NEET EXPERT BIOLOGY BY
DR A.KUMAR[
M.B.B.S.] ,DR. Y.KATIYAAR [MBBS,MD]& SANDHYA KATIYAAR
TOPIC ..PLANT MORPHOLOGY
Morphology is
the branch of science which deals with the study of external structural of the
plants.
The flowering plant
consists of An Axis, Root System and Shoot System.
Root
System
The flowering plants
consist of a long cylindrical axis which is differentiated into an
underground Root System.
Root is the descending, non green, underground
part lacking nodes, internodes, leaves and buds.
In Dicotyledon plants, the elongation of radicle leads to the
formation of primary root. The primary root grows inside the soil. Primary root
bears lateral roots known as Secondary Roots, Tertiary Roots etc.
Types
of Root System
The primary roots
along with its branches forms a Tap Root system. For Example: Mustard Plant. Primary roots have short life span and are
replaced by large roots later on.
The roots which arises
from the base of the stem are known as Fibrous Root system. For Example: Grasses. This type of system is common in Mocotyledon Plants.The roots which arises from the
part other than the radicle are known as Adventitious Roots. For Example: Banyan Tree, Monstera.
Regions
of the Root
The root has five main
regions which are as follows:
·
Root Cap is the structure that covers the
apex/tip of the root. It helps in protecting the apex of the root.
Root Cap is the structure that covers the
apex/tip of the root. It helps in protecting the apex of the root.
·
Region of Meristematic Activity located few millimeters above the root
cap. The cells of this region has the property of repeated divisions which is
needed for the growth of the plant.
·
Region of Elongation is located proximal to the region of meristematic
activity. This is required for the elongation of root.
·
Region of Maturation is located proximal to region of elongation. These cells
gradually differentiate and mature.
·
Root Hairs arise
from the region of maturation which helps in absorption of water and minerals
from the soil.
Modifications
of the Root
Roots modify in
structure to perform different functions as explained below:
·
Fusiform root is a modified form of tap root. The root is swollen from the
middle and tapers at both the ends. For Example: Radish
·
Napiform roots are also a modified tap root. They have swollen base and then it
tapers abruptly. For Example: Turnip
·
Prop roots are roots which are modified for aerial support. For Example: Corn
·
Stilt roots are modified roots for support. For Example: Maize
·
Pneumatophores are roots modified for respiration. They grow in swampy area and
grow vertically upwards. For Example:Rhizophora
·
To absorb water and
minerals from the soil.
·
To provide proper
anchorage to the plants.
·
To store reserve food
materials
·
Synthesis of plant
growth regulators
Shoot
System
It is an aerial
system, usually above the soil and originates from the plumule. It consists of
stem, branches, leaves, flowers and fruits.
Stem
It facilitates
conduction of water, mineral and food material. The flowers, fruits and seeds
form the reproductive parts of the plants. The stem bears the nodes and internodes. The
point from where the leaf arises is known as Node and the part between the two Nodes are known as Internodes.
Modification
of Stems
Similar to roots, the
stem is also modified to perform different functions:
·
Tendrils are
slender, twining strands that allow the plant to climb and helps in
support. For Example: Cucumber.
·
Thorns are modified
stem for protection of the plant. They are hard, woody and sharp outgrowths
from the plant. For Example: Rose.
Fig.6.
Modifications of Stem
·
Bulbils are
modified stem that becomes fleshy and store food.
Leaf
Leaf is a green
dorsoventrally flattened exogenous lateral outgrowth that arises from a node of
the stem or a branch. The leaf is a specialized organ of photosynthesis,
transpiration and gaseous exchange.
Parts of Leaf
The point of origin of
leaf is known as Node. It bears bud in its axil.
The leaf is attached to the stem by the leaf base and have two lateral leaf
like structures called Stipules.
In monocots, the leaf base is swollen to form pulvinus. The stalk of the leaf
is known as Petiole.
The green exposed part of the leaf is known as lamina. Lamina bears the veins
or veinlets. The arrangement of veins on the leaf is known as Venation.
There are basically
two types of Venation: Parallel Venation and Reticulate Venation.
Parallel venation is
observed when veins run parallel to each other. For Example: Monocots
Fig.8.
Types of Venation
Flower
The modified shoot is
known as Flower. The arrangement of
flower on the floral axis is known as Inflorescence.
There are two major
types of Inflorescence- Racemose and Cymose.
In racemose type of
inflorescence, new flowers are generated at the tip of the inflorescence. For Example: Snapdragon
In cymose type of
inflorescence, new flowers are generated at the base of the inflorescence.
Structure
of the Flower
Each flower has four
whorls- Sepals, Petals, Stamens and the Carpels. Sepals are green leafy
structures that covers the flower bud. Petals are bright colored to attract
insects for pollination.
