Classification of Plants

(A) On the Basis of Habit

On the basis of their form or habit, the flowering plants are usually classified into the following three categories

(a) Herbs: Herbs are small plants with soft stems. They may be annual (e.g., Mustard=Brassica campestris) , biennial (e.g. , Radish=Raphanus sativus) , or perennial (e.g., Canna).

(b) Shrubs: Shrubs are medium sized perennial woody plants which branch profusely from the base and attain a bushy appearance e.g., China rose (Hibiscus rosa-sinensis), Capparis decidua etc.

(c) Trees: A tree has a main stout and woody trunk which gives off branches only at some distance above the ground e.g., Mango (Mangifera indica), Shisham (Dalbergia sissoo) and banyan (Ficus benghalensis).

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On the basis of their branching, trees are classified into the following three categories.

(i) Caudex (Columnar). The stem is unbranched and usually bears a crown of leaves at the apex, e.g., Date palm (Phoenix dactylifera), Fan palm (Borassus flabellifer), etc.

(ii) Excurrent. The branches arise from the main stem in acropetal succession and the tree assumes a cone like appearance e.g., Pinus, Eucalyptus, Casuarina, etc.

(iii) Decurrent (Deliquescent). The lateral branches grow more vigorously and outcompetes the main trunk, giving a dome-shaped apperance, e.g., Mango (Mangifera indica), Shisham (Dalbergia sissoo) and Banyan (Ficus benghalensis).

(B) On the Basis of Mode of Nutrition

1. Autotrophs. These are photosynthetic plants synthesizing their own food, e.g., all green plants.

2. Heterotrophs. These plants can not synthesize their own food. They are of following types –

(a) Parasitic plants.

Depend on other plants for food and water. They have special structures for absorption of food and water. They may be

(i) Obligate or total parasite. Depend on other plants for both food and water.

Total stem parasites. e.g., Cuscuta, Cassytha and Arceuthobium (smallest among angiospermic parasite, only the flowers are visible externally, A. minutissimum is found on stem of Pinus wallichiana).

• Total root parasite. e.g., Orobanchae (Broom rape), Balanophora, Rafflesia, Sapria, Cistanche.

(ii) Partial or semi -parasites. Depend on other plants for water and minerals only.

• Partial stem parasites. e.g., Viscum (Mistletoe), Loranthus .

• Partial root parasites. e.g., Santalum, Striga, Thesium.

(b) Saprophytic plants.

Grow on dead organic matter e.g., Monotropa (Indian pipe), Neottia (Bird's nest). They are mycotrophic plants.

(c) Symbiotic plants.

Symbiosis or mutualism is obligatory beneficial partnership of two organisms e.g., lichens (algae and fungi), Rhizobium (N2 fixing bacteria and leguminous plants), mycorrhiza (fungi and roots of higher plants).

Symbiotic relationship between ants and some higher plants is another good example where the ants obtain food and shelter from the plant.

They protect the plant from other animals e.g., Acacia sphaerocephala (Stipules are hollowed to function as ant shelter, leaflet tips and rachis possess feeding materials).

(d) Insectivorous or carnivorous plants.

They grow in soil deficient in nitrogen.

They trap insects and digest their protein.

These are chlorophyllous plants thus can synthesize their own food.

So all insectivorous plants are producers and secondary consumers.

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Some insectivorous plants are:

(i) Utricularia (Bladder wort). It is a rootless aquatic plant with highly dissected leaves. Some of the leaf segments are modified into small bladders which have trap valves for catching small animals like Cyclops and Daphnia.

(ii) Drosera (Sundew). It is a herbaceous plant growing in water logged places. The upper surface of its leaves possess club shaped tentacles. The tentacle heads secrete sticky purple juice that shines in the sun (hence called sundew). An insect touching a tentacle is stuck up and trapped by bending of tentacles. The trapped insect is then digested by enzymes secreted by digestive glands and amino acids are absorbed by the leaf.

(iii) Dionaea (Venus fly trap). It is a herbaceous plant bearing rosette leaves. Leaves have winged petiole and lamina modified into two toothed jaws (bilobed) normally open at an angle forming a trap. Each jaw contains several teeth. Upper surface of each jaw (lobe) bears sensitive hair, spines or bristles (three in number) and digestive glands.

