Leaves

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Leaves, at least while still alive are generally described as flat, green and found on plants. There are however huge variations in size, shape, colour and purpose. For the most part though leaves are the factories of the plant, where photosynthesis occurs, fixing carbon with the aid of energy from sunlight. This makes leaves critical as primary produces of food, and form the base of many food chains including our own. They have many other uses to us including composting, insect deterants and medicine. Each leaf is very specific to the plant it grows on, though typically they consist of veins, one or more blades and the petiole (leaf stalk). Non-flowering plants(Gymnosperms)such as mosses have highly specialised leaves where as flowering plants (Angiosperms) have hugely varied ones. There are also two types of flowering plants; monocotyledonous plants such as grasses and dicotyledonous plants such as daisies and the way their leaves grow are very different from each other, as is explained later on.

Parts of the Leaf

Despite being generally very flat, a leaf has many layers, each with its own function.

  • The cuticle is a waxy skin that helps control water loss as well as help protect the plant from predators and pathogens
  • The epidermis is the skin of the leaf
  • Trichomes, perhaps better known as leaf hairs, protrude from the leaf surface.
  • The palisade layer is the layer of cells under the epidermis, on the upper side of the leaf. This is where the majority of photosynthesis occurs.
  • Spongy mesophyll cells make up the majority of the the rest of the leaf. They act as stores for the products of photosynthesis and water. Between the mesophyll cells are air spaces needed for gaseous exchange.
  • Stomata are pores found on both the upper and lower surface of the leaf, but mostly on the lower side. Stomata can be opened and closed by the guard cells.
  • Guard cells are kidney shaped cells that work in pairs to open and close the stomata. Their action is very important, as they regulate the water loss and gaseous exchange.
  • Vascular cells, made up of xylem and phloem cells, supply the leaf with water and distribute the products of photosynthesis.

Leaf Formation

Leaves of dicotyledonous plants grow from the shoot apical meristem1. The angle between one leaf forming and the next is normally 137.5 degrees; this is to maximise exposure to light. Three layers of tissue on the meristem are involved with the formation of the new leaf. The L1 layer will form the epidermis. L2 normally forms the outer part of the mesophyll, and L3 the inner part. Sometimes mutations in L2 and L3 occur, causing a fault in the production of chloropyll (the green photosynthetic pigment that forms a key part of photosynthesis). This gives rise to variegated leaves. As the meristem is the only place that leaves grow from, this enables gardeners to prune plants into shape, as the plant must grow a new meristem rather than keep growing in the same direction.

Monocotyledenous plants have leaves that continually grow from their base, so if the top of a leaf is cut off then the leaf can still keep growing. This is why a lawn keeps needing to be mown and a field can be continually grazed in.

Senescence and Shedding

Leaves eventually grow old and less efficient. As soon as a leaf is fully extended it starts to lose its productivity. At the point at which the leaf takes more energy to maintain than it creates through photosynthesis, it is normally shed. Leaves may also be shed if they become diseased or injured. Shedding a leaf also removes waste products of photosynthesis that accumulate in the leaf over time. Trees will shed their leaves all year round, but some trees, such as several broadleaved species in temperate climates or tropical areas with a pronounced wet and dry season, will shed all their leaves at the start of an unfavourable season.2

Leaf shedding (abscission) is a very complex process and involves several plant hormones. However, generally speaking, the contents of the leaf the plant wants to keep3 break down and are transported to the stem or woody tissue. Chlorophyll breaks down and the underlying pigments are revealed (chlorophyll is just one of many pigments, a leaf just looks green if chlorophyll is the dominant pigment). In most dicotyledonous species, the leaf is shed at the abscission zone, a weak area with few fibres at the base of the petiole.

Leaf Variation

There is huge variation in leaf form and structure as leaves are adapted to different environmental conditions. Leaves can also be hugely varied on the same plant, especially those on water plants where some leaves are submerged and others emergent above the water surface. The age of a plant can also affect leaf form; young plants tend to have simple leaves whereas adult plants will have more complex leaves as anyone who grows plants from seed will know. Some of the main leaf variations are given below.

Colour

For the most part, leaves are green due to the dominant photosynthetic pigments; chlorophyll a and chlorophyll b. These pigments absorb most light and reflect mostly green light, and so give plants their main colour. Other pigments include the orangey carotene, the yellow xanthophylls and in some plants red anthocyanins (this is a water-soluble pigment and its purpose is not currently known). Leaves can be different shades of green and even purple or red due to the differing amounts of the pigments they contain. Water plants will often have red leaves as red light doesn't penetrate very far through water. Some plants even colour some of their leaves instead of growing petals.