Gynoecium are female
reproductive structures whereas Androecium is the male reproductive
structure.
Fig.5.
Structure of the Flower
Gynoecium is composed
of carpels. Carpels comprises of three structures: Ovary, Style and Stigma. Ovary comprises of
ovules that forms the seed, whereas stigma is the place of landing of pollen at
the time of pollination. Ovary finally forms the fruit.
The male part of the
flower is known as Androecium.
Androecium is composed of stamens. Stamen is composed of anther and the
filament. Anthers contain the pollen grains, the male gamete.
ROOT….
The root is usually an underground part of the plant which helps
in fixation and absorption of water.
The root with its branches is known as the root system.
Characteristics of the Root
(i) The root is the descending portion of the plant axis and
is positively geotropic.
(ii) It is non-green or brown in colour.
(iii) The root is not differentiated into nodes and internodes.
(v) Usually the root tip is protected by a root cap.
(vi) The root bears unicellular root
hairs.
(vii) Lateral roots arise from the root which are endogenous in
origin (arises from pericycle).
Parts of the Root
From the tip of the root upwards, the following parts can be
traced in root.
(i) Region
of root cap : The tip of the root is called calyptra or
root cap. It is for protection of root tip against any
injury. It is formed from meristem called calyptrogen. Pandanus is
the only plant with multiple root caps. In the aquatic
plants like Pistia, Lemma and Eicchornia instead of root caps, they have root pockets for
buoyancy. The root caps are absent in parasites and mycorrhizal roots.
(ii) Region
of cell elongation : The region of cell division lies
partly within and partly beyond the root cap. This
is the meristematic region of the root which produces new cells by cell
division.
(iii) Region
of Root Hairs: This region is present above the region of
elongation. In this region the epidermal cells produce many tubular,
unicellular outgrowths called root hairs. This is also called Piliferous
region. Water absorption mostly takes place through this
region. The root hairs are absent in many aquatic plants.
(iv) Region
of Maturation : Above
the root hair zone, mature region is present. This region consists of permanent
cells. Lateral roots are produced endogenously from the mature region.
Conduction of water and mineral salts takes place through this region.
Types of Root System
The root system is generally of two types:
(i) Tap
root system
(ii) Adventitious
root system
Tap Root System
·
The tap
root system develops from radicle of the germinating seed.
The tap
root system develops from radicle of the germinating seed.
·
The radicle develops into a primary root which
grows vertically downwards and become the tap root.
·
The tap root is the true root that produces many lateral roots
endogenously which grow obliquely.
·
The tap root system is present in dicotyledonous plants.
Adventitious Root System
·
The root system that develops from any part of
the plant body other than the radicle is called the adventitious root system.
·
It is
mostly seen in monocotyledonous plants.
It is
mostly seen in monocotyledonous plants.
·
In grasses, fibrous root system is present.
·
In this case the primary root formed from the
radicle disappear soon.
·
Then many slender roots develop from the base of the stem as
cluster of fibres, hence called the fibrous root system.
·
Shrub like monocots needs additional support because of the
adventitious root. e.g., Stilt root in sugarcane.
Modification of Roots
Sometimes the root performs other functions other than fixation,
absorption and conduction so get modified structurally. Both tap roots and
adventitious roots may undergo such modifications. There are many types of root
modifications.
Modification
of Tap Roots
(i) Storage roots : In
some plants, the primary tap roots are modified for storing reserve food
materials. The secondary roots remain thin and they are absorptive in function.
The storage roots are usually swollen and assume various forms :
(a) Conical
: The swollen root is broad at the base and tapers
gradually towards the apex giving a shape of cone, e.g., Carrot.
(b) Fusiform
: The root is swollen in the middle and narrow towards both its base and apex giving a
shape of spindle, e.g., Radish (Raphanus
sativus). Half or less than half portion towards the base of
fusiform root is formed by hypocotyl.
(c) Napiform : The
root is nearly globular or spherical in shape. The basal portion of root is
much swollen which suddenly tapers towards the apex giving a top-shaped
appearance, e.g., Turnip (Brassica
napus, vern, Shalgam) and Beet (Beta vulgaris,
vern. Chukandar).
(d) Tuberous
: The storage root having no definite shape is called
tuberous, .e.g., Mirabilis jalapa(4
O’clock plant), Trichosanthes (vern.
Parwal), Echinocystis lobata (The
tuberous root is lobed and weighs as much as 22 kg.).
(ii) Branched
roots : They are following types :
(a) Nodular
roots : The primary tap roots and its branches of leguminous plants, i.e., plants belonging to sub-family papilionatae of the family leguminosae
(e.g., Pea, Gram, Ground nut, Beans etc.), bear nodule like
swellings, called root nodules.
