Gymnosperms Angiosperms

3.5 Gymnosperms

The gymnosperms (gymnos: naked, sperma: seeds) are the plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation are not covered, i.e., are naked.

Gymnosperms include medium-sized trees or tall trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest tree species. The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with N₂-fixing cyanobacteria.

Plant Body

The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind. In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

Reproduction in Gymnosperms

The gymnosperms are heterosporous; they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones:

• The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
• The microspores develop into a male gametophyte (pollen grain) which is highly reduced and confined to only a limited number of cells.
• The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.

The male and female cones may be borne on the same tree (Pinus) or on different trees (Cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form female cones.

From Pollen to Seed

The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia. Note: The female gametophyte is retained within the megasporangium throughout its life (does not have a free-living stage).

The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards the archegonia in the female gametophyte and discharges its contents near the mouth of the archegonia. Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered (naked).

3.6 Angiosperms

Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers. In angiosperms, the seeds are enclosed by fruits.

The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from the smallest Wolffia (~0.1 cm) to tall trees of Eucalyptus (over 100 metres). They provide us with food, fodder, fuel, medicines and several other commercially important products.

They are divided into two classes: dicotyledons (dicots) and monocotyledons (monocots).

Reproduction in Angiosperms

The male sex organs in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers, following meiosis, produce pollen grains.

The female sex organs in a flower is the pistil or the carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed embryo sacs. The embryo-sac formation is preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil. This is termed as pollination. The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule.

Double Fertilization (UNIQUE to Angiosperms!)

The pollen tubes enter the embryo-sac where two male gametes are discharged. One of the male gametes fuses with the egg cell to form a zygote (this is called syngamy). The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is termed as double fertilization, an event unique to angiosperms.

After Fertilization

The zygote develops into an embryo (with one or two cotyledons). The PEN (primary endosperm nucleus) develops into the endosperm which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilisation. During these events, the ovules develop into seeds and the ovaries develop into fruits.

Gymnosperms vs Angiosperms

3.6.1 Dicots vs Monocots

The seeds produced by angiosperms after fertilization, possess cotyledons. Based on the number of cotyledons, two classes are recognised:

Alternation of Generations Across Plants

Worked Examples