A leaf is green flattened lateral appendage of the stem or their branch that develops at the nodes Leaves ate arranged in acropetal succession and originates from the shoot meristem as lateral primordia.
All the leaves of a plant constitute foliage or phyllome.
Characteristics of Leaf:
(i) These are dissimilar, lateral outgrowths of the stem.
(ii) Leaves are exogenous in origin.
(iii) They are borne on the stem at the nodes.
(iv) Leaves generally have axillary bud in their axil.
(v) Unlike stem and root, an apical bud or a regular growing point is absent in leaves.
(vi) The leaf base may have two lateral outgrowths called stipules.
(vii) Leaves show limited growth.
(viii) A leaf is differentiated into three parts – leaf base, petiole and lamina.
(ix) The lamina possesses prominent vascular strands called veins. The veins traverse through length and breadth of the leaf lamina.
(x) The leaves are generally green because of the presence of chlorophyll. Hence, leaves are the organs of photosynthesis.
(xi) A large number of specialized openings, called stomata, are present in the lamina of leaves through which exchange of gases and transpiration lakes place.
1. Types of Leaves:
(i) Foliage leaves:
These are normal green leaves of a plant meant for photosynthesis, gas exchange and transpiration.
(ii) Seed leaves or Cotyledons:
These are embryonic leaves borne at the node of embryo axis (tigellum). They come out as first leaves during germination
(iii) Scale leaves or Cataphylls:
These are dry membranous non-green leaves that appear on buds and underground stem.
(iv) Floral leaves:
These arc modified leaves borne on thalamus of flower, e.g., sepals and petals.
These leaves bear spores or sporangia, e.g., stamens (microsporophyll’s) and carpels (megasporophylls).
(vi) Bract of Hypsophylls:
These are small leaf like structures in whose axil flowers develop.
These are first few leaves of a stem that differ from the- rest, e.g., in Citrus and pteridophytes.
2. Leaf duration:
On the basis of life span or duration of a leaf on the plant, it may of three types.
(a) Caducous or fugacious:
The leaves which fall soon alter their appearance, e.g., Opuntia.
(b) Deciduous or annual:
The leaves which fall off at the end of growing season or in winter, e.g., Mulberry, Poplar.
(c) Persistent or evergreen:
The leaves survive for more than one year and fall off individually at different times, e.g., Mango, Pinus, and Eucalyptus etc.
3. Leaf Insertion:
Leaves borne on the main stem, e.g., Rose, maize etc.
Leaves borne on the lateral branches.
Leaves arise from the much-condensed underground stem, as if arising from the root, e.g., carrot, aloe, raddish etc.