Plant kingdom and classification for neet

 

PLANT KINGDOM

Our understanding of the plant kingdom has changed over time. Member of fungi, Monera and Protista having cell wall is now excluded from plantae. The cyanobacteria or blue green algae are not algae any more. The earliest classification of plants was based on gross  superficial morphological characters.

ALGAE

Algae are largely aquatic (fresh and marine water). They occur in a variety of other habitat: moist stones, soils and wood. Some of them also occurs in association with fungi (lichens) and animals (e.g. on sloth bear).

The form and size of algae is highly variable. Algae are found in many forms like filamentous, colonial. A few of the marine forms such as kelp form massive plant body.

 Algae are surrounded by mucilagenous sheath and below the sheath cell wall is present which is made up of cellulose and pectin but mainly made up of cellulose, galactans, mannans and mineral, like calcium carbonate.

 On the basis of structure, algae are thalloid i.e. plant body is not differentiated into root, stem and leaves. Tissue system is also absent in algae.

On the basis of nutrition, algae are photoautotrophic. They have plastid in which photosynthetic pigments are present. Classification of algae is mainly based on pigments. Chl-a and B carotene are universal pigments of algae.

REPRODUCTION

Vegetative reproduction - By Fragmentation - Filaments break down into small pieces and each fragment develops into a thallus.

 Asexual reproduction :-  

 Asexual reproduction is by the production of different types of spores. The most common being is the zoospores. They are flagellated and on germination gives rise to new plants. Zoospores are formed in favourable conditions and aplanospores, hypnospores and akinetes etc. are formed in unfavourable condition.

Sexual reproduction :-

Male sex organ is called antheridium and female is called oogonium. The sex organs of algae are unicellular & jacketless. But exceptionally sex organs of green algae Chara are multicellular and jacketed.

The male sex organ of Chara is known as globule and female is known as nucule.

Sexual reproduction takes place by fusion of two gametes.

Plantbody of algae is haploid, so sexual reproduction take place through zygotic meiosis. Therefore, their life cycle is haplontic. But exceptionally brown algae are diploid. [e.g. In Fucus, life cycle is diplontic]

Algae reproduce by zygotic meiosis i.e. first division in zygote is meiosis so embryo is not formed.

 

The classification of algae is mainly based on the photosynthetic pigments.

Algae are divided into following divisions

(A)                  Chlorophyta – Green Algae

(B)                  Phaeophyta – Brown Algae

(C)                  Rhodophyta  - Red algae

 

(A)               Chlorophycea-:

       Green algae are the most advanced algae.

       Habitat: Green algae are cosmopolitan in nature.

   Different body forms of green algae: The plant body may be unicellular, colonial or filamentous. 

(i) Unicellular :-

(a) Chlamydomonas - Motile unicellular algae. These algae move with the help of flagella.

(b) Chlorella - Non motile unicellular alga.

(c) Acetabularia - It is the largest unicellular plant.

According to five kingdom system the algae described above should be placed in Protista but because their life cycle is similar to green algae so they are studied in Plantae.

(ii) Colonial - Some green algae are found in colonies. They form colony of cells. The number

of cells in a colony is fixed. Colony with fixed number of cells is called coenobium.

(iii) Multicellular filamentous - Mostly the green algae are multicellular and filamentous.

eg. Ulothrix, Spirogyra

• Green algae usually have a rigid cell wall, made up of inner layer of cellulose and outer layer of pectose. 

Pigments:

Chlorophyll - Chl 'a' and Chl 'b'

 Carotene - ẞ carotene

Xanthophyll - Yellow coloured

They are usually grass green due to the dominance of pigments chlorophyll a and b. Pigments are localised in definite chloroplasts.

Stored food - Most of the members of green algae have starch as stored food and some have oil droplets also.

Vegetative reproduction - Usually takes place by fragmentation.

Asexual reproduction - Asexual reproduction is by flagellated zoospores which are formed in favourable condition and aplanospores, hypnospores and akinetes are formed in unfavourable condition.

(B) Class: Phaeophyceae - Division: Phaeophyta

Habitat: Brown algae are found primarily in marine habitats.

Brown algae are multicellular filamentous. They show great variation in size and form. They range from simple branched filamentous forms (Ectocarpus) to profusely branched forms as represented by Kelps which may reach a height of 100 meters. The plant body is usually attached to substratum by a holdfast and has a stalk (stipe) and leaf like photosynthetic part, frond or lamina, so brown algae are known as leafy algae. (eg. Laminaria)

The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. In brown algae protoplast contains plastid, centrally located vacuole and nucleus. e.g. Fucus, Dictyota, Ectocarpus

Pigments :-

Chlorophyll - chl 'a' and chl 'b'

Carotene - B carotene 

Xanthophylls - fucoxanthin

They vary in colour from olive green to various shade of brown depending upon the xanthophyll pigment fucoxanthin.

Stored food: Laminarin and mannitol - both are complex carbohydrates.

Vegetative reproduction: Takes place by fragmentation.

Asexual reproduction: By biflagellated Zoospores that are pear shaped and have two unequal laterally attached flagella.

Sexual reproduction: Sexual reproduction in brown algae may be isogamous, anisogamous or oogamous Union of gametes may takes place in water or within the oogonium.

Gametes are pyriform (Pear - Shaped) and have two unequal laterally attached flagella. Life cycle of fucus is diplontic.  

(C)  Rhodophyceae - Division: Rhodophyta ( RED ALGAE)

Habitat:

Red algae mainly found in marine water with greater concentration found in the warmer areas. They occur in both well lighted regions close to surface of water, and also at great depth in ocean, where relatively little light penetrate. The red thalli of red algae are usually multicellular. Some of them have complex body organisation.Cell wall of red algae is complex because it is made up of cellulose & pectin with polysulphate esters. Some red algae may secrete and deposite calcium carbonate and appear like corals.

Pigments

Chlorophyll- Chl 'a' and Chl 'd'

                Carotenes -ẞ carotene

            Phycobilins - R-phycoerythrin (red coloured) and R- phycocyanin (blue coloured)

The members of rhodophyceae are commonly called red algae because of predominance of the red pigment, r-phycoerythrin in their body.

On the basis of pigments red algae are similar to blue green algae.

Stored Food:

The food is stored as floridean starch which is very similar to glycogen and amylopectin.

Vegetative reproduction - Fragmentation

Asexual reproduction - Non motile spores

Sexual reproduction - Occurs by non motile gametes

(i) Sexual reproduction is oogamous and accompanied by complex post fertilization developments.

(ii) Life cycle of Polysiphonia is Haplo-diplontic or diplobiontic.

Economic importance :-

Porphyra - It is an edible algae and used as a food.

Gelidium and Gracilaria - Agar-agar is a hydrocolloid (Phycocolloid) is obtained from these red algae. It is used to prepare culture medium to grow microbes and in prepartion of ice creams and jellies.