Leaf

LEAF 

Characteristics:

 

  • Leaves are flattened structures on stems, developing at nodes and bearing buds in axils that become branches.
  • Leaves originate from shoot apical meristems and are arranged acropetally (proceeding from the base toward the apex or from below upward), essential for photosynthesis.
  • A typical leaf consists of three main parts: leaf base, petiole, and lamina (leaf blade).
  • The leaf base attaches to the stem and may have stipules; in monocots, it can form a sheath, while in some legumes, it swells into a pulvinus.
  • Petioles secure leaf blades for optimal light exposure; long, flexible petioles allow fluttering in wind, cooling the leaf and promoting air circulation.
  • The lamina contains veins and veinlets, with a central midrib for support; veins transport water, minerals, and nutrients.
  • Lamina characteristics vary: shape, margin, apex, surface, and extent of incision differ among leaves.

 

 

 

Venation:

 

  • Vein arrangement in a leaf's lamina is known as venation.
  • Reticulate venation occurs when veinlets create a network pattern.
  • Parallel venation is seen when veins run parallel within the lamina.
  • Dicotyledonous plants usually have reticulate venation.
  • Most monocotyledons exhibit parallel venation.

 

Types:

 

  • Two types - simple and compound leaves.
  • A leaf is considered simple when it's lamina is either whole or if incised, the cuts don't reach the midrib.
  • A leaf becomes compound when incisions of the lamina extend to the midrib, forming separate leaflets.
  • Both simple and compound leaves have axillary buds on their petioles, except for the axil of leaflets in a compound leaf.

 



  • Compound leaves come in two types: pinnately and palmately compound.

 

Pinnately compound leaves have leaflets along a central axis (rachis), as seen in neem leaves.

 

Palmately compound leaves feature leaflets attached at a single point, typically the tip of the petiole, seen in silk cotton leaves.

 

 

Phyllotaxy:

 

  • Phyllotaxy refers to the arrangement pattern of leaves on stems or branches.
  • There are three main types of phyllotaxy: alternate, opposite, and whorled.

In alternate phyllotaxy, a single leaf emerges at each node in an alternating manner, seen in plants like china rose, mustard, and sunflower.

 

Opposite phyllotaxy involves pairs of leaves arising at each node, positioned opposite each other, seen in plants like Calotropis and guava.

 

Whorled phyllotaxy occurs when more than two leaves originate from a single node, forming a whorl, as seen in Alstonia.