which does not overheat as quickly because of its superior thermoregulation capabilities, is then able to stab the prey while it is incapacitated and cannot attack.

Tracking and running

Due to the complexity of following a fleeing animal, tracking methods must have been a prerequisite for the use of endurance running in persistence hunting. Scientists posit that early tracking methods were developed in open, sparsely vegetated terrain such as the Kalahari Desert in southern Africa. This "systemic tracking" involves simply following the footprints of animals and was most likely used for tracking grassland species on soft terrain. Skeletal remains suggest that during the Middle Stone Age, hominins used systemic tracking to scavenge for medium-sized animals in vegetation cover, but for hunting antelope in more open grasslands. From the Middle Stone Age into the Later Stone Age, tracking methods developed into what is termed "speculative tracking". When tracks could not easily be found and followed, Homo predicted where tracks were most likely to be found and interpreted other signs to locate prey. This advanced method of tracking allowed for the exploitation of prey in a variety of terrains, making endurance running for persistence hunting more plausible. 

The process of tracking can last many hours and even days in the case of very large mammals. Often times, the hunter(s) will have to run after the animal to keep up. The skeletal parameters of the tibia of early modern humans and Neanderthals have been compared with runners, and it surprisingly shows that these individuals were running even more than cross-country runners today. Particularly, European Neanderthals, the Skhul and Qafzeh hominins, and Late Stone Age Khoisan score very high compared to runners. This is consistent with modern observations of Khoisan, who routinely spend hours running after animals that have been shot with arrows.

Examples of persistence hunters

Although exact dates and methods of persistence hunting are difficult to study, several recent accounts of persistence hunting have been recorded. Tribes in the Kalahari Desert in Botswana have been known to employ endurance running to scavenge and hunt prey. In the open country, the !Xo and /Gwi tribes run down slow-moving animals such as aardvark and porcupines, while during the hotter part of the day, they target animals such as eland, kudu, gemsbok, hartebeest, duiker, steenbok, cheetah, caracal, and African wildcats. In addition to these existing African tribes, it has been suggested that the Tarahumara people in Mexico and the Paiute people and Navajo in the American Southwest, used persistence hunting to capture prey including deer and pronghorn. The Aborigines in Australia are known to have hunted kangaroo in similar ways. Due to the increased availability of weapons, nutrition, tracking devices, and motor vehicles, one may argue that persistence hunting is no longer an effective method of hunting animals for food. However, there are examples of the practice occurring in modern times: the !Xo and /Gwi in the central Kalahari, still practice persistence hunting and have developed advanced methods of doing so. Similarly, the Russian Lykov family that lived in isolation for 40 years also used persistence hunting due to a lack of weapons.

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  Red Hartebeest
  
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  Kudu
  
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  Gemsbok
  

Seasonal differences

In particular, these two tribes maximize the efficiency of persistence hunting by targeting specific species during different seasons. In the rainy season, prime targets include steenbok, duiker, and gemsbok, as wet sand opens their hooves and stiffens their joints. Hunting in the early rainy season is particularly advantageous because dry leaves form "rocks" in the animals' stomachs, resulting in diarrhea. Stiff joints and suboptimal digestion make the prey weaker and more available targets. In contrast, in the dry season, hunters run down kudu, eland, and red hartebeest because these species tire more easily in the loose sand. Hunters say that the best time to practice persistence hunting is near the end of the dry season when animals are poorly nourished and therefore more easily run to exhaustion.^[7] By targeting the most vulnerable prey during each season, the hunters maximize the advantages of endurance running.

Conditions/parameters

1. Persistence hunting must be performed during the day when it is hot, so that the animal will overheat.
2. Homo must have been able to track the animal, as they would have lost sight of it during the chase.
3. Such a long hunt requires high amounts of dietary sources of water, salt, and glycogen.
4. Although the success rate of recorded persistence hunts is very high (approximately 50%), unsuccessful hunts are very costly. Therefore, there would have had to be a social system in which individuals share food, so unsuccessful hunters could borrow food from others when necessary.

