Page 6 - Lesson Notes-Morphologyof flowering plant

The stem is the ascending part of the axis bearing branches, leaves, flowers and
fruits. It develops from the plumule of the embryo of a germinating seed.
It shows distinction into nodes and internodes; where node is the region from where
leaves are born and internodes are the region between two nodes. Its apex bears a
terminal bud for growth in length.
A bud can be defined as a condensed embryonic shoot that has a growing point
surrounded by closely packed immature leaves. When bud grows, the internodes
become longer and the leaves spread out, resulting in the formation of a young
shoot.
Note:
* The largest bud is cabbage.
* Bamboo is considered to be tallest herb, tallest shrub or arborescent grass.
Bamboos are called culms, after the jointed nature of their stems.
Forms of Stem
Stem may be aerial, subaerial or underground. In most of the plants, stems grow
above the soil. These are aerial stems. The aerial stems of some plants trail or creep
on the ground. They are called subaerial stems. In some plants, the stem grow in the
soil and are called underground stems.
i. Aerial Stems
The aerial stems have two forms, i.e., reduced stem and erect stem.
(a) Reduced Stems It is reduced to a small disc. The * nodes and internodes are not
distinguished, e.g., carrot, radish, turnip, etc. In some aquatic plants, the reduced
discoid stem is green and flattened to float on the surface of water. It does not bear
leaves, e.g., Lemna, Wolffia, Spirodela. In underground structures also a reduced,
non-green stem is found, e.g., Garlic, onion and lily.
(b) Erect Stems These stems are strong enough to remain erect or upright without
any external support.
ii. Subaerial Stem
In subaerial stems, some part lives underground, whereas, the remaining part of the
stem is aerial.
The subaerial stem are also divided into two forms
(a) Upright Weak Stems These stems are weak which climb up a support to expose
their foliage and reproductive organs.
These are of two types twiners and climbers
* Twiners These are long, slender and very sensitive and coil around an upright
support on coming in contact, e.g, Dolichos lab lab (bean), Clitoria (butterfly pea),
Cuscuta.
* Climbers These have weak and flexible stem which climb up a support with the
help of certain clasping or clinging structures, e.g., Bougainvillea.
(b) Prostate Weak Stems These weak stems take support of the ground for
spreading as growth occurs.
They are of following four types
* Trailers (creepers) They trail along the surface and do not climb up.
* Runners These are subaerial weak stems that grow horizontally along the soil
surface, e.g., Cynadon (lawn grass), Centella (brahmi booti), Oxalis, etc.
* Stolons These subaerial weak stems are horizontal or branched runners with long
internodes which can pass over small obstacless. Stolons, also propagate
vegetatively like runners, e.g, Fragaria verica (strawberry), Jasminum (jasmine),