68 BIOLOGY
Turnip Carrot Sweet
potato
Asparagus
(a) (b)
Figure 5.5 Modification of root for : (a) storage (b) respiration: pneumatophore in
Rhizophora
5.2 THE STEM
What are the features that distinguish a stem from a root? 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. The stem
bears nodes and internodes. The region of the stem where leaves are
born are called nodes while internodes are the portions between two nodes.
The stem bears buds, which may be terminal or axillary. Stem is generally
green when young and later often become woody and dark brown.
The main function of the stem is spreading out branches bearing
leaves, flowers and fruits. It conducts water, minerals and photosynthates.
Some stems perform the function of storage of food, support, protection
and of vegetative propagation.
5.2.1 Modifications of Stem
The stem may not always be typically like what they are expected to be.
They are modified to perform different functions (Figure 5.6). Underground
stems of potato, ginger, turmeric, zaminkand, Colocasia are modified to
store food in them. They also act as organs of perenation to tide over
conditions unfavourable for growth. Stem tendrils which develop from
axillary buds, are slender and spirally coiled and help plants to climb
such as in gourds (cucumber, pumpkins, watermelon) and grapevines.
Axillary buds of stems may also get modified into woody, straight and
pointed thorns. Thorns are found in many plants such as Citrus,
Bougainvillea. They protect plants from browsing animals. Some plants
of arid regions modify their stems into flattened (Opuntia), or fleshy
cylindrical (Euphorbia) structures. They contain chlorophyll and carry
MORPHOLOGY OF FLOWERING PLANTS 69
Stem modified
into tendril
(a) (b)
Stem modified
into spine
Roots arising
(c) (d) from nodes
Figure 5.6 Modifications of stem for : (a) storage (b) support (c) protection
(d) spread and vegetative propagation
out photosynthesis. Underground stems of some plants such as grass
and strawberry, etc., spread to new niches and when older parts die new
plants are formed. In plants like mint and jasmine a slender lateral branch
arises from the base of the main axis and after growing aerially for some
time arch downwards to touch the ground. A lateral branch with short
internodes and each node bearing a rosette of leaves and a tuft of roots is
found in aquatic plants like Pistia and Eichhornia. In banana, pineapple
and Chrysanthemum, the lateral branches originate from the basal and
underground portion of the main stem, grow horizontally beneath the
soil and then come out obliquely upward giving rise to leafy shoots.
5.3 THE LEAF
The leaf is a lateral, generally flattened structure borne on the stem. It
develops at the node and bears a bud in its axil. The axillary bud later
develops into a branch. Leaves originate from shoot apical meristems and
are arranged in an acropetal order. They are the most important vegetative
organs for photosynthesis.
A typical leaf consists of three main parts: leaf base, petiole and lamina
(Figure 5.7 a). The leaf is attached to the stem by the leaf base and may
MORPHOLOGY OF FLOWERING PLANTS 71
In palmately compound leaves, the
leaflets are attached at a common point, i.e.,
at the tip of petiole, as in silk cotton.
5.3.3 Phyllotaxy
Phyllotaxy is the pattern of arrangement of
leaves on the stem or branch. This is usually
of three types – alternate, opposite and (a) (b)
whorled (Figure 5.9). In alternate type of
phyllotaxy, a single leaf arises at each node
in alternate manner, as in china rose,
mustard and sun flower plants. In opposite
type, a pair of leaves arise at each node and
lie opposite to each other as in Calotropis
and guava plants. If more than two leaves
arise at a node and form a whorl, it is called (c)
whorled, as in Alstonia. Figure 5.9 Different types of phyllotaxy :
(a) Alternate (b) Opposite
(c) Whorled
5.3.4 Modifications of Leaves
Leaves
Leaves are often modified to perform modified
Leaf
functions other than photosynthesis. They tendril into spines
are converted into tendrils for climbing as
in peas or into spines for defence as in cacti
(Figure 5.10 a, b). The fleshy leaves of onion
and garlic store food (Figure 5.10c). In some
plants such as Australian acacia, the leaves
are small and short-lived. The petioles in
these plants expand, become green and
synthesise food. Leaves of certain
insectivorous plants such as pitcher plant,
venus-fly trap are also modified leaves.
(a) (b)
5.4 THE INFLORESCENCE
A flower is a modified shoot wherein the shoot
apical meristem changes to floral meristem.
Internodes do not elongate and the axis gets
condensed. The apex produces different Fleshy
kinds of floral appendages laterally at leaves (c)
successive nodes instead of leaves. When a Figure 5.10 Modifications of leaf for :
shoot tip transforms into a flower, it is always (a) support: tendril (b) protection:
solitary. The arrangement of flowers on the spines (c) storage: fleshy leaves
