Biology Today — January 2018

(Jacob Rumans) #1

Parts of a Typical Leaf


Leaf base
( Hypopodium)
Basal part of leaf by which
it is attached to the node of
stem. Swollen leaf base is called
pulvinus, e.g., legumes, Mangifera
indica etc. Sheathing leaf base
is broad and flat, found in
monocots, e.g.,
Zea mays.

Mid rib
Mid rib is a thick
middle rib from petiole
to the apex. Lateral vein
arises from it. It contains
vascular tissues.
Stipule
A pair of small lateral
outgrowths of the leaf base, which
protect the young leaves and their
axillary buds in the young state. Leaves
containing stipules are called stipulate
leaves (e.g., Hibiscus rosa-sinensis)
whereas those lacking stipules are
called ex-stipulate leaves
(e.g., Petunia, Nerium,
etc.)

Vein
Veins and veinlets
contain vascular tissues
for conduction of water,
mineral salts and prepared
food. They also provide
rigidity to lamina.

Petiole
(Mesopodium)
A stalk that joins the lamina
to the base. A leaf with a petiole
is called petiolate and the one
without it is called sessile. The
leaf with a very small petiole
is called as sub-sessile.
(e.g., Calotropis).

Lamina
(Epipodium
or leaf blade)
Normally a flat, thin, expanded,
green and conspicuous structure
where all the functions of leaf are
carried out like photosynthesis,
gaseous exchange,
transpiration, etc.

Fig: Parts of a typical leaf

Phyllotaxy (Phyllotaxis)



  • Phyllotaxy refers to the arrangement of leaves on the stem or its branches. The aim of phyllotaxy is to orient the leaves in
    such a manner that all of them get maximum exposure to sunlight to perform their main function of photosynthesis.


Opposite


  • A pair of leaves arise at each node on opposite sides.

  • It is of two sub-types:
    (i) Opposite and superposed: Leaves of successive
    nodes lie in the same plane,,e.g.,Quisqualis
    S,yzygiumetc.
    (ii) Opposite and decussate : Leaves of adjacent nodes
    lie at right angles,.e.g,,Calotropis, Ocimumetc.


Alternate or Spiral


  • When single leaf is borne at each node and the leaves are arranged in suchaway thataline
    drawn on the stem through the leaf bases takesaspiral course, the arrangement is called
    alternate or spiral or acyclic,e.g Hibiscus., , ango,m mustard,etc.

  • It may be of following sub-types:
    Distichous(1/2phyllotaxy),e.g., Poaceae; Tristichous (1/3 phyllotaxy),e.g.,Cyperaceae;
    Pentastichous (2/5 phyllotaxy),e.g., Hibiscus rosa-sinensis;Octastichous (3/8 phyllotaxy),
    and so on.

  • These phyllotaxic series are called Schimper-Brown
    Series.


1
2

,^1
3
,^2
5

,^3
8
,^5
13
,^8
21
)e.g., ,etc.)

Whorled or erticillatev
Three(.e.g Nerium,)or more than three
leaves(.e.g,)Alstonia are borne on a
single node inawhorl or circle.The
leaves of the whorl of one node generally
alternate with the leaves of the whorl of
adjacent nodes inorder to provide
maximum exposure.

Types of
Phyllotaxy
Fig.:Spiral or alternate

Fig.:Whorled

Fig.:Opposite
superposed

Fig.:Opposite
decussate
Free download pdf