Evolution of Seed Habit Part 2
Gymnosperms
are one of the successful groups of seed plants of worldwide
distribution. They constitute about one-third of the world’s forests.
The gymnosperms are heterosporous plants which produce seeds but no
fruits. The term gymnospermae literally means ‘naked seeded’
(Gymno=naked, spermae= seed). The ovules in these plants are usually
borne on the exposed surfaces of fertile leaves (megasporophylls). These
ovules, unlike those of angiosperms are not enclosed but lie naked on
the surface of fertile leaves.
Like
filicinae, they show regular heteromorphic alternation of generations.
They have independent, dominant sporophyte but less conspicuous,
dependent gametophyte. The female gametophyte is permanently retained
within the ovule. The two kinds of spores are microspores and megaspores
which develop on microsporophylls and megasporophylls respectively. The
megasporophylls bearing ovules are not folded and joined at the margins
to form an ovary. For this reason the seeds lie naked on the mega
sporophylls, (Fig. 9.20a).
The important genera are Cycas (sago-palm) (Figs. 9.20-a), Pinus (pine), taxus (Yew), picea (hemlock) and Cedrus (deodar) Ginkgo (Fig. 9.20=b) etc.
Pinus- Life Cycle
The
pine is a conifer. The main plant body is saprophyte which produces
spores after reduction division of spore mother cell in sporangia.
Conifers are heterosporous. Microspores and megaspores are produced in
microsporangia and megasporangia respectively. Sporangia (i.e, micro and
megasporangia) are produced on respective cones (male cones and female
cones) on the same plant.
The male cones are small in size and are produced in clusters on an
axis. Each male cone consists of microsporophylls which contain
microsporangia. Microspore germinates to form a small inconspicuous male
gametophyte (also called as microgametophyte) within the spore wall.
Such a microspore of seed plants that contains the microgametophyte
including the gametes is called a pollen grain (plural = pollen).
Pollen are produced in great numbers and are transported by wind. Pollen grain in pinus
has two wings attached to its lateral sides. Due to wings, pollen can
float in air for a longer period of time and can travel long distances.
The gymnosperms have successfully evolved this totally new mechanism of
transfer of male gamete to the female gametophyte through wind which has
made them independent of water for this purpose. This is an important
improvement and evolutionary adaptation to survive in the harsh dry
terrestrial (land) environment.
During pollination the pollen land directly on the ovules. Only few
pollen are able to germinate to form pollen tubes through which male
gametes are transferred to the embryo sac for fertilization.
More then one egg can be fertilized to form several zygotes, but one
zygote usually survives to form a single embryo. After fertilization the
ovule becomes the seed. The seeds now contains an embryo along with
some stored food material. The seed upon germination gives rise to a new
sporophyte plant.
In the life cycle of piuns, the dominant diploid sporphyte generation alternates with inconspicuous haploid gametophyte generation (Fgi. 9.21)
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