The Leaf: Meaning, Types and Modification

In this article we will discuss about:- 1. Meaning of Leaf 2. Functions of the Leaf 3. Simple and Compound Leaves 4. Phyllotaxy 5. Kinds 6. Modifications.

Meaning of Leaf:

The leaves are the most conspicuous vegetative organs of the plants. They are lateral dissimilar outgrowths or appendages of the stems or branches. Leaves are usually green, flat, expanded organs of limited growth. They develop from the nodes having invariably buds at their axils, and remain arranged in acropetal order.

Leaves originate as exogenous outgrowths from the grow­ing point of the stem. These protuberances are called leaf primordia, which, by continued growth, develop into the mature green leaves called foliage leaves.

Functions of the Leaf:

Leaves are of paramount importance as they are mainly res­ponsible for the manufacture of food. Photosynthesis, as the process is called, is the main function of the leaf. Owing to the presence of chlorophyll, the green colouring pigment, the leaves can manu­facture complex organic food matters (like sugar and starch), out of water and carbon dioxide gas absorbed from the soil and air respectively, with the aid of sunlight.

Leaves also carry on respira­tion, an energy releasing process, which involves intake of oxygen and outgo of almost equal volume of carbon dioxide. All living organs, of course, have this function. The third function of the leaf is transpiration or giving out of excess of water as water vapour.

Plants usually absorb water from the soil much in excess of their need. They get rid of the surplus water by transpiration. Besides these normal functions, leaves protect the axillary buds, often store up water and food matters and perform other special functions as well.

Parts of a Typical Leaf (Fig. 61):

A typical or an ideal leaf has usually three parts:

(i) Leaf base, by means of which the leaf remains attached to the stem or branch.

(ii) Leaf stalk or petiole, the cylindrical stalk which con­nects the leaf base with the flat blade.

(iii) Leaf blade or lamina, the green flat expanded part of the leaf. It goes without saying that the blade is the most impor­tant part.

The blade has usually a prominent rib running up to the tip. It is the midrib. The midrib has many branches and sub- branches distributed in the lamina. They are called veins. Veins really form the skeleton of the leaf on which softer materials remain inserted, and they are the channels for conduction of water and food. The outer edge of the leaf forms the margin, and the extreme tip, the apex.

Simple and Compound Leaves:

A leaf is called simple when it has a single blade with entire or incised margin. It is simple so long it presents a single appearance. According to the degrees of incisions, the suffixes ‘fid’, ‘partite’ and ‘sect’ are used.

A simple leaf with pinnate or uni­costate. Venation is called pinnatifid, when it has incisions less than halfway towards the midrib, as in Chrysanthemum (B. Chandramallika); it is pinnati-partite, if the incisions extend beyond halfway, e.g. Argemone (B. Shialkanta); and it is pinnati-sect, when the incisions almost reach the midrib, as in marigold, Ipomoea (B. Tarulata).

Similarly simple leaves with multicostate or palmate venation may be palmate-fid, e.g. lady’s finger; palmati- partite, e.g. bitter gourd (B. Uchche); and palmati-sect, as in morning glory (Fig. 70).

But when the incisions reach the midrib, breaking down the leaf into a number of segments or leaflets, the leaf is called compound.

The leaflets of a com­pound leaf may remain attached to a common axis, called rachis, like the pinnae of a feather; or may be jointed or articulated to a common point on an axis like the outstretched palm. The former types are called pinnate compound leaves; and the latter are known as palmate compound leaves.

Pinnate Compound Leaves (Fig. 71):

The leaflets of a pinnate compound leaf remain attached to an axis called rachis. If they are arranged in pairs, the rachis ends abruptly and the number is even, the leaf is called pari-pinnate as in tamarind, Cassia (B. Kalkasunde), but if a terminal leaflet is present on the rachis, naturally making the number odd, the leaf is called imparipinnate, as in rose, Clitoria (B. Aparajita).

Pinnate compound leaves may be unipinnate or once pinnate, bi-pinnate or twice pinnate, tri-pinnate or thrice pinnate, and so on.

Paripinnate leaves of tamarind and impari­pinnate leaves of roses are unipinnate, as they have leaflets attached directly to the rachis. In bi-pinnate leaves the rachis is branched and the leaflets are arranged pinnately on the secondary axes of the rachis, as in the sensitive plant (B. Lajjabati).

The tri-pinnate leaves have the third series of branches bearing leaflets, e.g. horse­radish. When the leaves are pinnate more than thrice they are called decompound, e.g. carrot.

Palmately Compound Leaves (Fig. 72):

Here the leaflets are jointed or articulated to a common point on the axis.

According to the number of leaflets present, palmate compound leaves may be:

(i) Uni-foliate, with only one leaflet, as in lemon;

(ii) Bi-foliate, with two leaflets, e.g. Balanites (B. Hinghan);

(iii) Trifoliate, with three leaflets, as in wood apple, Oxalis (B. Amrul);

(iv) Quadrifoliate with four leaflets, as in Marsilea (B. Sushni); and

(v) Digitate, with five or more leaflets, as in silk- cotton.

Differences between a Pinnate Compound Leaf and a Short Branch:

A pinnately compound leaf with many leaflets often closely resembles a branch with simple leaves.

The following characters would show the difference between the two:

(1) A compound leaf has no terminal bud which the branch always has.

(2) A compound leaf develops from the node of the stem or the branch, bearing buds at their axils; whereas the short branch is axillary in position.

(3) If stipules are present they are found at the base of the rachis of a compound leaf; whereas in a short branch the indivi­dual leaves bear the stipules.

