Plant Kingdom

Spirogyra is a freshwater green alga which belongs to Class Chlorophyceae. The sexual reproduction in Spirogyra is called conjugation. It involves the fusion of two morphologically identical but physiologically dissimilar non-ciliated gametes.

For development of gametes, some of the cells start to act like male and female gametangia in which the cell contents become separated from the cell wall, shrink and ultimately forms gametes. The fusion of these gametes takes place by scalariform conjugation or lateral conjugation.

Maize, mint and peepal are flowering plants or angiosperms but Pinus is a gymnosperm in which seeds are produced but flowers are not produced or seeds are not enclosed in flowers.

Vesselless angiosperms are Wintera, Trochodendron etc. Secondary growth is absent in some angiosperms. Angiospermic plants are autotrophic in nutrition. But some angiosperms are heterotrophic in nutrition. The 4 special modes of nutrition and their examples include:
(i) Saprophytic : e.g. Neottia, Monotropa.
(ii) Symbiotic – e.g. Mycorrhiza-between fungus and roots of higher plants.
(iii) Parasitic – Cuscuta.
(iv) Insectivorous plant – Nepenthes. Double fertilization is characteristic of all angiosperms.

Cycas belongs to Order Cycadales of gymnosperms because it has naked seed. It is not enclosed inside a fruit. It does not have double fertilization and so the endosperm formed is haploid in nature and not triploid. So it is not included in angiosperms as they have ovules (or seeds) produced inside fruit. This is the main difference between gymnosperms and angiosperms.

In green algae, the photosynthetic pigments are chlorophyll a & b, carotenes and xanthophylls. In phaeophyceae, the pigments are chlorophyll a, chlorophyll c and carotenes and xanthophylls. Phycoerythrin gives red colour to Rhodophyceae.

Gymnosperms are the most primitive seed plants. The plants are generally perennial, woody trees or shrubs. In general, tallest trees in gymnosperms, e.g., Sequoia sempervirens is 366 ft. in height. The male gametes of Cycas are largest (300 $\mu $) in size, they are visible to naked eye and are oval in form and top-shaped. The ovule of Cycas is also largest in the plant kingdom.

A fern plant body is sporophytic (2n) and is differentiated into roots, stems and leaves. On the ventral surface of leaves sporangia are borne in a group called sori. Inside the sporangium are present the spores which are formed by reduction division.

Thus the spores produced are haploid in nature and germinate to produce a prothallus that represents the gametophytic generation. Antheridium and archegonium are borne on this prothallus. Thus meiosis takes place at the stage of spore formation.