Page 24 - 11-Bio-9 Kingdom plantae

9. Kingdom Plantae eLearn.Punjab

Pinus- Life Cycle

The Pine is a conifer. The main plant body is sporophyte 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 ean 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.

The female cones are large and conspicuous. Each female cone is composed of large number of
spirally arranged scales, the megasporophylls which are woody in texture. At the base of each
scale two ovules are present. An ovule is actually a megasporangium which is protected by an
integument. Each megasporangium has a single diploid megaspore mother cell. The megaspore
mother cell divides meiotically to produce four haploid megaspores. The functional megaspore (n)
undergoes mitosis to produce female gametophyte or an embryo sac. The embryo sac contains
one to several archegonia. The archegonia contain the female gamete or an egg.

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 sporophyte generation alternates with inconspicuous

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