A leaf can be generally define
d as flat
from the stem
of a plant
whose primary purpose
. They are incredibly varied in size and shape, and can show some amazing adaptation
s (see Modifications
General description and appearance
Despite their incredible diversity
in appearance, can be loosely organized into three groups1, 2
: the microphyll
s, the angiosperm
leaves, and the sheath
leaves. Microphylls are seen in relatively few species
in nature, and are considered to be evolutionary relic
s. They are characterized by their extremely small size and lack of special form or differentiation
. They are generally not photosynthetic
. The most prevalent
and common example are the leaves of Equisetum
sp., the horsetail
s. Their leaves are little black triangles which lie around the circumference
of the stem, and look like a pointy collar
around each nodal
The angiosperm leaf, on the other hand, is the classical leaf we all know and love. Examples include the maple, oak, chestnut etc. These leaves are generally quite large, vascularized and differentiated (their vascular organization is called netted veination, see Morphology and histology below). They attach to the stem by way of the petiole (the stalk-like portion of the leaf). The remainder of the leaf is called the blade, or lamina. In many cases, the leaves are not simple (as in the maple) but are instead compound, where many smaller blades are attached to the petiole (as in ash or hickory).
Finally, there are the sheath leaves of the monocotyledons. These leaves are flattened and simple in construction. They are generally shaped like very high triangles, and are far wider than they are thick. The veinous structures in these leaves lie nearly parallel to the long axis of the triangle, and do not cross (this pattern is called parallel veination, see Morphology and histology below). The sheath leaves generally join to the stalk not with a petiole, but surround the stalk like a scarf. Perhaps the easiest example of this leaf type is the corn, where the leaves surround the ear and overlap one another.
Several plant species have two distinct kinds of leaves. These different leaves are either produced by environmental or genetic factors. In many tree species, for example, the leave in the upper part of the crown are thicker and smaller, and are adapted to high light conditions (they are hence called sun leaves). Further down on the plant are the shade leaves, which are far larger and thinner, and have maximal photosynthetic performance under the low light conditions of the understory. These two leaf types are produced as a result of differing light conditions, and the control of their form is not purely genetic. In some aquatic plants, on the other hand, the two leaf morphologies are also observed and their formation is directly controlled by genetic factors. For example, the common bassweed (Potamogeton sp.) has small, paddle-shaped leaves which float along the surface while the leaves submerged under water are elongated and shaped like grass shoots.
In almost all plants, the first leaves appear well after the stem
systems have begun growth. They are produced by the shoot apex
, the region of greatest cell production, at the tip of a stem. They appear, at first, to be no more than little bud
s growing off the stem of a plant (called leaf buttress
es). As the stem continues to grow upwards, the buttress develops into the leaf primordium
, which has a distinct peg
-like appearance. Further development at this point is mostly in cell division, ageing and growth, called intercalary growth, rather than new cell development from the leaf meristem
This general pattern does not always hold true, however. In the case of monocotyledon plants, such as the grasses (including corn and barley), the sheathing leaf base is developped quite early, and the initial growth occurs both laterally and longitunidally (meaning the leaf both wides and lengthens as it grows). As the leaf grows, is changes from a small collar-shaped protuberance, to a down-turned hood and finally the linear blade as in common grasses.
In most plant species common to temperate climates, the leaves are shed as the climate turns inhospitable in the winter months. The process of shedding leaves is called abscission, and is accomplished by structural and chemical changes at the base of the petiole. The abscission zone, where the petiole joins the stem, is comprised of two sections: the separation layer and the protective layer. The separation layer is comprised of small, weak-walled cells which are very effective at transporting water and nutrients. As the temperature cools and the daylight hours shorten, water and nutrients are transported away from the leaves back to the stem for storage. During this transporting process, the chloroplasts break down, eliminating the green colour of the leaf. The yellow and red pigments, however, are not degraded thus explaining why leaves change from green to yellow, red or orange in the fall. After this has been accomplished, specialized enzymes destroy the cells in the separation layer, causing the leaf to fall from the plant. The protective layer, during this time, develops highly dense cells with large cell walls, so as to protect the plant from the environment. After the leaf falls, this layer is clearly visible as the leaf scar.
Morphology and histology
Leaves are comprised of several tissue types, and have a remarkable design
. The upper layer of the leaf (that facing the sun
) is a one-celled layer called the epidermis
. Its primary purpose is protection of the leaf from harsh environmental conditions and insect pest
s. The cells of the epidermis are tightly packed and have a waxy cuticle
which reduces water loss due to evapotranspiration.
Beneath the epidermis lies the mesophyll, which has a large volume of loosely packed cells. It is in this area that photosynthesis occurs. The large air spaces permit the leaf to easily exchange gases required for photosynthesis and respiration. In most plants, the mesophyll is composed of two layers: the palisade parenchyma and the spongy parenchyma. The palisade cells are column-shaped, with the longer axis at right angles to the epidermis. These cells have up to four times as many chloroplasts than those in the spongy parenchyma, and thus perform the majority of the energy fixation. The spongy parenchyma is more useful in gaseous, water and nutrient transport. Note that in some species, particularly the grasses, the distinction between these two parenchymal types is either very subtle or non-existent. The primary vascular tissues of the leaf are also found in the mesophyll, and are bundled into veins (these are clearly seen in the veinous structure of the maple leaf, for example). These veins contain both the xylem and the phloem. These vascular tissues are never exposed directly to the environment, instead being sheathed by parenchymal cells. These cells have very few chloroplasts, and are adapted for water and nutrient transport.
The lower layer of each leaf is also covered by a single layer of epidermal cells, similar in construction to the upper epidermis. However, the lower layer is also covered with stomata, which are small cells used to promote gas exchange (note that the stomata are found on the upper epidermis in aquatic plants, as the atmosphere is only in contact with the upper surface of the leaf). This lower layer may also be covered in leaf hairs, or trichomes, which are used to reduce the air flow along the bottom of the leaf, so as to give the leaf more control over gaseous exchange. These hairs are often used in semi-arid climates to reduce water loss.
There are many leaf modifications that can be observed in nature. Already mentioned are those plants which have dimorphic
leaves, but there are far more unusual and fascinating adaptations. Consider the leaves of celery
, where the largest part of the plant is the petiole, swollen
and elongated to store water and nutrients. Even more fascinating is the example of the venus flytrap
. These plants have a single pair of leaves at the upper apex
of each stem. Each pair of leaves is shaped like a half-circle, and each leaf has numerous toothy projection
s. These leaves are used to capture insect prey
. The leaves secrete a sweet nectar
y-substance, which attracts insects like flies. Once the insect lands on one or both leaves, one or more of the three sensitive hair
s on the inner surface of each leaf is trigger
ed, and the leaves close shut capturing the insect. Digestive enzyme
s then liquify the prey item, and the nitrogen
contained in the insect's body is absorbed by the plant. These leaves are so highly adapted that the plant can differentiate between insect prey and inanimate matter
. If you poke one with a blade of grass of a small stick
, it will likely not close.
1 While they are biologically speaking leaves, I have chosen not to include the needles of gymnosperms (evergreens) in this write-up. They are quite different from the leaves discussed in this write-up, and will be treated more fully in their own node.
2 Again, while the byrophytes (mosses and liverworts) also have leaves, botanically speaking, their formation is quite distinct from those of the higher plants. Some discussion of their body form may be found in their own node.
Composed with help from Raven, P. H., Evert, R. F. and S. E. Eichorn. (1992) Biology
of Plants, 5th Ed. Worth Publishers.