In culture and folklore

In the oral traditions of the Hadza, an isolated aboriginal people of hunter-gatherers living in Tanzania, the Tlaatlanebe in their folk history's second epoch practiced this. 

In the first epoch, the world was inhabited by large hairy humanoids called Akakaanebe ("ancestors"), who did not yet possess tools or fire. They simply "stared" at game until it fell dead, referring to either scavenging or early persistence hunting without weapons, or a combination of the two. They did not build houses but slept under trees.

The Tlaatlanebe of the second epoch, however, were large but without hair and lived in caves. As animals had grown more wary of humans due to earlier hunting, they now had to be chased and hunted with dogs.

Criticisms

While there is evidence supporting selection on human morphology to improve endurance running ability, there is some dispute over whether the ecological benefits of scavenging and persistence hunting foraging behaviors were the driving force behind this development.

The majority of the arguments opposing persistence hunting and scavenging behaviors are linked to the fact that the paleohabitat and paleoecology of early Homo were not conducive to these behaviors. It is thought that the earliest members of Homo lived in African savanna-woodlands. This environment consisted of open grassland, as well as parts with dense vegetation—an intermediate between forest and open savannas. The presence of such tree covering would reduce visibility and so require tracking skills. This causes problems for the hypothesis of persistence hunting and running to aid scavenging.

Against persistence hunting

Ungulates are known from archaeological evidence to have been the main prey of the early Homo, and given their great speed, they would have easily been able to outrun early hominins. Ungulate speed, coupled with the variable visibility of the savanna-woodland, meant that hunting by endurance running required the ability to track prey. Pickering and Bunn argue that tracking is part of a sophisticated cognitive skill set that early hominins would not have had, and that even if they were following a trail of blood left by an injured ungulate—which may have been in their cognitive capacity—the ability to craft penetrating projectile technology was absent in early hominins.

It has been suggested that modern hunters in Africa do not use persistence hunting as a foraging method, and most often give up a chase where the trail they were following ends in vegetation. The rare groups of hunters who do occasionally participate in persistence hunting are able to do so because of the extremely hot and open environments. In these groups, a full day of rest and recovery is required after a hunt, indicating the great toll persistence hunts take on the body, making them rare undertakings.

Finally, in critique of Liebenberg's research on modern day persistence hunting, it was revealed that the majority of the hunts initiated were prompted for filming rather than spontaneous, and that few of these hunts were successful. The hunts that were successful involved external factors such as the hunters being able to stop and refill water bottles.

A response to these criticisms has been formulated by Lieberman et al., noting that it is unclear how humans could have grown to occupy a new niche as a diurnal social carnivore without persistence hunting, as the weapons preferred in modern hunter-gatherer tribes would not have been available at the time.

Against scavenging

The proposed benefit of endurance running in scavenging is the ability of early hominins to outcompete other scavengers in reaching food sources. However paleoanthropological studies suggest that the savanna-woodland habitat caused a very low competition environment. Due to low visibility, carcasses were not easily located by mammalian carnivores, resulting in less competition.

Leaf

From Wikipedia, the free encyclopedia

The diversity of leaves

 

Leaf of Tilia tomentosa (Silver lime tree)

 

A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage".

Diagram of a simple leaf.

1. Apex
2. Midvein (Primary vein)
3. Secondary vein.
4. Lamina.
5. Leaf margin
6. Petiole
7. Bud
8. Stem

Top and Right: Staghorn Sumac, Rhus typhina (Compound Leaf)
Bottom: Skunk Cabbage, Symplocarpus foetidus (Simple Leaf)
1. Apex
2. Primary Vein
3. Secondary Vein
4. Lamina
5. Leaf Margin
6. Petiole

A leaf is a dorsiventrally flattened organ, usually borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinct upper surface (adaxial) and lower surface (abaxial) that differ in color, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax and other features. Leaves are mostly green in color. This is due to the presence of a compound called chlorophyll. This compound is essential for photosynthesis as it absorbs light energy from the sun. A leaf with white patches or edges is called a variegated leaf. 