(4) Buds are never present at the axils of the leaflets of a com­pound leaf, but the simple leaves of a short branch invariably bear buds at the axils.

Phyllotaxy of Leaves:

The leaves are not haphazardly arranged on the stems and branches but they come out in a definite order. This arrangement of leaves on the stems and branches is known as phyllotaxy, meaning leaf order. The main object of phyllotaxy is to expose the leaves to proper illumination for different vital functions, avoiding shading as far as possible.

There are three principal types of arrangement (Fig. 73):

(1) Alternate, when a single leaf arises from a node and naturally the leaves are alternately placed. This arrangement is also called spiral, because if an imaginary line is passed through the bases of the leaves in the order of their development, it would form a spiral round the stem.

(2) Opposite, when two leaves develop from a node opposite to each other.

Opposite arrangement may be:

(i) Superposed, when one pair of leaves stands just above the lower pair. Here leaves are arranged in two vertical rows and the internodes are long to avoid shading as far as possible, e.g. guava, Rangoon creeper; and

(ii) Decussate, when one pair of leaves is at right angles to the next upper or lower pair, forming four vertical rows of leaves on the stem or branch, e.g. Ocimum (B. Tulsi), Ixora (B. Rangan), Calotropis (B. Akanda).

(3) Whorled or Verticillate, when more than two leaves arise from a node forming a whorl, e.g. Nerium (B. Karabi), Alstonia (B. Chatim).

Leaf Mosaic:

Plants growing in shady places with poor light exhibit a peculiar sort of arrangement of leaves. Here all the leaves come up and arrange them­selves side by side with a view to utilizing the maxi­mum amount of light. They form something like a mosaic (Fig. 74), hence called leaf mosaic, e.g. Oxalis (B. Amrul), Garden Nasturtium.

Heterophylly:

Leaves of different forms are sometimes found on the same plant. This condition is known as heterophylly (heteros=different; phylla=leaves). Hetero­phylly is particularly noticed in aquatic plants.

Sagittaria or arrow-head, a common water-plant, has two kinds of leaves; the submerged ones are ribbon- shaped, whereas the aerial ones are sagittate (arrow-shaped). Many other aquatic plants bear finely dissected submerged leaves and flat expanded floating or aerial leaves.

Heterophyllous condition in aquatic plants is an adaptation to two different environmental conditions. Even land plants often bear different environmental conditions. Even land plants often bears different types of leaves.

Leaves vary considerably in shape in plants like Pterospermun (B. Kanak champa), Artocarpus chaplasa (B. Chaplas), coriander, etc. The first formed leaves in wood apple, Mormodica (B. Uchche), etc., are quite different from later-formed ones.

Kinds of Leaves:

Besides the green foliage leaves, there are other types of leaves as well. Cotyledons of the embryo are the seed leaves, and so are the first leaves of the plants.

The scale leaves, as found in modified stems, are small mem­branous bodies. Bracts are specialised leaves often brightly coloured, bearing floral buds at the axils. The beautifully coloured petals and other floral parts are also specialised leaves meant for reproduction.

Modifications of Leaves:

Leaves are often modified for parrying on special functions.

An account of the modified leaves is given below:

1. Leaf Tendrils:

The whole leaf or more commonly the parts of the leaves are modified into slender sensitive tendrils for climbing. The whole lamina in Lathyrus or wild pea, the terminal leaflets in pea, the apex in Gloriosa (B. Ulat chandal), the petiole in Clematis and the stipules in Smilax (B. Kumarika) are modified into tendrils.

2. Spines:

The lamina or a part of it may be modified into hard sharp-pointed structure, called spine, for the purpose of self-defence. In common Opuntia (B. Fani manasha) the stem becomes green and flattened (phylloclade) and the leaves are converted into spines.

Similarly the apex of date-palm, the margin of Argemone (B. Shialkanta), stipules in Acacia (B. Babla) is modified into spines. (It should be noted that thorns and spines carry on the same function; but thorns are modified stems and spines are modi­fied leaves. The curved sharp-pointed outgrowths of roses are the prickles.)

3. Storage Leaves:

Many leaves become fleshy and succulent due to the storage of water and food. Here the leaves are partly modified. Leaves of Agave, Aloe, Portulaca (B. Nune-shak), the scaly leaves of onion are familiar examples.

4. Insect-Catching Leaves:

The insectivorous plants have peculiar leaves nicely adapted for catching insects (Fig. 176).

(a) Nepenthes or Pitcher Plant:

Here the leaf-base is modified into a flat lamina-like body, the petiole into a slender coiled tendril for climbing and the lamina into the pitcher proper. The pitcher has a coloured hood for attracting the insects.

(b) Utricularia or Bladder-Wort:

The bladder-worts (B. Jhanji) have much dissected compound leaves. Some of the leaflets are modified into bladders having special devices for catch­ing insects. Each bladder has a valve-like door which opens only inwards. The small aquatic insects push in the valve and are caught. In course of time they die and the bodies are digested.

(c) Drosera or Sundew:

It is another insectivorous plant where the spatula-shaped leaves bear many tentacles, the tips of which glisten in the sun and look like dew drops. Small insects are attracted, come and rest on the leaf only to find the tentacles bend and imprison them. By the secretion of enzymes the insect body is decomposed. After completing digestion the tentacles resume their original position and get ready for another prey.

5. Leaves of Bryophyllum (B. Pathar kuchi), Begonia bear epiphyllous buds and thus help in vegetative multiplication.