In this article we will discuss about:- 1. Venation in Leaves 2. Types of Leaves 3. Stipules 4. Phyllotaxy 5. Modifications.
Venation in Leaves:
The arrangement of veins and the veinlets in the lamina is known as venation. The veins are chiefly made of vascular tissues, the xylem and phloem.
There are two main types of venation:
(1) Reticulate Venation:
In this type of venation, the veins and veinlets are repeatedly branched and irregularly distributed, forming a complex network, e.g., dicotyledonous leaves. However, some monocotyledonous leaves like Smilax, Dioscorea and aroids also show reticulate venation.
Reticulate venation is divided into two types depending upon the number of the principle veins.
a. Pinnate or Unicostate Type:
In this type of venation there is a prominent midrib in the median region. This gives off lateral veins which reach the margin or apex of the leaf. These are then connected by smaller veins which pass in all directions, forming a network. E.g., peepul (Ficus), mango (Mangifera).
b. Palmate or Multicostate Type:
In this type there are a number of more or less equally prominent veins which arise from the tip of the petiole and reach outwards or upwards.
There are two types of palmately reticulate venation:
(i) Divergent Type:
When the main veins diverge towards the margin of the leaf. E.g., gourd, castor, China rose.
(ii) Convergent Type:
When the veins spread upward in a curved manner and converge towards the apex. E.g., bay leaf (Cinnamomum), Indian plum (Zizyphus).
(2) Parallel Venation:
In this type of venation, the veins and veinlets run parallel to each other.
Parallel venation is divided into two types depending upon the number of principle veins:
a. Pinnate or Unicostate Type.
b. Palmate or Multicostate Type.
a. Pinnate or Unicostate Type:
In this type of venation, the leaf has a prominent midrib and this gives off lateral veins which precede parallel to each other towards the margin or apex of the leaf-blade. E.g., banana, ginger, turmeric, Canna.
b. Palmate or Multicostate Type:
It is of two types:
(i) Divergent Type:
The veins arise from the tip of the petiole, diverge and reach the margin of the leaf-blade in a more or less parallel manner. E.g., Palmyra palm.
(ii) Convergent Type:
A number of more or less equally strong veins proceed from the leaf base and converge towards the leaf apex. E.g., Grasses, wheat, bamboo.
Types of Leaves:
1. Simple Leaf:
A leaf is said to be simple when it consists of a single blade which may be entire or incised (and, therefore, lobed) to any depth, but not cut down to the mid-rib or the petiole. E.g., Mango, guava, banyan, papaya etc.
2. Compound Leaf:
A leaf is said to be compound when the incision of the leaf-blade goes down to the midrib (rachis) or to the petiole so that the leaf is broken up into a number of segments, called leaflets. These are free from one another, that is, not connected by any lamina, and more or less distinctly joined (articulated) at their base. A bud (axillary bud) is present in the axil of a simple or a compound leaf, but it is leaf never present in the axil of the leaflet of a compound leaf. There are two types of compound leaves namely – pinnate and palmate.
3. Pinnate Compound Leaf:
A leaf which bears leaflets on either sides of the rachis (midrib) is called a pinnately compound leaf. E.g., gum tree (Acacia), Cassia.
Pinnately compound leaves are of the following types:
(a) Unipinnate:
When the rachis of a pinnately compound leaf bears the leaflets, it is said to be unipinnate. E.g., Rose. A unipinnate leaf is said to be paripinnate if the leaflets are even in number. E.g., tamarind. If the leaflets are odd in number then it is said to be imparipinnate. E.g., Murraya.
(b) Bipinnate:
If the rachis is branched once and the leaflets arise on the secondary rachii the leaf is said to be bipinnate. E.g., Caesalpinia.
(c) Tripinnate:
It is a leaf in which the secondary rachii are branched to form tertiary rachii which in turn bear the leaflets. Such a leaf is said to be tripinnate. E.g., drumstick (Moringa).
(d) Decompound Leaf:
When the tertiary rachii are further branched i.e., more than thrice pinnate. Such a leaf is said to be decompound. E.g., coriander (Coriandrum).
(e) Palmately Compound Leaf:
A leaf which bears a number of leaflets which seem to be radiating from a common point on the tip of the petiole. E.g., silk cotton tree (Bombax).
Palmately compound leaves are of the following types:
(i) Unifoliate:
A single leaflet is articulated to the petiole. E.g., Citrus.
