The following points highlight the three main parts of a leaf. The parts are: 1. The Leaf-Base 2. The Petiole 3. The Lamina.
Part # 1. The Leaf-Base:
It is by no means a conspicuous part of the leaf. Some leaves like those of the pea, sensitive plant, mango have swollen leaf-bases. Such a leaf- base is called pulvinus (Fig. 62, a). In many monocotyledons and a few dicotyledons the leaf-base grows into a sheath partly or wholly surrounding the stem. e.g. palms, grasses, banana (Fig. 62, b) coriander.
The Stipules:
The leaf-base often bears a pair of lateral appendages or outgrowths called stipules. If stipules are present, the leaves are called stipulate, e.g. rose, China-rose; and if stipules are wanting, they are called ex-stipulate, e.g. mango, guava. They normally protect the buds.
The stipules are of the following kinds (Fig. 63):
1. Free Lateral:
These are two small green stipules from the two sides of the leaf-base which remain quite free from each other, e.g. China- rose (Fig. 63, 1), cotton.
2. Adnate:
Adnate stipules are found in roses. The stipules unite with the petiole up to a certain distance and form two wings on the two sides (Fig. 63, 2).
3. Interpetiolar:
The stipules of two opposite leaves unite by their outer margins and form a structure lying in between the petioles of the two leaves, e.g. Ixora (B. Rangan, Fig. 63, 3), Anthocephalus (B. Kadamba).
4. Ochreate or Ochrea:
Here the two stipules unite to form a hollow tube surrounding a part of inter- node above, e.g. Polygonum (B. Panimarich, Fig. 63, 5), Rumex (B. Ban palang).
5. Bud Scales:
These are rather prominent stipules which protect the terminal bud when young. With the gradual opening of the bud the pale membranous stipules fall off, e.g. banyan, India- rubber, Jack fruit plant (Fig. 63, 4).
Modifications of Stipules:
Some stipules are modified for special functions.
They are:
I. Foliaceous:
These are large flat leaf-like stipules as found in the pea (Fig. 63, 6). Here the terminal leaflets of the compound leaf are modified into tendrils and the pair of foliaceous stipules take up the functions of leaves. In some varieties of wild pea the whole leaf is modified into a tendril.
II. Tendriller:
These stipules are found in Smilax (B. Kumarika) where a pair of tendrils develop from the leaf-bait obviously they are meant for climbing (Fig. 63, 8).
III. Spinous:
In Acacia (B. Babla) or Zjzyphus (B. Kul) a pair of pointed outgrowths are present at the leaf-base (Fig. 63, 7). These modified stipules are meant for defensive purpose.
Part # 2. The Petiole:
The petiole is the cylindrical stalk connecting the lamina with the base. It exposes the leaf lamina to proper light and air, and also conducts water from the stem to the leaf blade and elaborated food matters from the leaf to the stem.
If the petiole is present, the leaf is called petiolate, e.g. pipul, mango; and if the petiole is wanting, the leaf is called sessile, e.g. Calotropis (B. Akanda), Gloriosa (B. Ulat chandal). Normally the petiole lies in the same plane with the lamina, but in some leaves like those of the lotus, castor, etc., the petiole is attached at the back more or less at right angles to the blade. They are called peltate leaves.
Usually the petiole is a cylindrical stalk. It is grooved in banana, and swollen and spongy in water hyacinth due to the presence of abundant air chambers (Fig. 64). In lemon leaves it forms a pair of fiat wings on the two sides and is called winged petiole (Fig. 64).
Leaves of Australian Acacia (B. Akasmoni) have peculiar petioles. They become vertically flattened and lamina like in appearance. As a matter of fact, the leaves of these plants fall off soon and their functions are taken up by flat lamina-like petioles.
These modified petioles are called phyllodes (Fig. 65). In grasses a membranous hairy outgrowth, called ligule, is present at the point of the union of the sheathing leaf-base and the blade. These leaves are known as ligulate.
Part # 3. The Lamina:
The lamina is the most conspicuous and important part of the leaf. It is characterised by green colour, thinness and flatness. The structure is so nicely constructed that the maximum number of chlorophyll containing cells may be exposed to light and air, and the different functions involving gaseous exchange between the internal tissues and the outer atmosphere, may be conveniently carried on.
Lamina should be discussed with reference to the form or shape, the margin, the apex and the veins.
Forms of Lamina:
Leaves show wide variations as regards their forms or shapes. There are the long needle-like leaves of the pines and Casuarina (B. Jhau), and in the other extreme there are the round disc-like leaves of the lotus and Victoria regia. Transitional forms are numerous.