Leaves can have many different shapes, sizes, and textures. The broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In the clubmosses, with different evolutionary origins, the leaves are simple (with only a single vein) and are known as microphylls. Some leaves, such as bulb scales, are not above ground. In many aquatic species, the leaves are submerged in water. Succulent plants often have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls and spines. Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin. Some structures of non-vascular plants look and function much like leaves. Examples include the phyllids of mosses and liverworts.

General characteristics

Leaves are the most important organs of most vascular plants. Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis. They capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are then stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolized by cellular respiration to provide chemical energy to run cellular processes. The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf (epidermis), while leaves are orientated to maximize their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots. Vascular plants transport sucrose in a special tissue called the phloem. The phloem and xylem are parallel to each other, but the transport of materials is usually in opposite directions. Within the leaf these vascular systems branch (ramify) to form veins which supply as much of the leaf as possible, ensuring that cells carrying out photosynthesis are close to the transportation system.

Typically leaves are broad, flat and thin (dorsiventrally flattened), thereby maximising the surface area directly exposed to light and enabling the light to penetrate the tissues and reach the chloroplasts, thus promoting photosynthesis. They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. For instance, plants adapted to windy conditions may have pendent leaves, such as in many willows and eucalyptss. The flat, or laminar, shape also maximizes thermal contact with the surrounding air, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of transpiration, providing the energy required to draw the transpiration stream up from the roots, and guttation. 

Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost. These are interpreted as reduced from megaphyllous leaves of their Devonian ancestors. Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favor of protection from herbivory. For xerophytes the major constraint is not light flux or intensity, but drought. Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. and Bulbine mesembryanthemoides.

Leaves also function to store chemical energy and water (especially in succulents) and may become specialized organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia. Leaves are the fundamental structural units from which cones are constructed in gymnosperms (each cone scale is a modified megaphyll leaf known as a sporophyll) and from which flowers are constructed in flowering plants.

Vein skeleton of a leaf. Veins contain lignin that make them harder to degrade for microorganisms.

 

The internal organization of most kinds of leaves has evolved to maximize exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide while at the same time controlling water loss. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute (length and width measured in tens of µm) openings called stomata which open or close to regulate the rate exchange of carbon dioxide, oxygen, and water vapor into and out of the internal intercellular space system. Stomatal opening is controlled by the turgor pressure in a pair of guard cells that surround the stomatal aperture. In any square centimeter of a plant leaf, there may be from 1,000 to 100,000 stomata.

Near the ground these Eucalyptus saplings have juvenile dorsiventral foliage from the previous year, but this season their newly sprouting foliage is isobilateral, like the mature foliage on the adult trees above

 

The shape and structure of leaves vary considerably from species to species of plant, depending largely on their adaptation to climate and available light, but also to other factors such as grazing animals (such as deer), available nutrients, and ecological competition from other plants. Considerable changes in leaf type occur within species, too, for example as a plant matures; as a case in point Eucalyptus species commonly have isobilateral, pendent leaves when mature and dominating their neighbors; however, such trees tend to have erect or horizontal dorsiventral leaves as seedlings, when their growth is limited by the available light. Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide. In most plants, leaves also are the primary organs responsible for transpiration and guttation (beads of fluid forming at leaf margins). 

Leaves can also store food and water, and are modified accordingly to meet these functions, for example in the leaves of succulent plants and in bulb scales. The concentration of photosynthetic structures in leaves requires that they be richer in protein, minerals, and sugars than, say, woody stem tissues. Accordingly, leaves are prominent in the diet of many animals. 

A leaf shed in autumn.

 

Correspondingly, leaves represent heavy investment on the part of the plants bearing them, and their retention or disposition are the subject of elaborate strategies for dealing with pest pressures, seasonal conditions, and protective measures such as the growth of thorns and the production of phytoliths, lignins, tannins and poisons. 

Deciduous plants in frigid or cold temperate regions typically shed their leaves in autumn, whereas in areas with a severe dry season, some plants may shed their leaves until the dry season ends. In either case, the shed leaves may be expected to contribute their retained nutrients to the soil where they fall.

In contrast, many other non-seasonal plants, such as palms and conifers, retain their leaves for long periods; Welwitschia retains its two main leaves throughout a lifetime that may exceed a thousand years. 