 
·
They are red in colour due to the
presence of leg-haemoglobin.
·
The nodules are inhabited by nitrogen
fixing bacteria called Rhizobium
leguminosarum.
·
It converts atmospheric nitrogen into
nitrates and supply them to the plant.
·
In turn Rhizobium gets nutrients and shelter from the
plant. This type of association between the bacterium and leguminous plant
for mutual benefit is known as symbiosis and
the organisms involved are called symbionts.
·
This association is also called mutualism
which is obligatory for both i.e.,
for bacteria and leguminous root.
|
(b) Pneumatophores
or Respiratory roots :
·
Roots also breathe and as such they also require air for gaseous
exchange.
·
Normally, the soil has a large number of small air spaces
between the soil particles.
·
This air is utilized by the plants for their respiration. But
the roots of some plants growing in saline marshes (mangrove
plants) suffer from the lack of oxygen.
·
This is due to the water logged condition of the soil.
·
To cope with this situation some root branches grow vertically
upwards. They become aerial and negatively geotrophic.
·
Gaseous exchange takes
place through pneumathodes.
·
Such aerial, porous negatively geotrophic roots which help in
gaseous exchange are called breathing or respiratory roots, breathing roots or
pneumatophores roots or pneumatophores e.g., Sonneratia, Heritiera, Rhizophora, Avicennia and Ceriops etc.
and are found in sundarbans of West Bengal.
Modification
of Adventitious Roots
(i) For
physiological or Vital functions
(a) Storage
roots : The roots where adventitious roots become
swollen to store food. They are following types :
·
Tuberous roots : These adventitious roots are swollen without any
definite shape e.g., Ipomoea
batata or (sweet potato).
·
Fasciculated roots : These
are tuberous roots arising in cluster from the base of the stem. e.g., Dahlia, Ruellia (Menow
weed), Asparagus (Asparagus
fern) etc.
·
Nodulose roots : These
roots become swollen at their tips due to accumulation of food e.g., Maranta sp.
(Arrowroot), Curcuma
amanda (Mango – ginger).
·
Moniliform or Beaded roots
: These adventitious roots are swollen at frequent intervals. This
gives the root a beaded appearance. e.g., Portulaca (Rose moss) Momordica (Bitter
gourd) Cyperus (Guinea
rush).
·
Palmate tuberous roots : In Orchis there
is a pair of succulent tuberous root, one
of which perishes every year while another new one is formed by its side. Such
orchid roots may sometimes be of palmate shape, therefore, called palmate
roots.
·
Annulated roots : The
roots of a medicinal plant, Ipecac (Cephaelis
ipecacuanha) yields emetine that looks like discs placed one above
another, therefore, called annulated.
(b) Epiphytic
roots : These
roots are also called ‘hygroscopic roots’. These roots
develop in some orchids which grow as
epiphytes upon the trunks or branches of trees. They hang freely in the air and absorb moisture with the help of special sponge like tissue called velamen. Velamen is
modification of epidermis. e.g., Venda,
Dendrobium etc.
(c) Parasitic
or Haustorial roots : Plants
which depend on plant partially or totally for their food material are known as
parasites. The roots of parasitic plants, which penetrate into the host tissues
to absorb nourishment, are called haustorial roots. The
haustorial roots of Cuscuta (Dodder,
vern. Amarbel) penetrates the host upto phloem and xylem to absorb
organic food, water and minerals. The haustorial roots of partial parasite
– Viscum (Mistletoe) penetrate upto xylem of host to absorb water
and minerals.
- Parasite which absorb their nutrients from the host stem are
known as stem parasites while those which absorb their nutrients from the host
root are know as root parasite.
(d) Saprophytic
roots : They
are also called mycorrhizal roots as here roots are associated with fungal
hyphae either superficially (ectomycorrhizae) or internally (endomycorrhizae)
for absorption of water and minerals. e.g., Monotropa and Sarcodes.
(e) Photosynthetic
or Assimilatory roots : These
are green, aerial, adventitious roots which prepare food materials by
photosynthesis are called photosynthetic roots or assimilatory roots
e.g., Taeniophyllum,Trapa and Tinospora. In
some epiphytes like Taeniophyllum, the stem and leaves are absent. The entire
plant is represented by thin green, ribbon like roots which contain
velamen. These roots absorb moisture from the atmosphere and manufacture
food materials by photosynthesis. Since the roots are green and
perform photosynthetic activity, these roots are called photosynthetic roots or
assimilatory roots.
(f) Reproductive
roots : Some fleshy adventitious root develop buds which
can grow in to new plants. These are called reproductive root. These roots serve as means of vegetative propagation. e.g., Sweet potato,
Dahlia etc.
