Vegetative Shoot Apex is derived
from the meristem that is present in the plumule of the embryo and occurs at the tip of stem and
its branches in the form of a terminal bud. When in the inactive form, it occurs in
the axils of leaves as lateral buds. It is conical or dome shaped and covered
by young leaves arising from its side. Leaf primordia is produced periodically, and the period between appearance of two successive leaf primordial is called plastochron. Each leaf primordial has an axillary zone of meristematic activity
called axillary meristem or axillary bud which may remain dormant for some time
before adding new tissues resulting in growth of shoot.
There are various theories which
have been proposed pertaining to the shoot apex:
Apical Cell Theory
proposed by Nageli claims that only a
single apical cell is responsible for the development of the entire plant body.
This theory applies to higher algae, but not to gymnosperms and angiosperms.
Histogen Theory proposed
by Hanstein talks about three
distinct histological zones in the apical meristem called histogens. Dermatogen,
the outermost layer, gives rise to epidermis and epidermal tissue system. Periblem,
the middle layer, gives rise to the cortex of stem and roots. Plerome, the central
or innermost histogen, forms stele. Part of the plerome which forms the vascular
tissue is called procambium.
Tunica Corpus Theory was proposed
by Schmidt who suggested that the shoot apex consists of two distinct zones. The
tunica is single layered and forms the epidermis. The tunica has anticlinal
division and is responsible for surface growth. The corpus is the central core
with larger cells and since it is capable of division in all planes, it
provides voluminous growth. The cells derived from corpus give rise to
procambium and ground meristem.
Reproductive Shoot Apex
The vegetative shoot apex becomes
a reproductive shoot apex or floral bud during the reproductive phase and this apex develops into a single flower. The cells in the apical region which were dormant
during vegetative phase get activated and give rise to the primordial of sepals and
petals. The entire apical meristem undergoes a change during the reproductive
The root apex is found at the tip
of the main root and its branches. In the tap root system, it is derived from the radicle part of the embryo whereas in adventitious roots, it is produced from the derivatives of the shoot apex. The ground apical meristem gives rise to protoderm,
procambium and ground meristem. In monocots, the root cap is derived from the
calyptrogen. The root cap has a buffering role to play between the soil and the
apical meristem. The cells of the root cap secrete a mucilaginous substance
which act as a lubricant and help the tender root tips in entering the hard
surface of the soil. In many cases a
small region with very low mitotic activity called the quiescent centre is
present in the root apex. The cells in quiescent centre have very little
synthesis of new proteins, RNAs and DNAs.