The leaf-like organs of Bryophytes (e.g., mosses and liverworts), known as phyllids, differ morphologically from the leaves of vascular plants in that they lack vascular tissue, are usually only a single cell thick and have no cuticle stomata or internal system of intercellular spaces. 

Simple, vascularized leaves (microphylls) first evolved as enations, extensions of the stem, in clubmosses such as Baragwanathia during the Silurian period. True leaves or euphylls of larger size and with more complex venation did not become widespread in other groups until the Devonian period, by which time the carbon dioxide concentration in the atmosphere had dropped significantly. This occurred independently in several separate lineages of vascular plants, in progymnosperms like Archaeopteris, in Sphenopsida, ferns and later in the gymnosperms and angiosperms. Euphylls are also referred to as macrophylls or megaphylls (large leaves).

Morphology (large-scale features)

A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), and stipules (small structures located to either side of the base of the petiole). Not every species produces leaves with all of these structural components. Stipules may be conspicuous (e.g. beans and roses), soon falling or otherwise not obvious as in Moraceae or absent altogether as in the Magnoliaceae. A petiole may be absent, or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under morphology. The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The lamina is typically the location of the majority of photosynthesis. The upper (adaxial) angle between a leaf and a stem is known as the axil of the leaf. It is often the location of a bud. Structures located there are called "axillary".

Translucent glands in Citrus leaves

 

External leaf characteristics, such as shape, margin, hairs, the petiole, and the presence of stipules and glands, are frequently important for identifying plants to family, genus or species levels, and botanists have developed a rich terminology for describing leaf characteristics. Leaves almost always have determinate growth. They grow to a specific pattern and shape and then stop. Other plant parts like stems or roots have non-determinate growth, and will usually continue to grow as long as they have the resources to do so. 

The type of leaf is usually characteristic of a species (monomorphic), although some species produce more than one type of leaf (dimorphic or polymorphic). The longest leaves are those of the Raffia palm, R. regalis which may be up to 25 m (82 ft) long and 3 m (9.8 ft) wide. The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks. 

Prostrate leaves in Crossyne guttata

Where leaves are basal, and lie on the ground, they are referred to as prostrate.

Basic leaf types

Leaves of the White Spruce (Picea glauca) are needle-shaped and their arrangement is spiral

• Ferns have fronds
• Conifer leaves are typically needle- or awl-shaped or scale-like
• Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina
• Lycophytes have microphyll leaves.
• Sheath leaves (type found in most grasses and many other monocots)
• Other specialized leaves (such as those of Nepenthes, a pitcher plant)

Arrangement on the stem

Different terms are usually used to describe the arrangement of leaves on the stem (phyllotaxis): 

The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other (decussate) along the red stem. Note the developing buds in the axils of these leaves.

Alternate

One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem.

Basal

Arising from the base of the stem.

Cauline

Arising from the aerial stem.

Opposite

Two leaves, branches, or flower parts attach at each point or node on the stem. Leaf attachments are paired at each node and decussate if, as typical, each successive pair is rotated 90° progressing along the stem.

Whorled, or verticillate

Three or more leaves, branches, or flower parts attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc.). Opposite leaves may appear whorled near the tip of the stem. Pseudoverticillate describes an arrangement only appearing whorled, but not actually so.

Rosulate

Leaves form a rosette.

Rows

The term, distichous, literally means two rows. Leaves in this arrangement may be alternate or opposite in their attachment. The term, 2-ranked, is equivalent. The terms, tristichous and tetrastichous, are sometimes encountered. For example, the "leaves" (actually microphylls) of most species of Selaginella are tetrastichous, but not decussate.

As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves form a helix pattern centered around the stem, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence. There is a regularity in these angles and they follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to the golden angle, which is approximately 360° × 34/89 ≈ 137.52° ≈ 137° 30′. In the series, the numerator indicates the number of complete turns or "gyres" until a leaf arrives at the initial position and the denominator indicates the number of leaves in the arrangement. This can be demonstrated by the following:

• Alternate leaves have an angle of 180° (or 1/2)
• 120° (or 1/3): 3 leaves in 1 circle
• 144° (or 2/5): 5 leaves in 2 gyres
• 135° (or 3/8): 8 leaves in 3 gyres.