For
Mechanical Function
(a) Stilt
Roots:
·
The
aerial, adventitious obliquely growing roots
that develop from the lower nodes of the stem to give additional
support are called stilt roots.
The
aerial, adventitious obliquely growing roots
that develop from the lower nodes of the stem to give additional
support are called stilt roots.
·
This root bears several large overlapping root caps called multiple root caps. e.g., Sugarcane,
Pandanus,
Rhizophora, Sorghum and Maize. Pandanus (screw
pine) is a common sea shore plant.
·
They also help in the absorption of water and minerals from the
soil.
·
In monocots, these roots arise in whorls from a few basal nodes
of stem.
(b) Prop
Roots:
·
These adventitious aerial roots arise
from horizontal aerial branches of the trees like Ficus bengalensis (Banyan).
·
Initially, they are hygroscopic in function, become red in moist
condition and possess root-caps at their apices.
·
They grow vertically downward, penetrate the soil, become thick
and assume the shapes of pillars.
·
They provide support to the
spreading branches of tree.
·
Sometimes the main trunk dies and it is replaced by prop roots
which assume the shapes of trunks.
·
In India, the biggest banyan tree having large number of prop
roots is found at Indian Botanical Gardens, Kolkata and Kadiri (Andhra
Pradesh).
(c) Buttress
Roots:
·
The horizontal plank
like aerial, adventitious roots that develop at the base of the stem to give
additional support are called buttress roots or ballast roots, e.g., Terminalia and Salmalia.
·
In some huge and heavy trees, plank like roots develop at the
base of the stem on the soil surface.
·
These roots give additional support and act like ballasts. Hence
these roots are called ballast roots.
(d) Climbing
Roots:
·
The
aerial adventitious roots that arise from the nodes or internodes of weak
stemmed plants to climb up their support are called climbing roots, e.g., Pothos, Piper betel,
Vanilla and Hedera.
The
aerial adventitious roots that arise from the nodes or internodes of weak
stemmed plants to climb up their support are called climbing roots, e.g., Pothos, Piper betel,
Vanilla and Hedera.
·
Many weak stemmed plants climb up their supports in order to
expose their leaves efficiently to sunlight.
·
In Pothos and Hedera, climbing
roots develop all over the stem.
·
In Vanilla,
single tendril like root arise at each node. Hence they are called tendrillar
roots.
·
In Piper
betel, many short branched, adventitious roots arise at each
node. These roots are called clinging roots.
(e) Floating
Roots:
·
These roots develop from the nodes of floating aquatic plants
like Jussiaea (=Ludwigia).
·
They
store air, become inflated and spongy, project above the level of water, make
the plant light and function as floats.
They
store air, become inflated and spongy, project above the level of water, make
the plant light and function as floats.
(f) Contractile
or Pull Roots:
·
Some roots of plants with underground stems contract or swell so
that the aerial shoots are kept in a proper depth in the soil.
·
These roots are called contractile or pull roots, e.g., Canna, Crocus, Allium, Lilium,
Freesia, etc.
(g) Root
thorns : In aroids like Pothos and many palms (Acanthorhiza and Iriartea) the
adventitious roots become hard and pointed hence called root thorns.
Question 1 : Water is absorbed by
(a) Root hairs
(b) Root cap
(c) Root
(d) Root apex
Question 2 : Black pepper is a
(a) Tree
(b)
Climber
(c) Shrub
(d) Herb
Question 3 : Pneumatophores occur in plants
of
(a) Sandy soil
(b) Saline marshy
soil
(c) Marshy soil
(d) Water
Question 4 : Roots developing from plant
parts other than radicle are
(a) Epiphyllous
(b) Epicaulous
(c) Adventitious
(d) Fibrous
Question 5 : Roots are feebly developed
in
(a)
Hydrophytes
(b) Mesophytes
(c) Xerophytes
|
Q.1
|
Q.2
|
Q.3
|
Q.4
|
Q.5
|
|
A
|
b
|
b
|
C
|
a
|
STEM….
The
stem develop from the plumule of the germinating seed.
- Normally it is the
aerial part of the plant body. The stem with its branches, leaves, buds, flower
and appandages is known as Shoot System.
- The stem shows the
differentiation of nodes and internodes.
- The place where the
leaf develops on the stem is called the node.
- The portion of the
stem between two successive nodes is called the internode.
Characteristics of Stem
(i) Stem is an
ascending axis of the plant and develops from the plumule and epicotyl of the
embryo.
(ii) It is generally
erect and grows away from the soil towards light. Therefore, it is negatively
geotropic and positively phototropic.
(iii) The growing apex
of stem bears a terminal bud for growth in length.