An accounts of the more common forms of lamina is given (fig 66):
1. Acicular, the long needle-like leaves, as in the pine (Fig. 66, 1).
2. Linear, the lamina here is quite long and narrow; the two margins run more or less parallel to each other but meet at the apex, e.g. grass (Fig. 66, 2).
3. Lanceolate, the length of lamina is much greater than its breadth and it tapers towards both ends more or less like a lance, e.g. Polyalthia (B. Debdaru), custard apple (Fig. 66, 3).
4. Ovate or egg-shaped, when it is broad at the base but gradually becomes narrow towards the apex, e.g. banyan (Fig, 66, 6).
5. Obovate or inversely ovate, which is broad at the apex and narrow towards the base, e.g. almond, jack fruit leaf (Fig. 66, 7). –
6. Cordate or heart-shaped, it is broad at the base but forms two equal lobes due to the formation of a deep notch, as in betel vine (Fig. 66, 8).
7. Obcordate or inversely cordate (Fig. 66, 9), as in Oxalis (B. Amrul), Bauhinia (B. Kanchan).
8. Oblong, here the lamina is wide and long and the two ends are rounded, e.g. banana (Fig. 66, 5).
9. Reniform or kidney-shaped, it is rounded above but has a deep notch towards the base (Fig. 66, 10), e.g. Hydrocotyle (B. Thulkuri).
10. Sagittate or arrow-shaped, as in arum (Fig. 66, 11).
11. Hastate, it is like the sagittate leaf but two lobes are directed outwards (Fig. 66, 12), e.g. Ipomoea (B. Kalmi shak).
12. Elliptical or like an ellipse, as in guava, India rubber (Fig. 66, 13).
13. Orbicular or round, as in the lotus, Victoria regia (Fig. 66, 13).
The Margin of the Leaf (Fig. 67):
Margin may be:
(a) entire, i.e. even and smooth, e.g. banyan, banana;
(b) wavy, as in Polyalthia (B. Debdaru);
(c) serrate or like the teeth of a saw, e.g.China-rose;
(d) dentate, where the teeth are directed outwards, e.g. waterlily;
(e) crenate, having rounded teeth, as in Hydrocotyle (B. Thulkuri); and
(f) spiny, having spiny projections, as in Mexican poppy (B. Shialkantha).
The Apex of the Leaf (fig 68):
The apex may be:
(a) Acute, as in mango, China-rose;
(b) Acuminate, long tapering apex, as in pipul;
(c) Obtuse, as in banyan;
(d) Mucronate, apex ending in an abrupt sharp point, e.g. Vinca (B. Nayantara);
(c) Uspidate, long sharply pointed apex, e.g. date palm, pine-apple; and
(f) Tendrillar, as in Gloriosa (B. Ulat chandal).
Venation (Fig. 69):
Veins are the conducting strands distributed in the leaf lamina. They are solely responsible for the conduction of water and prepared food materials. Moreover, they form the skeleton of the leaf on which softer materials remain inserted. Veins can be conveniently observed by holding the leaf against light. The mode of arrangement of veins is called venation.
There are two principal types of venation:
(i) Reticulate or netted venation and
(ii) Parallel venation.
(a) Reticulate Venation:
The midrib or a few prominent ribs produce many branches and sub branches which ramify in the leaf lamina in such a manner as to form a network. This is reticulate venation (reticulum = net). Some of the veinlets are so small that they are not visible to the naked eye.
Reticulate venation may be unicostate or multicostate, according to the number of prominent ribs. If there is only one prominent rib, i.e. the midrib producing branches, as usual, it is unicostate or pinnate (uni = one; costa = rib), e.g. pipul, mango. But if more than one equally prominent ribs develop from the base, the venation is multicostate or palmate. Multicostate, again, may be convergent or divergent.
If the prominent veins converge (i.e. meet or tend to meet) towards the apex, it is called multicostate convergent, as in Zizyphus (B. Kul), Cinnamon leaf. But if the veins go away from one another or diverge, they are called multicostate divergent, has in castor-oil leaf, Cucurbita. Dicotyledonous leaves have usually reticulate type of venation.
(b) Parallel Venation:
This type of venation is usually found in the monocotyledons. Here the veins run more or less parallel to one another without forming a network. Parallel venation may also be unicostate or multicostate. Banana leaf is a good example of parallel unicostate venation.
In bamboos and grasses a good number of veins arise from the base and meet at the apex. So in these cases venation is parallel multicostate convergent. The palm leaves have many prominent veins but they diverge, showing parallel multicostate divergent venation.