Divisions of the blade

A leaf with laminar structure and pinnate venation

 

Two basic forms of leaves can be described considering the way the blade (lamina) is divided. A simple leaf has an undivided blade. However, the leaf may be dissected to form lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade being separated along a main or secondary vein. The leaflets may have petiolules and stipels, the equivalents of the petioles and stipules of leaves. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. The middle vein of a compound leaf or a frond, when it is present, is called a rachis.

Palmately compound

Leaves have the leaflets radiating from the end of the petiole, like fingers of the palm of a hand; for example, Cannabis (hemp) and Aesculus (buckeyes).

Pinnately compound

Leaves have the leaflets arranged along the main or mid-vein.

Odd pinnate

With a terminal leaflet; for example, Fraxinus (ash).

Even pinnate

Lacking a terminal leaflet; for example, mahogany. A specific type of even pinnate is bipinnate, where leaves only consist of two leaflets; for example, Hymenaea.

Bipinnately compound

Leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis. Each leaflet is called a pinnule. The group of pinnules on each secondary vein forms a pinna; for example, Albizia (silk tree).

Trifoliate (or trifoliolate)

A pinnate leaf with just three leaflets; for example, Trifolium (clover), Laburnum (laburnum), and some species of Toxicodendron (for instance, poison ivy).

Pinnatifid

Pinnately dissected to the central vein, but with the leaflets not entirely separate; for example, Polypodium, some Sorbus (whitebeams). In pinnately veined leaves the central vein in known as the midrib.

Characteristics of the petiole

The overgrown petioles of rhubarb (Rheum rhabarbarum) are edible.

 

Petiolated leaves have a petiole (leaf stalk), and are said to be petiolate.

Sessile (epetiolate) leaves have no petiole and the blade attaches directly to the stem. Subpetiolate leaves are nearly petiolate or have an extremely short petiole and may appear to be sessile.

In clasping or decurrent leaves, the blade partially surrounds the stem.

When the leaf base completely surrounds the stem, the leaves are said to be perfoliate, such as in Eupatorium perfoliatum.

In peltate leaves, the petiole attaches to the blade inside the blade margin.

In some Acacia species, such as the koa tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. There may or may not be normal pinnate leaves at the tip of the phyllode.

A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole, resembling a small leaf. Stipules may be lasting and not be shed (a stipulate leaf, such as in roses and beans), or be shed as the leaf expands, leaving a stipule scar on the twig (an exstipulate leaf). The situation, arrangement, and structure of the stipules is called the "stipulation".

Free, lateral

As in Hibiscus.

Adnate

Fused to the petiole base, as in Rosa.

Ochreate

Provided with ochrea, or sheath-formed stipules, as in Polygonaceae; e.g., rhubarb.

Encircling the petiole base

Interpetiolar

Between the petioles of two opposite leaves, as in Rubiaceae.

Intrapetiolar

Between the petiole and the subtending stem, as in Malpighiaceae.

Veins

Branching veins on underside of taro leaf

 

The venation within the bract of a lime tree

 

Micrograph of a leaf skeleton

 

Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics. The veins in a leaf represent the vascular structure of the organ, extending into the leaf via the petiole and provide transportation of water and nutrients between leaf and stem, and play a crucial role in the maintenance of leaf water status and photosynthetic capacity.They also play a role in the mechanical support of the leaf. Within the lamina of the leaf, while some vascular plants possess only a single vein, in most this vasculature generally divides (ramifies) according to a variety of patterns (venation) and form cylindrical bundles, usually lying in the median plane of the mesophyll, between the two layers of epidermis. This pattern is often specific to taxa, and of which angiosperms possess two main types, parallel and reticulate (net like). In general, parallel venation is typical of monocots, while reticulate is more typical of eudicots and magnoliids ("dicots"), though there are many exceptions.

