(a) Write a detailed account of homospory, anisospory, incipient heterospory and heterospory. Elaborate on the evolution from…

(a) Homospory, Anisospory, Incipient Heterospory, Heterospory and the Geological Sequence

Homospory. One morphological class of spore, from one class of sporangium. The gametophyte is typically exosporic and potentially bisexual (antheridia and archegonia on the same thallus). Ancestral in embryophytes. Living examples: most ferns (Pteris, Dryopteris), Lycopodium, Equisetum.

Anisospory. Spores of two sizes are produced, often within the same sporangium or from otherwise similar sporangia, but they are not yet committed as unisexual microspores and megaspores. Size dimorphism precedes sex dimorphism. The terms microspore and megaspore are properly reserved for true heterospory.

Incipient heterospory. Size difference is regular and the two spore classes begin to yield male-biased and female-biased gametophytes, but the separation is incomplete (a “female” gametophyte may still form antheridia if fertilisation fails). Platyzoma (a northern Australian fern) is the textbook living intermediate: smaller spores → filamentous male thalli; larger spores → spatulate thalli that start female and may later add antheridia.

Heterospory. Two spore classes produced in two sporangium classes: microspores in microsporangia germinate into male gametophytes; megaspores in megasporangia germinate into female gametophytes. Gametophytes are endosporic and reduced. Living examples: Selaginella, Isoetes, Marsilea, Salvinia, and all seed plants.

Geological time. Homospory is Silurian–early Devonian (early land plants). Heterospory appears independently in several lineages in the Middle–Late Devonian (progymnosperms, some lycopsids such as Barinophyton showing anisospory inside one sporangium). By the Carboniferous, arborescent lycopsids (Lepidodendron, Sigillaria) are fully heterosporous and some sphenopsids (e.g. Calamostachys casheana) are heterosporous, and monomegaspory plus retention of the megaspore in the megasporangium is the step toward the seed. Adaptive gain: endosporic, nutritionally provisioned female gametophyte; outcrossing; later, the ovule.

(b) Helobial Endosperm, its Variations, and Karyokinesis versus Cytokinesis

Endosperm in most angiosperms is triploid (2 polar nuclei + 1 male gamete). Exceptions exist (diploid in Oenothera-type sacs; pentaploid in Fritillaria-type). Helobial endosperm is a third developmental pathway, intermediate between nuclear and cellular, typical of many Helobiae / Alismatales and some Liliales. It is the primary endosperm nucleus (PEN), not the zygote, that divides first.

Sequence. The PEN moves toward the chalaza and divides. A wall is laid down, cutting a small chalazal chamber from a large micropylar chamber. The micropylar nucleus then undergoes free-nuclear divisions; the chalazal nucleus divides little or not at all and often becomes haustorial. Walls later appear in the micropylar chamber. Examples: Asphodelus, Eremurus, Hydrilla, Vallisneria. Sagittaria and rice, by contrast, have nuclear endosperm.

Variations. The chalazal chamber may stay uninucleate and ephemeral, or become a coenocytic haustorium. Timing of micropylar cellularisation differs among families. The pattern is almost confined to monocots.

Karyokinesis vs cytokinesis

• Nuclear: many rounds of karyokinesis with no cytokinesis → a coenocyte; walls form late, often centripetally (Cocos, Triticum, Helianthus, Sagittaria).
• Cellular: every karyokinesis is followed at once by cytokinesis; no free-nuclear phase (Impatiens, Adoxa, Datura, Petunia).
• Helobial: first mitosis is followed by a wall (cellular start); subsequent mitoses in the micropylar chamber are free-nuclear (nuclear middle); cellularisation last. Hence intermediate.

(c) Normal Embryo Sac and Structural Complexity of the Angiosperm Embryo

The normal (Polygonum-type) embryo sac is the starting point from which the embryo and endosperm develop.

Sac functions. Synergids: filiform apparatus attracts and receives the pollen tube; one synergid degenerates at discharge. Egg: fuses with one sperm → zygote (2n). Central cell: two polars fuse with the second sperm → primary endosperm nucleus. Antipodals: often ephemeral, sometimes haustorial.

Embryo. The zygote (not the PEN) divides, usually transversely, into a terminal (apical) cell and a basal cell (Arabidopsis and most dicots; first wall is not typically oblique). The basal cell builds the suspensor (and often the hypophysis that contributes to the root cap / radicle pole). The apical cell builds the proembryo → globular → heart (two cotyledon primordia in dicots) → torpedo, with a shoot apical meristem between the cotyledons and a radicle at the opposite pole. Monocot embryos form a single cotyledon and a lateral plumule. Compared with a free-living pteridophyte embryo, the angiosperm embryo is retained, nourished by endosperm, and patterned into embryonic organs before seed dormancy. That is its structural complexity: polar axis, meristems, cotyledon(s), suspensor, and dependence on a second fertilisation product.