Evolution of Seed Habit Part 2

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.

Fig. 9.20 (a) Cycas tree-habit and general organography

Fig. 9.20 (b) Ginkgo biloba

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)

Fig. 9.21 Life Cycle of pinus