The vein or veins entering the leaf from the petiole are called primary or first order veins. The veins branching from these are secondary or second order veins. These primary and secondary veins are considered major veins or lower order veins, though some authors include third order. Each subsequent branching is sequentially numbered, and these are the higher order veins, each branching being associated with a narrower vein diameter. In parallel veined leaves, the primary veins run parallel and equidistant to each other for most of the length of the leaf and then converge or fuse (anastomose) towards the apex. Usually, many smaller minor veins interconnect these primary veins, but may terminate with very fine vein endings in the mesophyll. Minor veins are more typical of angiosperms, which may have as many as four higher orders. In contrast, leaves with reticulate venation there is a single (sometimes more) primary vein in the centre of the leaf, referred to as the midrib or costa and is continuous with the vasculature of the petiole more proximally. The midrib then branches to a number of smaller secondary veins, also known as second order veins, that extend toward the leaf margins. These often terminate in a hydathode, a secretory organ, at the margin. In turn, smaller veins branch from the secondary veins, known as tertiary or third order (or higher order) veins, forming a dense reticulate pattern. The areas or islands of mesophyll lying between the higher order veins, are called areoles. Some of the smallest veins (veinlets) may have their endings in the areoles, a process known as areolation. These minor veins act as the sites of exchange between the mesophyll and the plant's vascular system. Thus, minor veins collect the products of photosynthesis (photosynthate) from the cells where it takes place, while major veins are responsible for its transport outside of the leaf. At the same time water is being transported in the opposite direction.

The number of vein endings is very variable, as is whether second order veins end at the margin, or link back to other veins. There are many elaborate variations on the patterns that the leaf veins form, and these have functional implications. Of these, angiosperms have the greatest diversity. Within these the major veins function as the support and distribution network for leaves and are correlated with leaf shape. For instance, the parallel venation found in most monocots correlates with their elongated leaf shape and wide leaf base, while reticulate venation is seen in simple entire leaves, while digitate leaves typically have venation in which three or more primary veins diverge radially from a single point.

In evolutionary terms, early emerging taxa tend to have dichotomous branching with reticulate systems emerging later. Veins appeared in the Permian period (299–252 mya), prior to the appearance of angiosperms in the Triassic (252–201 mya), during which vein hierarchy appeared enabling higher function, larger leaf size and adaption to a wider vaiety of climatic conditions. Although it is the more complex pattern, branching veins appear to be plesiomorphic and in some form were present in ancient seed plants as long as 250 million years ago. A pseudo-reticulate venation that is actually a highly modified penniparallel one is an autapomorphy of some Melanthiaceae, which are monocots; e.g., Paris quadrifolia (True-lover's Knot). In leaves with reticulate venation, veins form a scaffolding matrix imparting mechanical rigidity to leaves.

Morphology changes within a single plant

Homoblasty

Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages, in contrast to;

Heteroblasty

Characteristic in which a plant has marked changes in leaf size, shape, and growth habit between juvenile and adult stages.

Anatomy (medium and small scale)

Medium-scale features

Leaves are normally extensively vascularized and typically have networks of vascular bundles containing xylem, which supplies water for photosynthesis, and phloem, which transports the sugars produced by photosynthesis. Many leaves are covered in trichomes (small hairs) which have diverse structures and functions.

Small-scale features

The major tissue systems present are

• The epidermis, which covers the upper and lower surfaces
• The mesophyll tissue inside the leaf, which is rich in chloroplasts (also called chlorenchyma)
• The arrangement of veins (the vascular tissue)

These three tissue systems typically form a regular organization at the cellular scale. Specialized cells that differ markedly from surrounding cells, and which often synthesize specialized products such as crystals, are termed idioblasts.

Major leaf tissues

• 

  Cross-section of a leaf
  
• 

  Epidermal cells
  
• 

  Spongy mesophyll cells
  

Epidermis

SEM image of the leaf epidermis of Nicotiana alata, showing trichomes (hair-like appendages) and stomata (eye-shaped slits, visible at full resolution).

