MORPHOLOGY OF LEAF

THE LEAF –

Leaf is a green, dissimilar exogenous lateral flattened outgrowth which is borne on the node of a stem or its branches and is specialized to perform photosynthesis. Leaves originate from shoot apical meristem and areNArranged in an  acropetal order.

A typical leaf consists of three parts- Leaf base (phyllopodium, hypopodium), Petiole (mesopodium), Lamina (epipodium). Leaf is attached with stem by Leaf Base which may bear two small leaf like structure called stipule.

The pattern of arrangement of leaves on the stem or branch is called Phyllotaxy. The phyllotaxy is of three types;-

Alternate or spiral;- when one leaf is born at each node and are arranged alternately to form a spiral arrangement around the stem. Each spiral is called a genetic spiral and each vertical row is called an orthostichies. If the leaves are arranged in one circle and two vertical rows the phylotaxy is called distichous or is written as ½(spirals or circles/rows). Tristichous 1/3, pentastichous 2/5, octostichous- 3/8.

The arrangement is made as 1/2+1/3+2/5+3/8+5/13 this series is called schimper-brown series.

Opposite;- when two leaves are born at each node opposite to each other so that only rows are formed. The two leaves may form only two rows (superposed) or may form four rows (decussate).

Whorled;- when more then two leaves are born at each node it is called whorled.

VENATION

The Middle prominent vein of a leaf is called mid vein. Veins provide rigidity to the leaf blade and act as channels for transport of water and minerals. The arrangement of vein and veinlets in the lamina is called venation. It is of two types;-

Reticulate venation;- the Veinlets are irregularly distributed and form a network. It is present in all Dicotyledons like Gram, Pea, Beans, and Mango etc. if the lamina has only one principal vein, the venation is called reticulate unicostate (mango). If the lamina has more than one principal veins then it is called reticulate multicostate (smilex, cucurbita, papaya etc).

Parallel venation;- in this type the Veins are parallel to one another and do not form a network. It is present in Monocotyledons like Grass, Banana, Rice etc. if there is a single principal vein with lateral veins running parallel it is called parallel unicostate (banana). If more then one principal vein then it is called parallel multicostate (maize,sugarcane).

TYPES OF LEAVES;-A leaf having a single or undivided lamina it is called Simple leaf. The incisions do not touch the mid rib. Example- Mango, Guava etc.

When the incision of lamina reach up to the midrib and breaking it into a number of leaflets it is called Compound leaves.

In a Pinnately compound leaves, a number of leaflets are present on common axis called rachis. Example- Neem.

In Palmately compound leaves, the leaflets are attached at common point. Example- Silk cotton.

MEDIFICATION OF LEAVES;-

in plants like wild pea, gloriosa superba, clematis the leaves get modified into long, thin, and coiled tendrils. In nepenthes the petiole and rachis (clementis)  gets modified into tendril.

In some plants like agemon, opuntia, and barberry the leaves get modified into spines.

In plants like bignonia, asparagus the leaves get modified into hard and tapering hooks.

In some plants like accasia and parkinsonia the lamina is absent or becomes reduced. The rachis and the petiole gets modified into a leaf like structures called phyllodes.

In some plants like nepenthes (pitcher plant) and sarracenia the lamina of the leaf gets modified into a pitcher and petiole into a lid. In utricularia the leaf lobes get modified into sac like traps called bladders which trap insects. In drosera the leaf lamina gets modified into a spoon shaped blade that bears little touch sensitive glandular hairs which trap the pry. These plants are called insect eating or insectivorous plants.