Stimulation of a sensitive spine or hair by an insect causes folding of leaf and secretion of digestive juices.

(iv) Nepenthes (Pitcher plant). It is a climber. The pitcher is formed from the lamina of leaf and the lid is the modified leaf tip. The flattened leaf like part below the petiole is the leaf base. Petiole is elongated and tendrillar. A large number of glands are situated on the upper half of the inner wall of pitcher which secrete proteolytic enzymes. The enzymes hydrolyse the protein of insects and amino acids so produced are absorbed by the plant. Sarracenia, Darlingtonea and Cephalotus are other insectivorous pitcher plants.

(v) Aldrovanda (Water flea trap). It has a thin rootless floating stem, which bear whorls of modified leaves. Each leaf has a spathulate stalk and a folding two lobed lamina with teeth round the edges. The surface bears numerous sensitive joined hairs and digestive glands.

Leaf modified into insect catching structures:
A. Nepenthes, B. Drosera, C. Dionaea

(e) Epiphytes.

These plants grow on other plants for shelter only (for physical support).

They synthesize their own food.

They have special hanging roots called hygroscopic roots to absorb moisture from atmosphere by thin walled cells lying outside called velamen, e.g., Orchids like Vanda, Dendrobium, etc.

(C) On the Basis of Life Span

Three categories of plants are recognised on this basis

(i) Annuals:

The plants which complete their life cycle in a single season or few weeks or few months are called annuals.

They grow and produce flowers and fruits within this period and then die off, e.g., mustard (Brassica campestris), pea (Pisum sativum), wheat, maize, Euphorbia prostrata.

(ii) Biennials:

The plants which complete their life cycle in two growing seasons are called biennials.

In the first season, they grow vegetatively and in the next season, they produce flowers, fruits and seeds, e.g., carrot (Daucus carota), radish (Raphanus sativus) and turnip (Brassica rapa).

(iii) Perennials:

These are the plants which continues to grow for many years, e.g., peach (Prunus persica) and apple (Pyrus malus).

Perennials can be, monocarpic (which flower and fruit only once in life time) e.g., bamboo (Bambusa tulda), century plant (Agave), or polycarpic (which flower and fruit many times in life time), e.g., mango, pear.

Let us discuss various parts of a flowering plant


True roots develop from radicle of seed.

They are non green, underground, positively geotropic, positively_hydrotropic and negatively phototropic.

Roots usually do not bear buds, but buds are present for vegetative propagation in adventitious root of sweet potato (Ipomoea) and tap root of Indian red wood (Dalbergia).

They do not bear nodes and internodes.

They have unicellular roots hairs.

Lateral roots arise endogenously, i.e., from pericycle.

Zonation in Roots

(i) Root cap.

At the apex of root a smooth cap shaped structure is present which is called as root cap.

It is protective.

Multiple root cap is found in aerial roots of screwpine (Pandanus).

In hydrophytes, root cap is either absent or replaced by root pocket, e.g., Pistia, Lemna, Eichhornia.

(ii) Zone of cell formation or division.

The cells of this region are inactive state of division and their number increases continuously.

Vacuoles are small or absent.

(iii) Zone of cell elongation.

Maximum growth in the cells occurs in this zone.

Cells have a large central vacuole.

(iv) Zone of cell maturation.

The cells in this region are differentiated into permanent tissues depending upon the functions they have to perform.

Root hairs are also present in this zone which help in absorption of water.

In hydrophytes, root hairs are absent because they absorb water through general body surface.

Types of Roots

Roots are of two types :

(1) Tap roots. Primary root developing from radicle. The primary root grows and gives rise to secondary and tertiary roots forming the tap root system, e.g., dicots.

(2) Adventitious roots. They develop from any part of the plant body other than the radicle. They are called adventitious roots, e.g., monocots.

1. Modifications of Tap Root :

A. Storage or fleshy tap roots.

They store food and assume various shapes.

(i) Conical: Cone like, e.g., carrot.

(ii) Napiform: Swollen in the upper part and apruptly tapers in lower part, e.g., turnip and beet root.

(iii) Fusiform: Spindle shaped, e.g., radish.

