The arrangement of the veins in the lamina is called venation. Depending on the arrangement of the veins and veinlets, venation may be of two main types – reticulate venation and parallel venation.
This type of venation is a characteristic feature of the leaves of the dicotyledons plants. A number of lateral veins are developed from the midrib and give rise to many veinlets. The lateral veins and veinlets form a network of reticulum in the lamina and for this reason, such a venation is called reticulate venation. Reticulate venation may be of the following two types:
Unicostate or pinnate:
When there is only one prominent midrib, from which the lateral veins are developed, the venation is known as uicostate reticulate or pinnately reticulate. Examples, mango, sunflower, guava.
Multicostate or palmate:
When a number of prominent veins enter into the lamina from the petiole and spread out like the fingers of the palm, the venation is known multicostate reticulate or palmately reticulate venation. Palmate venation is of two types:
- Divergent: When the main veins arising at the base of the lamina, diverge from one another towards the margin of the blade, it is known as divergent. Examples, Hibiscus, castor.
- Convergent: When the main veins instead of diverging from one another, pass through the lamina in a curved manner towards the apex and finally, meet there, the venation is called convergent. Examples, Zizyphus.
This type of venation is characteristic feature of the leaves of the monocotyledonous plants. Here, the veins do not branch or form network but run parallel to one another. Parallel venation is of two types.
Unicostate or pinnate:
In this type, there is a single main vein, from which lateral veins are developed. The lateral veins run parallel to one another without branching or forming a network. Examples, banana, turmeric, ginger etc.
Multicostate or palmate:
In this type, several main veins arise in the main base of the lamina and run parallel to one another without branching. Multicostate Parallel venation may be of two types.
a). Divergent: When the prominent main veins diverse from one another from the base of the lamina and proceed towards the margin, being more or less parallel to one another, it is called multicostate parallel and divergent type of venation as in Borassus.
b) Convergent: In this type, the main veins after arising from the base of the lamina run parallel to one another and then get curved at the margin region so that they finally meet towards the apex of the leaf. Examples, rice, bamboo.
The mode of arrangement of leaves on the stem or branch is known as phyllotaxy. By means of various types of arrangements the leaves avoid shading over one another and get themselves better exposed to sunlight, so that they can carry out the function of synthesis of food. Mainly there are two types of phyllotaxy – spiral and cyclic.
When one leaf is developed at each node, having alternate arrangement, it is called spiral phyllotaxy as in case of Cynodon, Hibiscus etc. In such an arrangement, when an imaginary line is drawn passing through the base of successive leaves in order of their development, it forms a spiral around the stem. This imaginary spiral line is known as genetic spiral.
When the genetic spiral makes one complete turn, it subtends an angle of 360 degree at the centre. The angle which is formed between successive leaves is called angle of divergence. The number of vertical rows of leaves is known as orthostichy. Spiral phyllotaxy may be of the following types:
a) Distichous: In this type of phyllotaxy the third leaf always remains over the first leaf as in If the phyllotaxy in Cynodon be examined selecting any leaf as the first leaf, it will be seen that the third leaf remains just above it. Similarly, the fourth one remains above the second leaf. So the leaves remain in two vertical rows. If the imaginary line will be drawn starting from the first leaf, it will make one complete turn when it will reach the third leaf. For this reason, the phyllotaxy is expressed as 1/2.
b) Tristichous: In this case, the fourth leaf remains vertically above the first leaf, so that the genetic spiral makes one complete turn and the number of orthostichies is 3. The angular divergence will be 120 degree. Tristichous phyllotaxy is seen in case of Cyperus.
C) Pentastichous: In this case, the sixth leaf remains vertically above the first leaf, so that the genetic spiral makes two complete turns between the first and sixth leaves and the number of orthostichies is 5. The angular divergence will be 144 degree. It is seen in case of Hibiscus.
d) Polystichous: When the number of orthostichies exceeds 5,the phyllotaxy is called polystichous. They may be like 3/8, 5/13 etc. 3/8 phyllotaxy is seen in Thevetia and 5/13 in Plumeria
When two or more leaves are borne on the same node, the arrangement is known as cyclic phyllotaxy. It may be of two types: Opposite and whorled.
a) Opposite phyllotaxy: When the two leaves are developed at a node, the leaves are said to be opposite. Each of the successive pairs of opposite leaves may be arranged vertically or at right angle to its preceding ones. If one pair of opposite leaves remain vertically over the next pair of leaves, the arrangement is known as opposite superposed as in Quisqualis. When one pair of leaves is developed at right angle to the next pair of leaves, the arrangement is known as opposite decussate. In this case, the leaves are arranged in four vertical rows. Example, guava.
b) Whorled or verticillate: When more than two leaves are developed at each node forming a circle or whorl, the arrangement is known as whorled or verticillate phyllotaxy. Example, Nerium.