plant morphology part 0

                           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.

·         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

Fig.4. Modifications of Roots

·         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

Functions of the Root

·         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.

Fig.5. Shoot System

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

·         Cladodes are modified stem that perform the function of photosynthesis.

·         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

Reticulate venation is observed when veinlets form network. For Example: Dicots

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.

Fig.9. Types of Inflorescence

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.

(iv) As a rule the root does not bear leaves and true buds.

(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.

·         It is also called the normal root system.

·         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.

·         In grasses, fibrous root system is present.

·         It is a type of adventitious root system.

·         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.

·         These roots bear many minute pores called pneumathodes towards their upper ends.

·         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.

·         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.

·         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.

(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            

(d)      Halophytes

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

         

                                                              

 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.

·         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

Fig.4. Modifications of Roots

·         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

Functions of the Root

·         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.

Fig.5. Shoot System

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

·         Cladodes are modified stem that perform the function of photosynthesis.

·         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

Reticulate venation is observed when veinlets form network. For Example: Dicots

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.

Fig.9. Types of Inflorescence

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.