Leaves are the important part of the plant that perform the major functionality of the plant. They are the ones which perform the function of photosynthesis and even the foods. Being the part of the shoot system, they originate from the apical meristems and are arranged in a particular manner like alternate, opposite, sub-opposite, etc.
Basically, leaves are divided into three parts namely Petiole, Leaf Base and the Leaf blade or Lamina. The petiole is that part through which the leaf attaches itself to the stem or the branch from where it has originated. In some instances the petiole is absent. Then such a leaf is known as Sessile else it is known as Petiolate. The region where the petiole attaches to the leaf is known as leaf base and are of various shapes too. These shapes form an important point of observation in the process of identification. Lastly, the lamina or the leaf blade which is a broad flattened part of the leaf which we normally call as leaf is the one that performs the function of photosynthesis, respiration, transpiration, etc.
Other than these there are other parts that are connected with leaf. They are the stipules. The stipules are the lateral appendages present on either side of the leaf base or the petiole where it attaches to the branch or stem. When the stipules are absent the leaf is known as ex-stipulate else it is called as stipulated leaf. The stipules are of different varieties namely interpetiolar stipules, intrapetiolar, Foliaceous stipules, Scaly stipules, Spiny, ochreate, adnate, tendrillar and free lateral. Among this some are modification to carryout a specific function also.
The arrangement of veins in the leaves are an important point of observation with respect to the purpose of identification. This arrangement of veins and veinlets in leaves (Lamina) is known as Venation. It is of two types namely Reticulate and Parallel. In Reticulate venation, the veins are divided into many branches to form a net-like structure where there is one or more principle vein that gives out branches where the principle vein is the one that arises from the tip of the petiole. In a parallel venation all the veins run parallel to each other and hence there is no network of the same.
Another important point of observation for identification is the arrangement of leaf which is called as Phyllotaxy. There are basically three types of arrangement namely alternate, Opposite and Whorled. Alternate is where only one leaf arises in each node where the side of arise is opposite successively. The opposite arrangement is where two leaves arise on each node with the pair being opposite to each other. The opposite is again of two types namely Superposed and Decussate. The superposed is where the pair of leaves are arranged on a same plane such that one pair is exactly below or above the other. The decussate arrangement is where the pair of leaves of successive nodes are at right angles to each other.
Based on duration, leaves are of three types namely Evergreen, Deciduous and Caducous which stands for presence in all seasons, fall in certain seasons and shed when new buds are formed respectively. But broadly the leaves are of two types namely Simple and Compound. A simple leaf is the one where the lamina is either incised but not separated and is continuous whereas a compound leaf is where the leaf blade is incised such that the lamina is separated into several parts known as leaflets.
The compound leaves are again of two varieties namely pinnately compound and palmately compound. A pinnately compound leaf is the one where the mid-rib is known as the Rachis and the leaflets are arranged on both of its sides while palmately compound is where the leaflets are attached at the upper end of the petiole forming an appearance like a palm. Again, the pinnately compound leaves are of four types namely Unipinnate, Bipinnate, Tripinnate and Decompound leaves which is depending on the division of the rachis to which the leaflets are attached. Accordingly, it is one, two, three and more than three divisions respectively. Similarly palmate leaves are of five types namely unifoliate, bifoliate, trifoliate, tetrafoliate and multifoliate which is depending on the number of leaflets that the leaf has. Accordingly, it has one, two, three, four and more than four leaflets respectively.
MACROSCOPICAL CHARACTERS
Generally, leaves are known as the Kitchen of the plant which is due to the fact that it is the part of the plant that carries out the major functionalities. It is also known as the major Laboratory of the plant even the world too. Many of the phytochemicals are formed in the leaves as it is the part that gets majority of the exposure to sunlight which is necessary for the process of Photosynthesis. But at times it performs some of the extra functions for which just like any other parts even leaves too undergo modifications. These modifications are as explained below:
MODIFICATION OF LEAVES
The different modification of leaves to serve different purposes are:
- Leaf Tendril – Here the leaves are modified into tendrils which are awire like structures. For eg. Wild pea
- Leaflet Tendrils – Here the leaflets are modified into tendrils. For eg. Garden Pea
- Spines – For the purpose of defence and to conserve water, the leaves become spines or prickles. For eg. Cactus.
- Scales – The leaves become thin and dry resulting in the formation of a membrane like structure known a scale. this is basically to protect the buds and sometimes to store food and water. For eg. Ficus
- Pitcher – They form a pitcher like structure. This is mainly in carnivourous plants. For eg. Pitcher Plant
- Bladder – Leaves take the shape of the bladder. For eg. Utricularia
- Hooks – The leaves especially tips will modify to the shape of curved hooks mainly to facilitate climbing. For eg. Cat’s nail.
- Phyllode – The petile becomes flat like a leaf and functions similar to that of leaf after the withering of the leaf. For eg. Australian Acacia.