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What is the basis of classification of Algae?

Algae are classified depending on their characteristics and structure; including photosynthetic pigmentation, storage products, flagella, the structures of the cell wall, nucleus, chloroplast, pyrenoid (part of the chloroplast used in producing starch) and the eyespot (organelle composed of tightly packed lipids). Algae can be either prokaryotic (lacking a cellular membrane; classified in the monera kingdom) or eukaryotic (the cellular organelles are bound by a nuclear membrane; classified in the protista or plantae kingdoms). There are about sixteen different types of alga known or defined and are grouped depending on their structure and location of growth.


Write a note on economic importance of algae.

Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. 

Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) are used commercially. Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements even by space travelers.


What is heterospory?

Plant producing two types of spores of differing in size and sex is called heterospory.


Explain briefly the following terms with suitable examples:-

(i) protonema

(ii) Antheridium

(iii) Archegonium

(iv) Diplontic

(v) Sporophyll

(vi) Isogamy

(i) protonema
The predominant stage of the life cycle of a moss is the gametophyte, which consists of two stages. The first stage is the protonema stage, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage. Example: Sphagnum.

(ii) Antheridium:
The male sex organ is called antheridium. They produce biflagellate antherozoids. Example: Selaginella.

(iii) Archegonium:
The female sex organ is called archegonium. They produce biflagellate antherozoids. Example: Cycas.

(iv) Diplontic:
The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seed-bearing plants gymnosperms and angiosperms are following this pattern. Example: Pinus.

(v) Sporophyll:
The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. Example: Nephrolepis.

(vi) Isogamy:
Reproduction by the union or fusion of gametes of the same size and structure is called isogamy. Example: Spirogyra.


Write the difference between the liverworts and moss.

The differences between mosses and liverworts are,

(i) Capsule opening mechanisms
In most liverworts, when a spore capsule is ripe, it merely bursts open along four longitudinal axes; moss capsules nearly always have rather more complex opening mechanisms.

(ii) Structure of rhizoids
In liverworts, the rhizoids are nearly always composed of a single, elongated cell; in mosses the rhizoids are multicellular.

(iii) Early development of the gametophyte
In mosses, following spore germination, but preceding development of the mature gametophyte plant, a juvenile stage known as a protonema forms. This is absent or much reduced in liverworts.

(iv) Other differences are,

(a) All mosses consist of a stem (branched or not) bearing leaf-like structures. Many liverworts do not have this type of structure.

(b) Many liverworts have leaves which are deeply lobed or divided. No mosses have leaves like this.

(c) All liverworts with a leafy structure have their leaves arranged in a two- or three-rowed pattern. Very few mosses have this sort of arrangement of leaves.

(d) Many mosses have leaves with a thickened nerve. No liverworts have leaves with such nerves


How would you distinguish monocots from dicots?




Embryo with single cotyledon.

Embryo with two cotyledons.

Pollen with single furrow or pore.

Pollen with three furrows or pores.

Flower parts in multiples of three.

Flower parts in multiples of four or five.

Major leaf veins parallel.

Major leaf veins reticulated.

Stem vascular bundles scattered.

Stem vascular bundles in a ring.

Roots are adventitious.

Roots develop from radicle.

Secondary growth absent.

Secondary growth often present.



Describe the important characteristics of gymnosperms.

The gymnosperms (gymnos : naked, sperma : seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation, are not covered, i.e., are naked. Gymnosperms include medium-sized trees or tall trees and shrubs. The roots are generally tap roots. The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound. The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind. In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls, which are arranged spirally along an axis to form lax or compact strobili or cones. The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation, which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains takes place within the microsporangia. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male or female cones or strobili may be borne on the same tree (Pinus) or on different trees (Cycas). The megaspore mother cell is differentiated from one of the cells of the nucleus. Envelopes protect the nucleus and the composite structure is called an ovule. The ovules are borne on megasporophylls, which may be clustered to form the female cones.

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