Gymnosperms

The gymnosperms are seed producing land plants. However, the seeds are not enclosed in fruits. Most of them have now become extinct and only about 900 species are known to be surviving. The living gymnosperms are widely distributed in the cold climates where snow, rather than rain, is the source of water. Only one group called cycads thrive in warmer regions.

The adult plant body is a sporophyte. It is represented by a perennial, evergreen, woody plant. Most of them are trees and some are shrubs. There are no herbs.

Gymnosperms include one of the world's tallese trees - Sequoia sempervirens (the Giant Red Wood Tree) measuring about 125 meters in height and 30 meters in girth. A 4000 year old red wood tree is still found in California!

The plants possess well-developed tap root system. In some cases the roots are symbiotically associated with algae (coralloid roots of Cycas) or with fungi (mycorrhizal roots of Pinus). The stem is erect and woody, may be branched or unbranched. Gymnosperms are heterosporous, producing two kinds of spores microspores and megaspores produced by separate sporangia. The two types of sporangia - microsporangia and megasporangia are found on special leaf like structures called sporophylls. The microsporangia represent the pollen sacs and the megasporangia represent the ovules. Microsporangia occur on microsporophylls (stamens) and megasporangia occur on megasporophylls (carpels).

The sporophylls are usually aggregated to form structures called cones or strobili. Male cones (with microsporophylls) and female cones (with megasporangia) occur on separate plants (unisexual).

The microsporangium (pollen sac) produces numerous pollen grains which are anemophilous. The megasporangium (ovule) remains exposed on the megasporaphyll. The pollen grain which represents the male gametophyte comes in contact with the ovule, develops a pollen tube and brings about pollination. The generative cell present in the pollen tube produces two male gametes. One of them unites with the egg cell in the ovule to form a zygote.

The diploid zygote develops into an embryo which remains enclosed within the ovule. The ovule then transforms itself into a seed. The seed remains attached to the megasporophyll and is not enclosed by a fruit. The ripe and dry seeds are shed. Under suitable conditions the seeds germinate and develop into new plants.

fig. 8.32 - Life Cycle of Pinus (A Gymnosperm)

Gymnosperms are grouped into seven divisions of which four divisions include examples which have become extinct. Among the existing forms the most familiar ones are the Cycads. (E.g., Cycas) and the Conifers (E.g., Pinus). Some examaples like Gnetum and Ephedra show certain characteristics close to angiosperms.

Pinus as well as other conifers are every green trees. They have monopodial growth represented by one main axis. As the axis grows taller it increases in diameter due to secondary growth. Stem branches are of two types:

1. Long shoots or branches of unlimited growth.

2. Dwarf shoots or branches of limited growth.

fig. 8.33 - Examples of Gymnosperms