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

**(a) Gametophytes, pollination and fertilisation in *Pinus***

Pinus (for example P. roxburghii, chir pine) is heterosporous and monoecious. Male cones bear microsporophylls with two microsporangia each. Female cones bear ovuliferous scales, each carrying two ovules on its upper surface.

Male gametophyte

• Microspore mother cells in the microsporangium divide by meiosis to form microspores (n). Each develops two air sacs formed by separation of the exine from the intine.
• Development begins inside the microspore wall. The first two divisions cut off two small prothallial cells, which soon degenerate. The next division gives a small generative (antheridial) cell and a large tube cell.
• Pollen is shed at this 4-celled stage.
• In the ovule, the generative cell divides into a stalk cell and a body cell, and the tube cell forms the pollen tube. Just before fertilisation, the body cell divides into two unequal, non-motile male gametes.
• The mature male gametophyte therefore has only a few cells. It is highly reduced and endosporic.

Female gametophyte

• The ovule is orthotropous, with one thick integument and a nucellus (the megasporangium). A megaspore mother cell in the nucellus divides by meiosis to give a linear tetrad. The lowest (chalazal) megaspore is functional and the other three degenerate.
• The functional megaspore enlarges, and its nucleus undergoes many free-nuclear divisions. Walls then form from the periphery inwards, producing a cellular female gametophyte (n). This tissue, often called "endosperm" in gymnosperms, is haploid and is formed before fertilisation.
• At the micropylar end, two to five archegonia develop. Each has a short neck of two tiers of four cells (eight cells), a ventral canal cell that soon degenerates, and a large egg.
• There are no antipodal or synergid cells, which are angiosperm features.

Pollination

• Pollination is by wind (anemophily), in spring.
• The ovule secretes a pollination drop at the micropyle, which traps pollen. As the drop dries it draws the pollen into the pollen chamber at the tip of the nucellus.
• The micropyle then closes and the pollen germinates. The tube grows slowly through the nucellus, which it feeds on, and development pauses over winter.

Fertilisation

• Fertilisation takes place about 12–14 months after pollination, when the archegonia are mature.
• The pollen tube reaches an archegonium, and its tip bursts, releasing the male gametes, tube nucleus and stalk cell into the egg cytoplasm.
• The larger male gamete fuses with the egg nucleus to form the zygote (2n). The other degenerates. This is siphonogamy with no double fertilisation.
• Several archegonia may be fertilised (simple polyembryony), and the zygote's proembryo splits into several embryos (cleavage polyembryony), but usually only one survives.
• The embryo is nourished by the female gametophyte, the integument becomes the seed coat, and the ovuliferous scale forms the seed wing.

**(b) Plant body of Psilotum; primitive characters**

Plant body

• Psilotum nudum (whisk fern) is a small perennial herb that grows on the ground or on tree trunks and rocks in the tropics and subtropics. In India it is found in the Western Ghats, the Eastern Himalaya and the North-East.
• Aerial shoots: green, photosynthetic and repeatedly dichotomously branched, with ridged stems.
• Enations: the stems bear small, scale-like enations without any vascular supply, spirally arranged. They are not true leaves.
• Synangia: in the axils of small forked (bifid) appendages on the upper branches it bears synangia, three fused eusporangia (trilocular). These produce many identical spores (homosporous).
• Underground parts: an underground, branched rhizome with rhizoids and mycorrhizal fungi. There are no true roots.

Anatomy:

• The aerial stem has an epidermis with stomata and a cortex with chlorenchyma and sclerenchyma.
• The stele is a protostele with star-shaped, exarch xylem (an actinostele), often with a sclerenchymatous pith higher up.
• There is no secondary growth.
• Gametophyte: subterranean, cylindrical, non-green and mycorrhizal. It bears both antheridia and archegonia, and its multiflagellate sperm need water.

Primitive characters (note: Psilotales is the living order, containing Psilotum and Tmesipteris; its resemblance is to the extinct Devonian Rhyniophyta/Psilophytales, such as Rhynia and Horneophyton)

• The sporophyte has no roots and no true leaves; it is a dichotomously branched axis with rhizoids.
• The enations are veinless, like those of *Asteroxylon*-grade plants.
• The protostele (actinostele) has exarch xylem and no cambium.
• Sporangia are thick-walled (eusporangiate), borne on the stem (cauline), and homosporous.
• Stems do the photosynthesis, and mycorrhizal association is very ancient, known from the Rhynie chert.
• The subterranean gametophyte sometimes contains vascular tissue, recalling the similar axes of the sporophyte.

Critical note: molecular phylogenies place Psilotales as sister to Ophioglossales within the ferns. Many of their "primitive" features, such as the absence of roots and leaves, are therefore probably due to secondary reduction rather than direct descent from rhyniophytes.

(c) Myxomycetes

(i) What they are. Myxomycetes (plasmodial or acellular slime moulds, class Myxogastria) were long classified with fungi. They are now placed in the protist supergroup Amoebozoa.

• They are heterotrophic organisms that feed as a wall-less, multinucleate plasmodium but reproduce by spores formed in sporangia.
• About 1,000 species live on decaying logs, leaf litter and dung, and help recycle nutrients.
• Examples are Physarum polycephalum, Stemonitis, Fuligo septica ("dog vomit" slime mould) and Lycogala.
• They differ from the cellular slime moulds (Dictyostelium), whose amoebae aggregate without fusing.

(ii) Life cycle

• Spores and swarm cells: haploid spores germinate in moist conditions into a naked myxamoeba, or into a biflagellate swarm cell in water. These can interconvert, feed on bacteria and multiply by fission. Under adverse conditions they form resting microcysts.
• Syngamy: two compatible cells fuse to form a diploid zygote.
• Plasmodium: the zygote nucleus divides repeatedly without cell division, forming a large, multinucleate, often brightly coloured plasmodium. It creeps by rhythmic shuttle streaming of the cytoplasm and engulfs bacteria, yeasts and spores. In dry or cold conditions it becomes a hard sclerotium.
• Fruiting: starvation and light trigger the plasmodium to move to an exposed site and form sporangia, which are often stalked. Each has a peridium, a network of threads (capillitium) and, in Physarum, lime granules. Spores are cleaved inside, with meiosis occurring in the young spores.
• Dispersal: mature spores have walls and are carried by wind.

(iii) Protozoa-like and fungus-like characters

Protozoa-like:

• Wall-less amoeboid and flagellate cells, and a naked plasmodium.
• Movement by cytoplasmic streaming and pseudopodia.
• Phagotrophic (holozoic) nutrition, engulfing food.
• A diploid vegetative phase, as in animals.
• Mitochondria with tubular cristae, as in amoebae.

Fungus-like:

• Spores with walls (often containing cellulose and some chitin) formed in sporangia with a peridium and capillitium, as in fungal fruiting bodies.
• Wind dispersal of spores.
• Growth on decaying organic matter.
• Resistant sclerotia.

These mixed features explain why they were called "fungus-animals" (Mycetozoa). Molecular data now show that they are amoebozoans, not related to true fungi, whose cell walls are chitinous and whose nutrition is absorptive.