 

The epidermis is the outer layer of cells covering the leaf. It is covered with a waxy cuticle which is impermeable to liquid water and water vapor and forms the boundary separating the plant's inner cells from the external world. The cuticle is in some cases thinner on the lower epidermis than on the upper epidermis, and is generally thicker on leaves from dry climates as compared with those from wet climates. The epidermis serves several functions: protection against water loss by way of transpiration, regulation of gas exchange and secretion of metabolic compounds. Most leaves show dorsoventral anatomy: The upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.

The epidermis tissue includes several differentiated cell types; epidermal cells, epidermal hair cells (trichomes), cells in the stomatal complex; guard cells and subsidiary cells. The epidermal cells are the most numerous, largest, and least specialized and form the majority of the epidermis. They are typically more elongated in the leaves of monocots than in those of dicots.

Chloroplasts are generally absent in epidermal cells, the exception being the guard cells of the stomata. The stomatal pores perforate the epidermis and are surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts, forming a specialized cell group known as the stomatal complex. The opening and closing of the stomatal aperture is controlled by the stomatal complex and regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Stomata therefore play the important role in allowing photosynthesis without letting the leaf dry out. In a typical leaf, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and are more numerous in plants from cooler climates.

Mesophyll

Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the primary location of photosynthesis in the plant. The products of photosynthesis are called "assimilates".

In ferns and most flowering plants, the mesophyll is divided into two layers:

• An upper palisade layer of vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis, with intercellular air spaces between them. Its cells contain many more chloroplasts than the spongy layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight separation of the cells provides maximum absorption of carbon dioxide. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil are single-layered.
• Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more branched and not so tightly packed, so that there are large intercellular air spaces between them for oxygen and carbon dioxide to diffuse in and out of during respiration and photosynthesis. These cells contain fewer chloroplasts than those of the palisade layer. The pores or stomata of the epidermis open into substomatal chambers, which are connected to the intercellular air spaces between the spongy and palisade mesophyll cells.

Leaves are normally green, due to chlorophyll in chloroplasts in the mesophyll cells. Plants that lack chlorophyll cannot photosynthesize.

Vascular tissue

The veins of a bramble leaf

The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. The pattern of the veins is called venation. In angiosperms the venation is typically parallel in monocotyledons and forms an interconnecting network in broad-leaved plants. They were once thought to be typical examples of pattern formation through ramification, but they may instead exemplify a pattern formed in a stress tensor field.

A vein is made up of a vascular bundle. At the core of each bundle are clusters of two distinct types of conducting cells:

Xylem

Cells that bring water and minerals from the roots into the leaf.

Phloem

Cells that usually move sap, with dissolved sucrose(glucose to sucrose) produced by photosynthesis in the leaf, out of the leaf.

The xylem typically lies on the adaxial side of the vascular bundle and the phloem typically lies on the abaxial side. Both are embedded in a dense parenchyma tissue, called the sheath, which usually includes some structural collenchyma tissue.

Leaf development

According to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots, being derived from leaf primordia of the shoot apex. Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions. On the basis of molecular genetics, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties."

Ecology

Biomechanics

Plants respond and adapt to environmental factors, such as light and mechanical stress from wind. Leaves need to support their own mass and align themselves in such a way as to optimize their exposure to the sun, generally more or less horizontally. However, horizontal alignment maximizes exposure to bending forces and failure from stresses such as wind, snow, hail, falling debris, animals, and abrasion from surrounding foliage and plant structures. Overall leaves are relatively flimsy with regard to other plant structures such as stems, branches and roots.

Both leaf blade and petiole structure influence the leaf's response to forces such as wind, allowing a degree of repositioning to minimize drag and damage, as opposed to resistance. Leaf movement like this may also increase turbulence of the air close to the surface of the leaf, which thins the boundary layer of air immediately adjacent to the surface, increasing the capacity for gas and heat exchange, as well as photosynthesis. Strong wind forces may result in diminished leaf number and surface area, which while reducing drag, involves a trade off of also reduces photosynthesis. Thus, leaf design may involve compromise between carbon gain, thermoregulation and water loss on the one hand, and the cost of sustaining both static and dynamic loads. In vascular plants, perpendicular forces are spread over a larger area and are relatively flexible in both bending and torsion, enabling elastic deforming without damage.