(iv) In flowering plants,
stem is differentiated into nodes and internodes. A node occurs where leaves are attached to the stem. Internode is the
portion of stem between the two nodes.
(v) The lateral organs
of stem (i.e., leaves and branches) are exogenous in origin (from cortical
region).
(vi) The young stem is
green and photosynthetic.
(vii) Hair, if
present, are generally multicellular.
(viii) In mature
plants, stem and its branches bear flowers and fruits.
Diverse forms of Stem
(i) Reduced stems : In some plants, the stem is in the form of a
reduced small disc which is not differentiated into nodes and internodes. e.g., (a) A reduced green-coloured disc-like stem
lies just above the base of fleshy roots of Radish, Carrot and Turnip ; (b)
Green-coloured small discoid stem occurs in free-floating Lemna, Spirodela and Wolffia; (c) Highly reduced non-green discoid stem occurs at the base
of Onion and Garlic bulbs, etc.
(ii) Erect stems : Majority of angiosperms possess upright,
growing-ascending, vertically-erect stems. They are fixed in the soil with the
help of roots. Erect stems belong to four categories :
(a) Culm : Erect stems with solid nodes and hollow
internodes. The nodes are
swollen giving the stem a jointed appearance e.g., Bamboo (Bambusa arundinacea) and wheat (Triticum vulgare).
(b) Caudex : The main stem remains unbranched and bears a
crown of leaves at its top. e.g., Coconut (Cocos nucifera), Palm, etc.
(c) Excurrent : The main stem is trunk like. It is thickest at
the base and gradually tapers towards the apex. The branches arise in acropetal
succession, i.e., oldest at the base and youngest at the apex. The tree appears
cone-shaped. e.g., Casuarina, Eucalyptus, etc.
(d) Decurrent or Deliquescent : The apical bud of main stem is weak as compared
to the buds of lateral branches. Thus, the lateral branches are prominent and
spreading. The main stem grows upto a certain height after which it gives
several branches. These branches dominate by giving the branches of several
orders. The whole tree looks like dome-shaped. e.g., Banyan (Ficus bengalensis).
(iii) Weak stems : They are thin, soft and delicate which are
unable to remains upright without any external support. They are of two types :
upright weak stems and prostrate weak stems.
(a) Upright weak stem
·
Twiners : The stems are long, slender, flexible and very sensitive.
They twin or coil around an upright support on coming in its contact
due to a special type of growth movement called nutation. They may
coil the support to the right (anticlockwise from the top or sinistrorse)
e.g., Convolvulus sp., Ipomoea quamoclit Clitoria ternatea, etc. or to the left (clockwise or
dextrorse), e.g. Lablab.
·
Climbers : The stem is weak and unable to coil around a support. They
usually climb up the support with the help of some clasping or clinging
structure. They are of four types :
(i) Tendril climbers : Tendrils are thread like structure which help
in climbing the plants.
They may be modified stem (e.g., Vitis), stem branches (e.g., Passiflora) and inflorescence (e.g., Antigonon).
(ii) Root climbers : Adventitious roots arise from the nodes and
penetrate into the upright support so that the climber climbs up,e.g., Betel
vine (Piper betel), Tecoma, Ivy, etc.
(iii) Scramblers or Hook
climbers : These weak stemmed
plants slowly grow over other bushes and rest there. They attain this position
with the help of curved prickles (e.g., Rose), curved
hooks on flowering peduncle (e.g., Artabotrys), prickles on stem (e.g., Lantana), spines (e.g., Climbing Asparagus) or spinous stipules (e.g., Zizyphus).
(iv) Lianas : These are woody perennial climbers found in
deep forests. At first, they are just like ordinary twiners but once they reach
to the top and get sunlight, become woody. e.g., Tinospora, Ficus,
Bauhinia, Bignonia, etc.
(b) Prostrate weak stem
·
Trailers : The stem creep on the ground but roots do not arise at the
nodes. They are of three types :
(i) Procumbent : The stem creeps on the ground totally
e.g., Tribulus, Bassela, Evolvulus.
(ii) Decumbent : Branches, after growing horizontally for some
length, grow vertically upwards, e.g., Portulaca, Tridax, Lindenbergia, etc.
(iii) Diffuse : Branches grow profusely in all directions,
e.g., Boerhaavia.
·
Creepers : These weak-stemmed plants grow prostrate and develop
adventitious roots from their nodes. Creepers are of three kinds – runners,
stolons and offsets.
(i) Runners : This prostrate aerial stem has a long internode and creeps horizontally. Axillary buds
arise from nodes to form aerial shoots and roots. Several small (daughter) plants are thus linked by
runner which may break off later. e.g., Cynodon (doob grass) and Oxalis.