B. Respiratory root.

Some plants like Avicennia and Sonneratia, which grow in salty marshes (mangroves) develop special kinds of roots for respiration.

These roots are called respiratory roots or pneumatophores.

They arise in conical shape from the branches of underground tap root and grow vertically upwards (i.e., negatively geotropic) into the air.

The upper portions of these roots have numerous aerating pores, called pneumatothodes.

Pneumatophores (respiratory roots) of a mangrove tree:

A. Main plant with emerging pneumatophores; B. Pneumatophores enlarged

C. Nodulated roots

These are found in members of family Papilionaceae for nitrogen fixation. Symbiotic bacteria of the genus Rhizobium are present in nodules to fix atmospheric nitrogen.

2. Modifications of Adventitious Root

A. Storage adventitious roots

(i) Tuberous. Single root arises from node of stem and becomes tuberous and fleshy for storage of food, e.g., sweet potato.

(ii) Fasciculated. Roots arise in bunch (cluster) from lower node of stem and become fleshy, e.g., Dahlia, Asparagus.

(iii) Nodulose. Root apex becomes swollen and fleshy, e.g., mango ginger (Curcuma amada).

(iv) Beaded or Moniliform. Roots swell up at regular intervals forming beaded structure, e.g., Portulaca, Momordica (bittergourd).

Modifications of adventitious roots : A. Tuberous roots of sweet potato;

B. Fasciculated roots of Dahlia; C. Nodulose roots of mango ginger

(v) Annulated. Roots having series of ring like swellings e.g., Ipecac (Psychrotia).

Modifications of adventitious roots:

A. Moniliform roots of Momordica; B. Annulated roots of Ipecac.

B. Adventitious roots that provide extra support

They are of following types:

(i) Prop roots. They arise from the branches of stem for providing mechanical support to heavy branches, as pillars, e.g., old banyan tree (Ficus benghalensis).

Modification of adventitious root: Prop roots of banyan

(ii) Stilt roots. They arise from lower nodes of stem to support main axis and enter the soil obliquely, e.g., sugarcane, maize, screwpine (Pandanus).

Modifications of adventitious root: A. Stilt root of sugarcane; B. screw pine

(iii) Climbing roots. They arise from nodes and help the plants in climbing, e.g., Pothos, Piper.

Modification of adventitious roots: Climbing root of Piper

(iv) Buttress roots. They arise from basal parts of main stem and spread in different directions in the soil, e.g. , Bombax, Ficus religiosa .

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Adventitious root with special functions

(i) Floating roots. In aquatic plants (e.g., Jussiaea) white spongy roots arise from branches and help in floating and respiration.

(ii) Assimilatory roots. The aerial roots of Tinospora and submerged roots of Trapa (Water chestnut) become green and synthesize food. Podostemon also has green assimilatory roots.

(iii) Sucking or haustorial roots. These roots suck food and water from host and are found in parasitic plants e.g., Cuscuta, Orobanche, Viscum.

(iv) Hygroscopic roots. These are found in epiphytes, specifically orchids and help in absorption of moisture from the atmosphere using special tissue called velamen.

(v) Contractile roots -They shrink 60 -70% of the original length and bring underground organ at proper depth in the soil e.g., corm of Crocus (saffron), Freesia .

(vi) Root thorns -These are hard, thick and pointed thorns e.g., Pothos armatus and Acanthorhiza.

(vii) Clinging roots - These are non absorptive adventitious roots arising either from nodes (e.g., Tecoma, betel), internodes (Ficus pumila) or both (e.g., juvenile stage of Ivy).

(viii) Reproductive roots - These are fleshy, adventitious roots used for vegetative reproduction e.g., sweet potato (Ipomoea batatas) , Dahlia.

(ix) Leaf roots - In Salvinia, one leaf of each node modifies into root like structure for balancing the plant in water.

(x) Epiphyllous roots - These roots arise from the margins of leaf lamina for vegetative reproduction e.g., Bryophyllum.

Modification of adventitious root: Epiphytic roots of Vanda (an orchid)

Functions of Root

The root performs various functions like –

Fixation, Absorption, Conduction, Storage, Reproduction, Assimilation, Nitrogen fixation, Floating and Balancing and provides Mechanical support.