In this article, we propose to discuss about the development and parts of foliage leaf.
The three parts of a typical foliage leaf are: (1) The Leaf-Base (Hypopodium) (2) The Petiole (Mesopodium) and (3) The Lamina (Epipodium).
Foliage leaves are essential for the plant as leaf is the seat of several very important physiological processes like photosynthesis (which is related to the green colour), transpiration and respiration.
Besides these important functions, leaves also protect the buds in their axils and, in special cases, may help in climbing, store food or water (as in Aloe indica), serve for vegetative propagation as in the leaf of Kalanchoe absorb food material from water as in some aquatic plants and absorb protein as in some insectivorous plants.
Nevertheless, there are a number of plants which are completely or partially leafless. In some of these, the function of leaves is taken up by the stem which remains- green and sometimes becomes flat and leaf-like (phylloclades).
Others become saprophytic or parasitic so that this organ for food manufacture becomes unnecessary. As against these leafless plants, we may consider the case of Lemna where, according to some, the whole plant is a leaf, there being no stem.
The duration of leaves varies. It is caducous or fugdcious when the leaves fall off as soon as they are formed as in some Opuntia. Many trees are characterised by the fact that the leaves fall off at the end of a particular season. These leaves are deciduous or annual. Some trees shed all their leaves during winter, and then the trees become bare. New leaves cover the plant on the advent of spring. In most tropical trees, however, the leaves last for a long time and are termed persistent. Such trees are evergreen.
Development and Parts:
A typical foliage leaf has got three parts—(a) Hypopodium or the leaf- base, (b) Mesopodium or the petiole, i.e., the stalk of the leaf, and (c) Epipodium or the leaf lamina or blade. The entire leaf axis is called the phyllopodium.
In the development of the leaf, the leaf primordium first develops into the leaf-base and then a rudimentary lamina develops on it. Next, the leaf-base completes its development and any outgrowth on it (e.g., stipule) is formed.
Simultaneously, the lamina expands to form a flat structure. The petiole develops last as a constriction between the base and the lamina and in many cases it does not develop at all. In some rare cases, the development of the leaf may be checked after the formation of the leaf-base.
Ordinarily, the leaf blade lies in the same plane as the petiole, the lamina-axis being a prolongation of the petiole. But, in some cases, the petiole is attached to the centre of the lower surface of the lamina, the latter being placed at right angles to the former as in lotus, water-lily or garden nasturtium. This type of leaf insertion is called peltate.
1. The Leaf-Base (Hypopodium):
The leaf-base is the point of attachment of the leaf to the stem. Usually, it protects the small bud as its axil. The base is a prolongation of the tissue of the stem and cannot be demarcated from the petiole and the stem unless it is specially developed.
In many instances, as in the leguminous plants (e.g., the sensitive plant Mimosa pudica of Mimoseae or Dolichos of Papilionaceae) or in plants like mango , etc., the leaf-base has a swollen appearance forming a sort of a cushion and is called the pulvinus. Such a leaf can be easily plucked off and leaves a scar on the stem.
In most monocotyledons and in a few dicotyledons the leaf-base becomes very prominent and winged. In that case it clasps and sheaths the stem forming a sheathing leaf-base. Sometimes the sheathing leaf-base does not clasp a stem inside but the sheath of the next younger leaf and so on.
This is the case in banana where the apparent stem is only a series of sheathing leaf-bases, one inside another. The sheathing leaf- base may clasp and surround the stem completely when it is termed amplexicaul (e.g., grasses, Polygonum orientate, Aethusa of Umbelliferae
) or such clasping may be incomplete (as in coriander, palms, banana, etc.) when it is called semi-amplexicaul.
In a few cases, the petiole as well as the leaf-base is winged. These wings extend down the stem which also appears to be winged and all these wings extend down to the lower node. This is known as a decurrent leaf-base as found in Laggera of Compositae and Symphytum of Boraginaceae .
The leaf-base frequently bears a pair of lateral appendages or outgrowths called stipules.
Stipules are outgrowths from the leaf-base which usually protect the young axillary buds. The appearance of a stipule is changed by cohesion, adhesion or by the assumption of special functions. While they are very common among the dicotyledons, they are rather rare among the monocotyledons. When a leaf has got the stipular appendages it is called stipulate, when stipules are absent it is exstipulate (e.g., mango).
The duration of the stipules may be caducous (shed very early as in Michelia champaca, Dipterocarpus scabes, etc.), deciduous (lasting one season as in Dillenia indica, Cassia tora, etc.) or persistent (in pea, rose, etc.) as in foliage leaves. Stipules may be highly branched as in Ipomoea quamoclit.
The following types of stipules are commonly met with:
These stipules are of the simplest type. They are two soft filiform stipules hanging freely on the two sides of the leaf-base as in the china-rose, cotton, etc. (plants of the family Malvaceae) and also in some other plants.
The four following stipules are modified because of adhesion (i.e., union between dissimilar organs, e.g., between stipule and petiole) or cohesion (union between different organs of the same type, e.g., stipule and stipule, petal and petal).
(2) Adnate or Adherent:
Here the two stipules are adherent to the sides of the petiole to a certain distance so that the base of the petiole appears to be winged. This is seen in the compound leaf of rose.