Many leaves rely on hydrostatic support arranged around a skeleton of vascular tissue for their strength, which depends on maintaining leaf water status. Both the mechanics and architecture of the leaf reflect the need for transportation and support. Read and Stokes (2006) consider two basic models, the "hydrostatic" and "I-beam leaf" form (see Fig 1). Hydrostatic leaves such as in Prostanthera lasianthos are large and thin, and may involve the need for multiple leaves rather single large leaves because of the amount of veins needed to support the periphery of large leaves. But large leaf size favors efficiency in photosynthesis and water conservation, involving further trade offs. On the other hand, I-beam leaves such as Banksia marginata involve specialized structures to stiffen them. These I-beams are formed from bundle sheath extensions of sclerenchyma meeting stiffened sub-epidermal layers. This shifts the balance from reliance on hydrostatic pressure to structural support, an obvious advantage where water is relatively scarce. Long narrow leaves bend more easily than ovate leaf blades of the same area. Monocots typically have such linear leaves that maximize surface area while minimising self-shading. In these a high proportion of longitudinal main veins provide additional support.

Interactions with other organisms

Some insects, like Kallima inachus, mimic leaves

 

Although not as nutritious as other organs such as fruit, leaves provide a food source for many organisms. The leaf is a vital source of energy production for the plant, and plants have evolved protection against animals that consume leaves, such as tannins, chemicals which hinder the digestion of proteins and have an unpleasant taste. Animals that are specialized to eat leaves are known as folivores.

Some species have cryptic adaptations by which they use leaves in avoiding predators. For example, the caterpillars of some leaf-roller moths will create a small home in the leaf by folding it over themselves. Some sawflies similarly roll the leaves of their food plants into tubes. Females of the Attelabidae, so-called leaf-rolling weevils, lay their eggs into leaves that they then roll up as means of protection. Other herbivores and their predators mimic the appearance of the leaf. Reptiles such as some chameleons, and insects such as some katydids, also mimic the oscillating movements of leaves in the wind, moving from side to side or back and forth while evading a possible threat.

Seasonal leaf loss

Leaves shifting color in autumn (fall)

 

Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). This mechanism to shed leaves is called abscission. When the leaf is shed, it leaves a leaf scar on the twig. In cold autumns, they sometimes change color, and turn yellow, bright-orange, or red, as various accessory pigments (carotenoids and xanthophylls) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production. Red anthocyanin pigments are now thought to be produced in the leaf as it dies, possibly to mask the yellow hue left when the chlorophyll is lost—yellow leaves appear to attract herbivores such as aphids. Optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.

Evolutionary adaptation

Poinsettia bracts are leaves which have evolved red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by petals (which are themselves leaves highly modified by evolution).

 

In the course of evolution, leaves have adapted to different environments in the following ways:

• Waxy micro- and nanostructures on the surface reduce wetting by rain and adhesion of contamination.
• Divided and compound leaves reduce wind resistance and promote cooling.
• Hairs on the leaf surface trap humidity in dry climates and create a boundary layer reducing water loss.
• Waxy plant cuticles reduce water loss.
• Large surface area provides a large area for capture of sunlight.
• In harmful levels of sunlight, specialized leaves, opaque or partly buried, admit light through a translucent leaf window for photosynthesis at inner leaf surfaces (e.g. Fenestraria).
• Succulent leaves store water and organic acids for use in CAM photosynthesis.
• Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalypts).
• Inclusions of crystalline minerals deter herbivores (e.g. silica phytoliths in grasses, raphides in Araceae).
• Petals attract pollinators.
• Spines protect the plants from herbivores (e.g. cacti).
• Stinging hairs to protect against herbivory, e.g. in Urtica dioica and Dendrocnide moroides (Urticaceae).
• Special leaves on carnivorous plants are adapted for trapping food, mainly invertebrate prey, though some species trap small vertebrates as well (see carnivorous plants).
• Bulbs store food and water (e.g. onions).
• Tendrils allow the plant to climb (e.g. peas).
• Bracts and pseudanthia (false flowers) replace normal flower structures when the true flowers are greatly reduced (e.g. spurges and spathes in the Araceae.