(ii) Stolons : They are special kinds of runners which
initially grow upwards like ordinary branches and then arch down to develop new
daughter plants on coming in contact with the soil. e.g., Strawberry (Fragaria vesica), Peppermint (Mentha piperita), Jasminum (Jasmine).
(iii) Offsets : They are weak, elongated, horizontal branch of
one internode that arises in the axil of a leaf. At the tip, it produces cluster of leaves above and tuft of roots below.
The offset may break off from the parent plant and act as individual
plants. They are found usually in aquatic plants and
rarely is terrestrial plants. They are helpful for vegetative propagation.
e.g., Eichhornia (water hyacinth), Agave, Pistia.
Modification
of Stems
(1) Underground stem
: In many plants the
stems remain underground. There are many advantages for the underground stems.
(i) They can store
plenty of food material.
(ii) The underground
stems are well protected from herbivorous animals.
(iii) They can live for longer time (perennation).
(iv) The underground
stems can carryout vegetative propagation very easily.
The underground stems
lack green colour because of their geophillous nature. They can be identified as stems because of the presence of nodes,
internodes, scale leaves, buds and branches. Based on the type of
growth (transverse/vertical/oblique) and the part that stores food (main stem/
branch/ leaf base), the underground stems are classified into several types :
(i) Sucker : This is a sub aerial branch that arises from
the main stem. Initially it grows horizontally below the soil
surface and later grows obliquely upward. They are shorter and
stouter than the runners. e.g., Mentha arvensis (mint vern. Podina) and Chrysanthemum.
(ii) Rhizome : The rhizome is a thickened, underground
dorsiventral stem that grows horizontally at
particular depth within the soil. The rhizome is brown in colour and shows
cymose branching. It can be distinguished from the modified
root by the presence of nodes, internodes, terminal bud, axillary bud and scale leaves. The terminal bud
develops aerial shoot that bears inflorescence. Adventitious roots develop on
the ventral surface of the rhizome. The rhizomes are perennial and
vegetatively propagating structures. It is of following types :
(a) Rootstock : They are upright or oblique with their
tips reaching the soil surface. e.g., Alocasia indica and Banana.
(b) Straggling : They are horizontal in position and
generally branched (Sympodial or Monopodial), e.g., Nelumbo nucifera(Lotus), Zingiber
officinale (Ginger), Curcuma domestica (Turmeric), Saccharum etc.
(iii) Corm : The corm is an underground modification of
main stem. It grows vertically at particularly depth in the
soil. The corm stores food materials and becomes tuberous. It is non green in
colour and conical, cylindrical or flattened in shape. The corm bears scale leaves at each node. In the axils
of these scale leaves axillary buds arise which grow into daughter corms.
The terminal bud of the corm is large. It grows into aerial
shoot and bears leaves and flowers. Adventitious roots normally develop from
the base or all over the body of the corm. With the help of some special
adventitious roots called the contractile roots or pull roots, the corm remains
constantly at a particular depth. The corm propagates
vegetatively by daughter corms. e.g., Amorphophallus,
Colocasiaand Crocus (Saffron).
(iv) Tuber : Stem tuber is the tuberous tip of an
underground branch. It occurs beneath the soil at any depth. The axillary
branches (stolons) that are produced near the soil surface grow
into the soil and their tip become swollen due to accumulation of starch and
proteins e.g., Solanum tuberosum(potato). In potato, the stem nature is evident by the presence of ‘eyes’
on its brownish corky surface. Each eye is a pit like structure and represents
the node. At the rim of the eye, scale leaf scar is seen. Axillary bud is situated in the pit of the eye.The stem
tubers are differentiated from the tuberous roots by the presence of vegetatively propagating eyes.
(v) Bulb : A bulb is a specialized
underground stem bears roots on it’s lower side and rosette of
fleshy leaf bases or fleshy scales on
the upper side. In a bulb, the stem is reduced and becomes discoid. On the
lower side of the disc adventitious roots develop in clusters. The upper side
of the disc shows compactly arranged fleshy leaf bases or
scale leaves so as to form an underground bulb. The leaf
bases or scales become fleshy due to accumulation of food (carbohydrates) and
water. The terminal bud grows into inflorescence or aerial shoot (scape), while
some of the axillary buds develop into daughter bulbs. Bulbs are of two types,
tunicated bulb and scaly bulb.
(a) Tunicated bulb : In tunicated bulb, the fleshy leaf bases are
arranged in a concentric manner. The entire bulb is covered by peripheral dry
membranous leaf bases called tunics, hence called the tunicated bulb. e.g., Allium cepa (Onion), Narcissus and Tulip. Compound tunicated bulbs as in Allium sativum (garlic).
