Modification of stems
Can you guess what is common between
ginger, onion bulb, potatoes. All three are stems. Some plants have their stems
modified for storing food and for vegetative propagation. Modified stem may be
aerial, sub aerial or underground stems.
1. Aerial Modifications
Phylloclade :In dry climate, conserving
water is a challenge. Water evaporates from the surface. If the surfa ceareaislarger,
evaporation would be more; and if the surface area is smaller, the evaporation
will be less. Plants with many leaves have more surface area. Cactus hence has a thick stem which does
most of the food production through photosynthesis and leaves are reduced to small
spines with less surface area.
2. Sub – aerial Modifications
Stem of
some plants remains sub – aerial which
grow horizontally on the surface of the soil for the
purpose of reproduction. There are four
types.
i. Runner : The
stem grows laterally on the surface of the soil, breaks up to produce roots
where it touches the ground to give rise to new plants. E.g: Centella (Vallarai)
ii. Stolon : Stolon
is a slender branch of the stem that grows upwards to some distance and then
bends towards the ground. Upon touching the ground, it gives rise to a new plant.
E.g:Wild strawberry.
iii. Sucker : Sucker is a short and weak
lateralbranch that grows diagonally upwards and directly gives rise to a new
shoot. E.g : Chrysanthemum.
iv. Offset : An
offset is a short and thick branch that arises from the axial part of a leaf.
It has thick internodes. It produces a tuft of leaves and cluster of small
roots below. E.g: Eichhornia.
ACTIVITY 6
Aim: To study the modification of stem Materials Required: Specimens of
Ginger, Potato, Onion, Mint, Bougainvillea–, Acacia, Opuntia and locally
available specimens.
Procedure: Observe the external morphology of each specimen.
Observation: Draw diagram and bring out the differences and their function in each type
of stem modifications.
3. Underground modifications
In aerial and sub aerial
modifications, stem has indefinite growth. In underground modified stem, whole
stem is burried under the ground and it has definite growth. Usually stem grows
above the ground, but there are some stems that grow under the ground to store
food. These underground stems swell and become thick.
There are four types of underground
stems:
1. Rhizome; 2. Corm; 3. Tuber; 4.
Bulb
i. Rhizom: It is an underground thick stem
withnodes and internodes with scale leaves at the node. It grows horizontally
and has an irregular shape. Rhizome have buds. If give rise to new stem and
leaves. E.g: Ginger and Turmeric.
ii. Corm:
This underground stem is round in shape and flat at the top and bottom. It is a
condensed form of rhizome and bears one or more buds in the axils of scale
leaves. Daughter plants arise from their buds. E.g: Colocasia.
iii. Tuber:
It is an enlarged, spherical underground stem that stores food. It has many
dormant buds on its surface known as “Eyes”. If we plant a part of tuber with
the bud, it grows into a new plant. E.g. Potato.
iv. Bulb: It is a condensed stem which is
disc like and stores food in the fleshy leaves. The bulb has two types of
leaves.
i.
Fleshy Leaves
ii.
Scaly Leaves
The
upper part of the stem has a terminal bud and it is covered by many scaly
leaves. The inner fleshy leaves store food as seen in Garlic and Onion.
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