Mostly in many-celled animals (metazoans), diploid parents produce diploid offspring. The haploid state is confined to the gametes (eggs and sperm) which are produced through meiosis. After fertilization, a diploid zygote develops into a diploid adult. Plants however display some very interesting departures from the common situation in animals.
Other than algae, many-celled plants display some interesting variations on an alternation of generations theme. The least complicated land plants, those which display fewer specializations for survival on the land, are the mosses, liverworts and hornworts. The plant which you observe in the above groups, is haploid. It produces gametes in special sexual organs. The plant is typically dependent on rainwater to allow sperm to swim to the female plant which harbours an egg. After fertilization, a small diploid structure (sporophyte) develops growing on the haploid gametophyte parent plant. The diploid sporophyte produces haploid spores (after meiosis) and the spores develop on the soil into a dominant haploid plant again. This lifestyle is haploid dominant (gametophyte). [Definitions: “phyte” means plant and “gameto” refers to the plant producing gametes.]
The haploid dominant plants do not exhibit any conducting tissue, or roots, or real leaves and other features which help survival on the land. Not surprisingly these haploid plants survive best growing close to the ground in very moist habitats. There are no intermediate designs between these plants and the tracheophytes (plants with conductive tissue).
The tracheophytes display a huge amount of diversity. All of the land plants except for the mosses, liverworts and hornworts, are tracheophytes. The interesting thing is that the tracheophytes are diploid dominant (sporophyte). In this case, the gametophyte is inconspicuous or dependent on the sporophyte. [Definition:” sporo” spore producing]
There are many groups of land plants which display an obvious alternation of generations. Consider the ferns for example. The large graceful plants with which we are familiar, are the diploid dominant generation. On the lower side of the fronds (leaf) we find clusters of sporangia which produce spores. The spores are haploid, having been produced by meiosis. The delicate spores fall to the soil and germinate into a tiny heart shaped haploid plant lying on the ground. On the lower side of the plant, archegonia develop, each harbouring an egg, and antheridia which produce lots of tiny sperm cells. In rainwater the sperm swim to an archegonium. The fertilized egg develops in situ into the large plant (sporophyte) that we see and the tiny gametophyte soon disintegrates. (In the land plants, the retention of the zygote or fertilized egg within the archegonium and gametophyte parent plant is permanent until the parent plant disintegrates.) [Definitions: archegonium – a structure of only a few cells which produces an egg in its interior, and artheridium – a very small structure which produces many sperm in its interior]
In club mosses the gametophyte is tiny and partly or completely buried in soil when the spore germinates, possibly several years later. Other plant groups which show similar life style patterns to the ferns are the whisk ferns (Psilotum), club mosses (Lycopodiales), and horsetails (Sphenophyta). These typically produce spores which are all the same size (homospores) and which germinate into gametophytes with reproductive structures of both sexes.
Not all sporangia produce homosporous spores. There are some groups which exhibit differentiation of spore sizes (heterospores): some larger spores (megaspores) and some smaller spores (microspores). The development of separate male and female gametophytes is the inevitable result of heterospory, declared H. C. Bold in his 1972 text Morphology of Plants. Many specialists consider this development to be a major step toward the seed plants. However, the pattern of heterospory is spotty at best and not easy to interpret in evolutionary terms.
There are some large spores that germinate so quickly that the developing female plant (gametophyte) is still inside the partially split open wall of the megaspore. The archegonia may even project from the captive gametophyte plant inside the spore wall as it lands on the ground. This is observed in some taxonomically unpromising groups for evolution like Selaginella (grouped with club mosses) and the tree forming lycopods such as Lepidodendron, all of which are considered “dead ends” for evolution. Nevertheless, many experts interpret the seed as a megasporangium containing a megaspore containing a megagametophyte which produces several eggs. We could imagine the situation like a series of Russian stacking dolls in which the megaspore is the largest doll and each next stage is a smaller doll inside the previous one. The resulting seed inside them all, contains a fertilized egg which begins to develop into the embryonic stage of the diploid adult plant. This is just an interpretation or metaphor. At the creation, God created seed plants and flowering seed plants at the same time as all the other plants. The one group did not develop from the other.
The problem for this theory of evolutionary advancement from homospores to heterospores to fancier heterospores, is that the taxonomic distribution of plants exhibiting heterospory is more patchwork than a linear upward trend would lead one to expect. Bateman and DiMichelle, in an article on this very topic, declared that the appearance of heterospory among vascular plant taxa was “highly iterative” [i.e. it appeared again and again]. Thus, they declared that “current evidence suggests a minimum of eleven origins of heterospory.” In addition “Heterospory reflects the convergent attainment of similar modes of reproduction in phylogenetically disparate lineages.” “No origin of heterospory coincides with the origin of (and thus delimits) any taxonomic class of tracheophytes [vascular plants].” [Richard M. Bateman and William A. DiMichelle. 1994. Heterospory: the most iterative key innovation in the evolutionary history of the plant kingdom. Rev. Camb. Philos. Soc. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1994.tb01276.x ] We look in vain for evolutionary trends based on heterospory.
Other important non-seed plants are the woody “progymnosperms”. These fossil plants are called progymnosperms because they are considered to be somehow related to the seed plants such as the gymnosperms including the cycads, Ginkgo and the conifers. The ability to produce wood is a major new design feature in these plants involving several innovations requiring lots of new information. For a start these plants exhibit a vascular cambium (like a meristem only around the circumference of the stem.) The cambium can divide indefinitely and new cells produced on its outer circumference develop into phloem, while new cells on the inner circumference develop into wood. Both the phloem and xylem (wood) consist of more than one type of cell. Their vertical orientation is very important and the xylem cells are programmed to die at maturity so that they can conduct water. [A meristem is a group of unspecialized cells which can continue to divide indefinitely.] How Wood is Made
Among the “progymnosperms” the Aneurophytales are homosporous and the Archaeopteridales are heterosporous. It is their possession of wood which encourages biologists to connect them with the appearance of seed plants, specifically the gymnosperms, which are all woody plants. However, a close examination of land plants reveals a wild array of theories on what the evolutionary relationships might be. It is much less discouraging to view these groups as separate designs.