(a) Describe the male and female gametophytes of Pinus. How are the processes of pollination and fertilization accomplished in Pinus? 15+5=20

(b) Illustrate the plant body of Psilotum. Add a note on the primitive characters of Psilotum which indicat

Question

(a) Describe the male and female gametophytes of Pinus. How are the processes of pollination and fertilization accomplished in Pinus? 15+5=20

(b) Illustrate the plant body of Psilotum. Add a note on the primitive characters of Psilotum which indicate its close relation to extinct group Psilotales. 10+5=15

(c) What are myxomycetes? Outline the life cycle of a typical myxomycete. Mention its protozoa-like and fungus-like characters. 5+5+5=15

UPSC Answer Check · Accepted Answer

Part (a): Gametophytes, Pollination, and Fertilization in Pinus

The male gametophyte of Pinus is highly reduced, existing as a pollen grain. It is a 2- to 3-celled structure comprising a large prothallial (tube) cell, a generative cell, and often a small haustorial cell. The prothallial cell facilitates pollen tube growth, while the generative cell divides mitotically to form two sperm cells. The female gametophyte develops within the nucellus of the ovule. It begins as a megaspore mother cell that undergoes meiosis to produce four megaspores; three degenerate, and the functional one undergoes free-nuclear divisions to form a coenocytic prothallus. At the micropylar end, the prothallus differentiates into archegonia, each containing a ventral canal cell and a single egg cell at the apex.

Pollination is anemophilous. Winged pollen grains are carried by wind to the receptive cone. Upon landing near the micropyle, the pollen grain absorbs fluid from the nucellus, causing the prothallial cell to elongate into a pollen tube. The pollen tube grows through the nucellus, guided by the archegonial chamber, eventually reaching the archegonium. Fertilization is siphonogamous. The pollen tube discharges its contents into the archegonial chamber. The generative cell divides to release two sperm cells. One sperm fuses with the egg to form the zygote, while the other degenerates. The zygote develops into the embryo, and the nucellus becomes the seed coat.

Part (b): Plant Body and Primitive Characters of Psilotum

Psilotum is a small, herbaceous, dichotomously branched plant. The aerial stem is green and photosynthetic, bearing small, scale-like enations (microphylls) that lack veins. At the tips of branches, synangia (fused sporangia) are produced. Underground, the plant possesses a rhizome with rhizoids for anchorage and absorption, but lacks true roots. The vascular tissue is organized as a protostele, specifically an actinostele, where xylem and phloem are arranged in a star-like pattern around a central pith.

Psilotum exhibits several primitive characters indicating its close relation to extinct Psilotales like Rhynia and Horneophyton. It is homosporous, producing only one type of spore. The absence of true roots and leaves, replaced by rhizoids and enations, is a plesiomorphic trait. The synangial structure, where sporangia are fused into a single unit, is a key feature shared with early Devonian fossils found in the Rhynie chert. The protostelic vascular organization further supports its position as a basal lineage in vascular plant evolution.

Part (c): Myxomycetes: Life Cycle and Dual Nature

Myxomycetes, or plasmodial slime molds (Class Myxogastria, Phylum Mycetozoa), are eukaryotic organisms that exhibit characteristics of both protozoa and fungi. Their life cycle begins with spore germination, releasing motile swarm cells or myxamoebae. These cells aggregate to form a coenocytic, multinucleate plasmodium, which is the vegetative stage. Under unfavorable conditions, the plasmodium migrates to a suitable site and differentiates into fruiting bodies called sporangia. Within the sporangium, the plasmodium differentiates into a spore mass, which is released upon maturity.

Myxomycetes display protozoa-like characters in their vegetative phase. The plasmodium moves by cytoplasmic streaming, and the swarm cells are ciliated and amoeboid. Nutrition is phagotrophic (holozoic), as they engulf bacteria and other organic particles. They lack a cell wall during the plasmodial stage. Conversely, they exhibit fungus-like characters in their reproductive phase. They produce spores within sporangia, which have cellulosic walls. The sporangia often possess stalks, and the spores are released passively, resembling fungal spore dispersal. This dual nature places them in a transitional group between protozoa and fungi.

In conclusion, Pinus demonstrates advanced siphonogamous reproduction, Psilotum retains primitive vascular traits linking it to early land plants, and Myxomycetes bridge the gap between protozoan and fungal biology, highlighting evolutionary transitions in eukaryotic life.

(a) Describe the male and female gametophytes of Pinus. How are the processes of pollination and fertilization accomplished in…

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