Each ovule is actually the female spore-producing organ
(megasporangium, called nucellus) surrounded by a protective
envelope called integument. Inside the ovule, a cell undergoes
meiosis to produce four megaspores (female spores). Three
of them degenerate; the remaining megaspore is retained
inside the megasporangium and gives rise to a female gametophyte
(megagametophyte) inside the spore. The gametophyte produces
female sex organs (archegonia), each of which contains
an egg.
|
|
 |
| a gymnosperm ovule |
|
young pine cone |
Because the ovules are naked, pollen grains can enter
the ovules through an opening (micropyle) on the integument.
Each pollen grain germinates inside the ovule into a pollen
tube, which delivers and releases two male nuclei or two
flagellate sperms. One of the male nuclei or sperms fertilizes
an egg in a female sex organ, resulting in the formation
of a zygote. The zygote develops into an embryo, whereas
the ovule matures into a seed and the integument serves
as the seed coat. A seed in gymnosperms, therefore, is
a package composed of the diploid sporangium tissue from
the mother plant, the haploid female gametophyte tissues
serving as food reserves in the seed, and a diploid embryo
representing the next sporophyte generation.
Because the seeds are not enclosed in an ovary or fruit,
gymnosperms are generally regarded as bearing 'naked'
seeds. The 'naked' ovules do not mean that they do not
have any protection. In fact, the ovules are generally
borne on stalks or flattened structures (megasporophylls)
that cluster together to form cones. The cones do offer
some protection but not exclusion of pollen grains from
direct contact with the ovules.
|
