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
, are isogamous
, meaning their gamete
s are equal in size. Most plants, though,
: their gametes come in two flavors that differ in size, motility
, and in
amount of metabolic
resources (stored sugar
s and protein
s) associated with
the gamete. In such anisogamous plants, which include fern
and others, the larger, female gamete is
non-motile or minimally motile and usually incorporates more
Given that a species
of plant reproduces sexually and has gametes that are differentiated
into male and female, how is gender
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
- 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
- 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
s, but rather are spore
s 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
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.