(ii) Bifoliate:
When two leaflets are articulated to the tip of the petiole. E.g., Balanites, Hardwickia.
(iii) Trifoliate:
A leaf with three leaflets articulated to the tip of the petiole. E.g., wood apple (Aegle).
(iv) Quadrifoliate:
A leaf with four leaflets articulated to the tip of the petiole. E.g., Paris.
(v) Multifoliate or Digitate:
When five or more leaflets are joined to the tip of the petiole and are spreading like fingers from the palm. E.g., silk cotton tree (Bombax).
Stipules in Leaves:
The leaf base in many plants is associated with two minute appendages called stipules. The stipules may be either attached to the leaf base or are present on both sides as lateral outgrowths. Leaves with stipules are called stipulate and those without are called exstipulate.
Types of Stipules:
Stipules may be classified into the following three types based on the duration for which they remain attached to the leaf base:
1. Caducous:
When the stipules fall off before the unfolding of leaf, they are called caducous. E.g., Michelia champaca.
2. Deciduous:
When the stipules fall off soon after the leaf unfolds, they are called deciduous. E.g., Cassia.
3. Persistent:
These are permanent stipules which remain attached to the leaf for the whole of its life. E.g., rose, pea.
On the basis of their structure and relation to the leaf, stipules may be of the following types:
1. Free Lateral:
The two stipules are free and are borne on the two sides of the leaf base. E.g., Hibiscus.
2. Adnate:
Two lateral stipules grow adhering to the petiole upto a certain height, thus making it somewhat winged. E.g., rose.
3. Interpetiolar:
In this type two stipules lie between the petioles of opposite or whorled leaves. E.g., Ixora, Mussaenda.
4. Intrapetiolar:
These stipules are situated between petiole and axis. E.g., Tabernaemontana.
5. Ochreate:
These stipules form a hollow tube around the internodes up to a certain height. E.g., Polygonum.
6. Foliaceous:
When the stipules are large and green leafy structures. E.g., sweet pea (Lathyrus), pea (Pisum).
7. Spinous:
Here stipules are modified into spines. E.g., Zizyphus.
8. Convolute:
Stipules occur on the ventral side of the petiole and the margins of the stipules meet one another to serve as bud scales. E.g., Ficus, Magnolia.
Phyllotaxy of Leaves:
The arrangement of the leaves in various modes at the nodal region of the stem is called phyllotaxy.
There are three principal types of phyllotaxy:
(1) Alternate or Spiral:
Here a single leaf is produced at each node alternately on the stem. E.g., Hibiscus, mustard.
(2) Opposite:
Here two leaves are produced at each node opposite to each other.
Opposite phyllotaxy is of two types:
a. Opposite Superposed:
A pair of leaves at one node stands directly over the lower pair in the same plane. E.g., rangoon creeper (Quisqualis).
b. Opposite Decussate:
A pair of leaves at one node stands at right angles to the next upper or lower pair. E.g., Ixora, madar (Calotropis).
(3) Whorled:
When there are more than two leaves at each node which are arranged in a circle or a whorl. E.g., oleander (Nerium) devil’s tree (Alstonia).
Modifications of Leaves:
1. Leaf Tendrils:
In some plants, the entire leaf is modified into a tendril. E.g., Lathyrus. In Pisum the terminal leaflets are modified into tendrils. In Gloriosa superba the leaf apex is modified into a tendril.
2. Phyllode:
In some plants the petiole of the leaf or a part of the rachis gets modified into flattened or winged leaf like structure called as a phyllode. It is green in colour and carries out photosynthesis. E.g., Australian acacia. In this plant the leaf falls off at the seedling stage and the phyllode does the function of a leaf.
3. Pitcher:
In the pitcher plant Nepenthes, a terrestrial insectivorous plant, the lamina gets modified into a pitcher-like structure. This is used to capture small insects. The petiole is modified into a tendril to hold the pitcher upright. The leaf base is expanded to form a laminar structure to carry out photosynthesis. The leaf apex is modified into a lid.
4. Bladder:
In Utricularia which is an aquatic insectivorous plant, segments of the leaf modify into bladder like structures which trap small insects present in the water.
5. Leaf-Spines:
The entire leaf or a part of a leaf may be modified into a pointed structure called a spine. E.g., Opuntia.
6. Scale-Leaves:
These are thin membranous leaves found at the nodal region. Each scale leaf contains an axillary bud in its axil. E.g., ginger.
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