(b) Scaly or Imbricated
bulb : In scaly bulb, the
fleshy scale leaves are arranged loosely overlapping one another. Such bulbs
are not covered by any tunics, hence called naked bulbs or scaly bulbs. e.g.,
Lily.
(2) Aerial stem : The aerial stems are exposed to different
environmental conditions. Hence they show many modifications. The vegetative
and floral buds of many plants instead of growing into branches and flowers,
undergo metamorphosis to form new structure.
(i) Tendrils : The tendrils are thin, wiry, leafless and
spirally coiled branches. The terminal part of a tendril is sensitive. It holds
the support by coiling round it. The tendrils help the weak stems to climb the
support. In some weak stemmed plants, the axillary bud or terminal bud may modify to form tendrils which are
specially called stem tendrils. Stem tendrils are following four types:
(a) Axillary :
e.g., Passiflora.
(b) Extra-axillary :
e.g., Luffa (vern. Ghiatori), Cucurbita (vern. Kaddoo), Lagenaria (vern. Lauki).
(c) Apical bud
tendrils : e.g., Grape Vine (Vitis vinifera).
(d) Floral bud or
Inflorescence tendrils : e.g., Antigonon.
(ii) Stem thorns : The axillary buds of
some plants become arrested and get modified into stiff, sharp and pointed
structures, called thorns. They are
deep seated structures having vascular connections with
stem. Besides reducing transpiration, they protect the plant from
browsing animals. e.g., Citrus, Duranta,
Bougainvillea, Pomegranate, Flacourtia, Aegle marmelos etc.
(iii) Phylloclades
(Cladophyll) : The
phylloclade is special modified photosynthetic stem present mostly in
xerophytes. It is green,
flattened or cylindrical structure which has distinct nodes and internodes.
Xerophytes show many adaptations to check the rate of transpiration. Reduction
of leaf size, early leaf fall, formation of scale leaves, spines, thorns, thick cuticle, presence of fewer
stomata are some of the xerophytic characters. In such cases, the stems become
flattened to carryout photosynthesis. These modified stems are called phylloclades or cladophylls. Usually the phylloclades retain water in the
form of mucilage. e.g., Opuntia, Casuarina, Cocoloba and Ruscus.
In Opuntia, the leaves are modified into spines and
the stems becomes fleshy leaf like phylloclade. In Casuarina the leaves are modified into scales. The
phylloclade in Ruscus is
leaf like and bear flowers. In Cocoloba, after the modification of leaves into scales the stem becomes
ribbon like phylloclade with distinct nodes and internodes.
(iv) Cladodes : These are modifications of stem and branches
of limited growth. It has one internode only.
Each cladode is green, flat or cylindrical, leaf like
structure which performs photosynthesis. In Asparagus, the leaves are reduced to curved spines. In Ruscus aculeatus, the leaf like cladode are borne in the axils
of scale leaves.
(v) Thalamus : Thalamus of a flower is a modified stem
apex. The other floral parts (sepals, petals, stamens and carpels) are born on
the thalamus. It may be convex (Ranunculus), concave (Lathyrus)
or flask shaped (Rosa).
Question 1: In Amorphophallus and Colocasia (Ariods) an extremely enlarged
underground vertical stem meant for vegetative reproduction and storage is
(a)
Tuber
(b)
Corm
(c)
Bulb
(d)
Rhizome
Question 2: Vegetative reproduction occurs by bulbil in
(a)
Agave
(b)
Colocasia
(c)
Zingiber
(d)
Vallisneria
Question 3: Primary function of stem is to
(a)
Bear and hold out leaves
(b)
Absorb water and minerals
(c)
Fixation of plant
(d)
Help in vegetative propagation
Question 4: Plants of tropics bear
(a)
Winter buds
(b)
Summer buds
(c)
Naked buds
(d)
Adventitious buds
Question 5: Accessory buds occur at
(a)
Stem tip
(b)
Branch tip
(c)
Leaf axil
(d)
Side of axillary bud
|
Q.1
|
Q.12
|
Q.3
|
Q.4
|
Q.5
|
|
B
|
a
|
a
|
c
|
d
|
DR A.KUMAR[
M.B.B.S.] ,DR. Y.KATIYAAR [MBBS,MD]& SANDHYA KATIYAAR
TOPIC ..PLANT MORPHOLOGY
Morphology is
the branch of science which deals with the study of external structural of the
plants.
The flowering plant
consists of An Axis, Root System and Shoot System.
Root
System
The flowering plants
consist of a long cylindrical axis which is differentiated into an
underground Root System.
Root is the descending, non green, underground
part lacking nodes, internodes, leaves and buds.
