Plant Kingdom Test - 6

Plants given in the figure are A- Selaginella and B-Ginkgo respectively. Selaginella is a trailing, hanging or erect evergreen lycopod(Pteridophyte).
Ginkgo biloba(a Gymnosperm) has not changed for the last several millions of years since its appearance in triassic period. It is also called living fossil. The plant has survived due to interest shown by horticulturists. It has natural immunity to several plant diseases.

Main plant body is gametophytic in bryophytes and sporophytic in pteridophytes.

• Pteridophyta includes primitive vascular plants. The adult plant body in these plants is a sporophyte. It shows differentiation into true roots, stem, and leaves. The stem is mostly herbaceous.
• The life cycle includes an alternation of generation of a diploid prophetic generation with a haploid gametophytic generation. Sporophyte generation is dominant.

In pteridoghytes, spore is a haploid structure, which develops after meiosis of spore mother ceil ."On germination it gives rise to a green haploid grothallus (gametophyte) which is monoecious i.e. has both antheridia (male sex organs) and archegonia (female sex organs).

Heterospory is the production of spores of two different sizes and of two different developmental patterns - smaller male or microspores and larger female or megaspores. Heterospory is the most important evolutionary development in the vascular plants because it has ultimately led to seed development, e.g., Selaginella, Salvinia, Azolla, etc.

The spreading of living pteridophytes is limited and is restricted to narrow geographical region because its gametophytes require cool, damp, shady places to grow and also it requires water for fertilization.

In Equisetum plant body is differentiated into stem, leaves and roots. Adventitious roots develop in whorls over the nodes of rhizome. Aerial branches develop terminal strobili. In E. arvense, there are two types of aerial branches, green sterile and brown fertile. Terminal strobili occur only on fertile branches.

Salvinia is an aquatic fern with both annual (e.g., S. nutans) and perennial species (e.g., S. motesta). The plant body consists of a floating stem bearing two rows of large green hairy leaves on the upper surface and highly branched leaf roots on the lower surface.

Genera like Selaginella and Salvinia which produce two kinds of spores, are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is the precursor to the seed habit which is considered as an important step in evolution.

In pteridophytes, dominant free-living generation is diploid sporophytic generation. Sporophyte is differentiated into true roots, stem and leaves. Gametophyte develops from spores and is a short-lived generation.