- INTRODUCTION
The structure of the leaf can be described from two viewpoints; from the external overall structure and the angle of internal leaf structure.
EXTERNAL LEAF STRUCTURE
From Figure 1 of an average leaf one can observe that the leaf consists of a stalk and a flattened area. The stalk is the petiole. The larger part of the leaf is called the blade or leaf proper. In some plants the leaf is not attached to a petiole but attached directly to the stem; such leaves are said to be sessile. See Figure 2. The veins of leaves are made up of the xylem and phloem tissue.
INTERNAL LEAF STRUCTURE
The leaf bulk is enclosed between two layers of cells called the epidermis. On top is the upper epidermis and underneath the lower. At regular intervals in the epidermis two oval shaped cells can be seen, between which is a hole leading to the interior of the cell. The two oval cells are called guard cells while the hole is called the stoma. The stomata are more frequent in the lower epidermis than the upper. The upper epidermis is covered by a very thin, waxy cuticle. This waxy cuticle waterproofs the leaf and helps prevent water loss from actual cells. Immediately below the upper epidermis is a layer of oblong cells, usually one cell layer thick called the palisade layer.
The palisade cells contain many small structures called chloroplasts which contain a green pigment called chlorophyll. Just below the palisade layer is a thick layer of loosely packed cells called the spongy mesophyll. Contained between these cells are many air spaces. The vascular bundles are located in this layer and contain fewer chloroplasts. Finally, one reaches the lower epidermis. See Figure 3.
Figure 1: Petiolate leaf.
Figure 2: A sessile leaf (no petiolate).
Figure 3: A cross-section through a leaf.
- THE FUNCTION OF LEAVES
The leaves are the food factories of the plant and use sunlight energy to convert carbon dioxide gas and water into sugars. The chemical reaction for this is as follows:
SUNLIGHT
6CO2 + 12H2O C6 H12O6 + 6H2O + 6O2
This process is called photosynthesis
Sugar is the basic foodstuff of the plant, specifically glucose (C6 H12 O6). The chloroplasts are responsible for photosynthesis and the bulk found in the palisade cells. The leaves give off vast quantities of water, oxygen and carbon dioxide including taking both in. This is where the stomata are important; they can be closed, open or partly open. The guard cells swell up. When fully swollen are banana shaped and the stoma open. However, when they are soft and floppy the stoma is closed. See Figure 4.
Figure 4: A surface view of Stomata.
| Cessation: the fact or process of ending or being brought to an end. Peculiar: different to what is normal or expected; strange. |
THE DEVELOPMENT OF LEAVES
Leaves develop from lateral outgrowth from apical meristems. The growth of the leaf is generalised, and does not occur as a specific point, as in the root and stem. Growth by cell division actually stops when the leaf is very small. The dramatic increase in size is due entirely to cell enlargement and differentiation. The only exception to the cessation of cell division is in the grasses where the base of the leaf remains meristematic and it is for this reason that lawns have to be mowed.
TERMS RELATING TO LEAF SHAPE AND ARRANGEMENT
Figure 5 on the next page illustrates the different terms. From Figure 5 you will see that there are terms that describe the shape of the leaf, the margin of the leaf and whether the leaf is a whole unit, or consists of a number of smaller units.
Whole unit leaves are termed simple leaves, whereas leaves made up of numerous sub-units are termed compound leaves. There are three types of Compound leaves.
THE ROLE OF LEAVES IN PLANT IDENTIFICATION
Botanists regard the flowering parts of a plant as more useful than the leaf, stem or root parts in plant identification. This is because they do not vary as much with conditions. However, leaves are used in identification, especially in trees. From Figure 5 you will conclude that there are numerous combinations of leaf shapes and margins and each plant species will have its own peculiar combination. This fact can be used in identifying different species of plants.
Figure 5: Terms relating to leaf appearance.