Pests, diseases and nutrient deficiency can also lead to colour changes. For example, leaf miners can tunnel through leaves leaving silvery tracks. Powdery mildews can smother a leaf and coral spot causes orange spots. If a plant is deficient in a particular nutrient then the way the leaf changes colour can be a clue as to what nutrient it is, though in many cases the edge of the leaf just dies and turns brown.

Aging can also cause a leaf to change colour, as found in the autumn foliage of deciduous trees. Chlorophyll breaks down revealing the oranges, reds and yellows of the remaining pigments causing a great display of colour. Eventually the leaf dies and darkens and is shed.

Sun and Shade Leaves

In broadleaved trees in particular, a plant may possess sun and shade leaves. Photosynthesis is actually not all that efficient, and the process becomes saturated at 1/4 strength of full sunlight. Leaves near the canopy are therefore often small and thick so that many can be placed close together and at different angles. They can be thick as the solar radiation that penetrates through the upper layers of the leaf will still be strong enough for photosynthesis to occur. Shade leaves however tend to be large and thin in order to maximise the surface are exposed to the only just strong enough light.

Shape

Simple leaves, such as grass, only have a single blade per leaf where as compound leaves are broken up into leaflets. Leaves are very varied in shape and so help in identifying a plant such as the spiky edges of a holly leaf or the lobed edges of an oak.

Trichomes

Trichomes are leaf hairs. The cells that form them are a lot larger than most other leaf cells. Often, trichomes are composed of a single cell, though are normally made up of 3-5 cells. Trichomes have a variety of functions and there may be many different types on one leaf. For example, some plants such as mint emit a fragrance if you rub your hand over the leaves - this is a result of breaking the trichomes and releasing the oils within. If you rub a nettle, you get stung, as the trichomes dig into the skin and release irritants. Sometimes a trichome's function is simply to reduce water loss by reducing the windspeed over the stomata and thereby reducing evaporation. Trichomes may also make it difficult for insects to walk on the leaf (by making it very hairy) and therefore reduce its chances of being eaten. Some trichomes can be used to excrete excess salt absorbed from salty soils.

Spines and Tendrils

In some plants the leaves have hardened and become pointed to form spines. These are common in some desert species such as cacti. Spines provide protection from predators, for in cacti the stem is succulant so attractive to predators but is very important to the plant as it has become the main organ for photosynthesis. Tendrils help support climbing plants that can't support their own weight. In some cases, such as grape vines and ivy, tendrils are specialised stems. However, in other plants such as peas and cucumbers, tendrils are in fact modified leaves.

Bulbs and Succulents

Bulbs, formed by plants such as onions, tulips and daffodils, consist of a short underground stem with several large, fleshy leaves. These leaves are adapted for the storage of water and food. Succulents such as aloes also have fleshy leaves for the storage of water as an adaptation to living in arid conditions. The leaves of succulents also function in photosynthesis.

Carnivorous Plants

Some plants, especially those that live in boggy areas, are unable to acquire all the minerals they require, especially nitrogen. Some highly specialised plants have consequently evolved to be able to catch insects as a means of acquiring these minerals. In these carnivorous plants, the leaves have adapted to form passive or active traps. Pitcher plants have passive traps, where the leaves form bowls to collect rainwater. Pitcher plants secrete acid into the water to kill and digest the unfortunate insects and other small invertebrates that fall into the trap. The pitcher plant also has downward pointing hairs above the water level and some species attract insects to them with nectar. When an insect lands on the leaf it normally slips into the acid where it is digested by the plant.

Venus flytraps have leaves which have been modified to form active traps. The traps have three short hairs and in an insect brushes against two of the hairs or one in quick succession, the trap springs shut. The struggling of the insect stimulated the trap to tighten, and once fully shut the indect is digested inside the leaf. The trap then reopens and the indigestible exoskeleton is blown away.

Needles

Needles, such as pine needles, are the specialised leaves of conifers, most of which are Gymnosperms (non flowering plants). For many conifers, it is essential to keep water loss to a minimum for during winter months water is frozen and therefore unavailable to the plant. Needles therefore have grooves on the underside in which the stomata are sunk, minimising airflow over the stomata and therefore reducing water loss. Although present all year, needles do not photosynthesise during the winter when water retention is critical. Conifers retain their needles so that as soon as water is again available, they can start photosynthesising again and so their growing season is maximised.

1The growing point at the top of dicotyledonous plants2Such as Autumn3The sugars, amino acids and essential minerals

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