Sometimes the two stipules are coherent by their inner margins when a single fused stipule appears to be present at the axil of a leaf (intra means within). Examples are found in Gardenia (with opposite leaves), Houttuynia , etc.
In certain plants showing opposite phyllotaxy each of the two leaves at a node has stipules from its base. Cohesion takes places between stipules of opposite leaves by their outer margins so that the four stipules are transformed into two fused stipules which lie on two sides of the stem between (inter-) the petioles of the two opposite leaves. This is clearly seen in Ixora , Anthocephalus, Cephalanthus, etc.
The two stipules of leaf-base fuse to form a tube-like structure covering the internode up to a certain height. The tubular covering is called an ochrea. Ochrea is found in different species of Polygonum . In Astragalus the ochrea remains open at one side.
Stipules are sometimes modified to carry on special functions as in the following cases:
In peas the stipules of the compound leaf are modified into large leaf-like green structures which take up all the functions of foliage leaves. The true leaf lamina (the leaflets) is sometimes absent altogether being replaced by tendrils.
The stipules are modified into tendrils which help in climbing as in Smilax . (N.B. The stipule nature of Smilax tendrils is doubtful. It has been shown recently that it rather originates from the petiole.)
Stipules may be modified into two spines as in the Acacias or in Zizyphus mauritiana . Such spines serve as defensive armature.
(9) Convolute or Bud-Scales:
Bud-scales which protect the buds (discussed in connection with protection of buds) are sometimes modified stipules. The bud-scales fall off as the bud opens. Such bud-scales may be seen in the jack-fruit (Artocarpus).
It is explained later that the leaf lamina is broken up into small bits (leaflets) in compound leaves. The stipules are always borne on the leaf-base of the whole compound leaf and so no such stipule is expected at the bases of small leaflets.
But, sometimes, such stipule-like appendages are found to grow on the two sides of the leaflet bases. These appendages are called stipels and are regarded by some as rudimentary leaflets which failed to develop. Stipels are present in many leguminous plants like Dolichos lablab , Desmodium, Phaseolus, Clitoria. Vicia faba, etc.
2. The Petiole (Mesopodium):
The petiole develops last in the leaf and sometimes may not develop at all. A leaf with a petiole is petiolate (mango, etc.) and one without any is sessile (Calotropis, etc.).
A petiole or leaf-stalk is usually solid and cylindrical but may be more or less flattened, grooved and spongy as in banana or it may be hollow as in papaw (Carica papaya). Sometimes, however, the petiole is modified to a much greater extent.
(1) In lemon and shaddock (Citrus spp) the petiole becomes winged and resembles the leaf lamina.
(2) In Nepenthes the petiole is partly winged and partly tendrillar. (N.B. There is some difference of opinion as to the true nature of these organs as discussed in connection with the pitcher plants).
(3) In Clematis the petioles are tendrillar.
(4) In water hyacinth and Trapa bispinosa the petiole is swollen and spongy. This type of petiole encloses much air and helps the plant to float.
(5) In Quisqualis the leaf blades sometimes fall off and the petioles become hard Spines.
(6) The most interesting modification of the petiole is the phyllode. These are found in Australian Acacias of which there are several species. In Acacia moniliformis the seedling plants bear normal compound leaves.
As these leaves become mature, the leaflets drop off and the petioles (or sometimes any part of the rachis) become flattened and leaf-like. These flattened petioles which look like ordinary leaves are called phyllodes.
In mature A moniliformis plants the leaflets and rachis do not develop at all. In Acacia recurva even the mature plant shows leaflets on young leaves but these soon drop off and the petioles flatten into phyllodes. A phyllode is usually borne vertically so that both the surfaces are equally lighted. These plants are of xerophytic habit.
Ligule and Auricle:
In many plants of the family Gramineae where there is no petiole some peculiar outgrowths grow at the junction of the sheathing leaf-base and the lamina. This is a tongue-like membranous structure on the inner surface and is called the ligule.
The ligule may sometimes be incised and may even be a hairy growth. The ligule of Oryza sativa is strengthened by a vascular supply. The ligule completely closes the sheath so that the growing organs within the sheath are better protected. A ligule is also developed in the palms which covers up the growing parts like a mat.
In rice and some other plants, the ligule is further strengthened by two outgrowths from the two sides of the junction between the sheath and the lamina, growing like two ears. These are called auricles. The auricles clasp the younger parts inside the sheath affording even better protection.
3. The Lamina (Epipodium):
The lamina is the most important part of the leaf and that is the place where all the functions of leaf are carried on. It is the seat of gaseous interchange, starch formation and soon. The sheathing leaf-base is considered as a sheathing petiole by some botanists.
The leaf lamina is normally a flat structure. If the upper Surface (i.e., the surface facing the stem, also called adaxial or ventral surface) differs in structure from the lower (abaxial or dorsal) surface, the leaves are called dorsiventral.
In many monocots and shade plants the leaf is placed in such a manner that both the surfaces receive equal light and there is no difference between the two surfaces. Such leaves are called isobilateral. In exceptional cases (e.g., the leaf of onion ) the leaves are found to be cylindrical or centric when there is no question of two surfaces.
Leaves vary widely in the appearance of the lamina and to describe a plant the lamina must be correctly described.