In Dicotyledon plants, the elongation of radicle leads to the
formation of primary root. The primary root grows inside the soil. Primary root
bears lateral roots known as Secondary Roots, Tertiary Roots etc.
Types
of Root System
The primary roots
along with its branches forms a Tap Root system. For Example: Mustard Plant. Primary roots have short life span and are
replaced by large roots later on.
The roots which arises
from the base of the stem are known as Fibrous Root system. For Example: Grasses. This type of system is common in Mocotyledon Plants.The roots which arises from the
part other than the radicle are known as Adventitious Roots. For Example: Banyan Tree, Monstera.
Regions
of the Root
The root has five main
regions which are as follows:
·
Root Cap is the structure that covers the
apex/tip of the root. It helps in protecting the apex of the root.
Root Cap is the structure that covers the
apex/tip of the root. It helps in protecting the apex of the root.
·
Region of Meristematic Activity located few millimeters above the root
cap. The cells of this region has the property of repeated divisions which is
needed for the growth of the plant.
·
Region of Elongation is located proximal to the region of meristematic
activity. This is required for the elongation of root.
·
Region of Maturation is located proximal to region of elongation. These cells
gradually differentiate and mature.
·
Root Hairs arise
from the region of maturation which helps in absorption of water and minerals
from the soil.
Modifications
of the Root
Roots modify in
structure to perform different functions as explained below:
·
Fusiform root is a modified form of tap root. The root is swollen from the
middle and tapers at both the ends. For Example: Radish
·
Napiform roots are also a modified tap root. They have swollen base and then it
tapers abruptly. For Example: Turnip
·
Prop roots are roots which are modified for aerial support. For Example: Corn
·
Stilt roots are modified roots for support. For Example: Maize
·
Pneumatophores are roots modified for respiration. They grow in swampy area and
grow vertically upwards. For Example:Rhizophora
·
To absorb water and
minerals from the soil.
·
To provide proper
anchorage to the plants.
·
To store reserve food
materials
·
Synthesis of plant
growth regulators
Shoot
System
It is an aerial
system, usually above the soil and originates from the plumule. It consists of
stem, branches, leaves, flowers and fruits.
Stem
It facilitates
conduction of water, mineral and food material. The flowers, fruits and seeds
form the reproductive parts of the plants. The stem bears the nodes and internodes. The
point from where the leaf arises is known as Node and the part between the two Nodes are known as Internodes.
Modification
of Stems
Similar to roots, the
stem is also modified to perform different functions:
·
Tendrils are
slender, twining strands that allow the plant to climb and helps in
support. For Example: Cucumber.
·
Thorns are modified
stem for protection of the plant. They are hard, woody and sharp outgrowths
from the plant. For Example: Rose.
Fig.6.
Modifications of Stem
·
Bulbils are
modified stem that becomes fleshy and store food.
Leaf
Leaf is a green
dorsoventrally flattened exogenous lateral outgrowth that arises from a node of
the stem or a branch. The leaf is a specialized organ of photosynthesis,
transpiration and gaseous exchange.
Parts of Leaf
The point of origin of
leaf is known as Node. It bears bud in its axil.
The leaf is attached to the stem by the leaf base and have two lateral leaf
like structures called Stipules.
In monocots, the leaf base is swollen to form pulvinus. The stalk of the leaf
is known as Petiole.
The green exposed part of the leaf is known as lamina. Lamina bears the veins
or veinlets. The arrangement of veins on the leaf is known as Venation.
There are basically
two types of Venation: Parallel Venation and Reticulate Venation.
Parallel venation is
observed when veins run parallel to each other. For Example: Monocots
Fig.8.
Types of Venation
Flower
The modified shoot is
known as Flower. The arrangement of
flower on the floral axis is known as Inflorescence.
There are two major
types of Inflorescence- Racemose and Cymose.
In racemose type of
inflorescence, new flowers are generated at the tip of the inflorescence. For Example: Snapdragon
In cymose type of
inflorescence, new flowers are generated at the base of the inflorescence.
Structure
of the Flower
Each flower has four
whorls- Sepals, Petals, Stamens and the Carpels. Sepals are green leafy
structures that covers the flower bud. Petals are bright colored to attract
insects for pollination.
Gynoecium are female
reproductive structures whereas Androecium is the male reproductive
structure.
Fig.5.
Structure of the Flower
Gynoecium is composed
of carpels. Carpels comprises of three structures: Ovary, Style and Stigma. Ovary comprises of
ovules that forms the seed, whereas stigma is the place of landing of pollen at
the time of pollination. Ovary finally forms the fruit.
The male part of the
flower is known as Androecium.
Androecium is composed of stamens. Stamen is composed of anther and the
filament. Anthers contain the pollen grains, the male gamete.
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