The many and sundry systems for sexual reproduction exhibited by the plant kingdom lend new meaning to the term "variations." A few plants, such as certain kinds of algae, are isogamous, meaning their gametes are equal in size. Most plants, though, anisogamous: their gametes come in two flavors that differ in size, motility, and in amount of metabolic resources (stored sugars, fats and proteins) associated with the gamete. In such anisogamous plants, which include ferns, mosses, conifers, flowering plants and others, the larger, female gamete is non-motile or minimally motile and usually incorporates more resources.

Gender Expression

Given that a species of plant reproduces sexually and has gametes that are differentiated into male and female, how is gender distributed among individuals of that species?
  • Some species are hermaphrodites. A single plant of one of these species produces both male and female gametes. This can happen several ways:
    • All flowers or sex organs produce both kinds of gametes, either at the same time, or separated in time. This is one form of sexual monomorphism (a term which also applies to, e.g., species of animals in which the males and females can't be told apart by outward appearance).
    • Monoecious plants have two different kinds of flowers or sex organs; each is unisexually male or female.
  • Other species of plants are dioecious. A single individual of such a species is either male or female. There are even some variations of this:
    • In gynodioecy, most individuals are hermaphroditic, but some produce only female gametes.
    • In androdioecy (which is very rare), most individuals are hermaphroditic but some produce only male gametes.

Mating in Plants

Mating in plants involves the fusion of gametes into a zygote. This is one step in the plant reproductive life cycle, which is more complicated than that of animals in that it involves alternation of generations -- the products of meiosis are not themselves gametes, but rather are spores that develop into haploid plants (the gametophyte, or gamete-producing generation). The gametophyte then produce and release gametes. When these fuse to form a zygote, the zygote develops into a (usually) diploid plant that produces spores (the sporophyte generation), and so on.

In most lower plants (ferns, mosses, algae) about all there is to mating is the release of gametes into the environment; these are actively motile, seeking out other gametes to fuse with. They fuse, and a zygote is formed.

In seed plants (conifers and flowering plants) it's more complicated. The male gametophyte is very reduced in size and is not released at the spore stage -- it remains dependent on its sporophyte parent until it develops into a mature gametophyte and is released. Another name for the male gametophyte of seed plants is " pollen grain". The female gametophyte is also reduced in size and remains inside the ovule, parasitic upon its parent throughout its existence.

Mating in the seed plants involves new steps -- pollination and fertilization. Pollen is released into the environment, where it is transported by wind or by animals until it lands on the stigma of a compatible mate. (Hermaphroditic flowers can also pollinate themselves, but see self-incompatibility.) The pollen grain germinates, and grows a tube down through the tissue of the stigma to reach the ovule where the female gametophyte resides. The process of pollen tube growth can be quick (a few hours, in some flowering plants) or lengthy (it can take 15 months in pine trees!). The pollen grain contains two sperm cells (gametes). The female gametophyte includes one egg cell (also a gamete) and several other cells. The two sperm cells are discharged into the female gametophyte, and one fertilizes the egg cell to produce a zygote. (It's a little more complicated in flowering plants (they do double fertilization) than in conifers, but I will leave that for another story.)

Once fertilization has occurred, the zygote develops into an embryo as the whole ovule goes on to develop into a seed. A mature seed of pine includes the dipoid tissue of the embryo, the haploid tissue of the female gametophyte which serves a nutritive role, and the diploid tissue of the parent derived from the ovule, which forms the seed coat.

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