Detailed Notes -3 -Chapter3-Plant Kingdom

3.3 PTERIDOPHYTES

1. Description:

   • Pteridophytes include horsetails and ferns, and they are utilized for medicinal purposes, soil-binding, and ornamental gardening.
   • Evolutionarily, they are the first terrestrial plants to possess vascular tissues, namely xylem and phloem.

2. Habitat:

   • Pteridophytes are commonly found in cool, damp, and shady places, although some species may thrive in sandy-soil conditions.

3. Life Cycle:

   • In contrast to bryophytes, where the dominant phase is the gametophyte, the main plant body in pteridophytes is the sporophyte.
   • The sporophyte is differentiated into true roots, stems, and leaves, each possessing well-differentiated vascular tissues.
   • Leaves in pteridophytes may be small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
   • Sporangia, which produce spores by meiosis, are borne on leaf-like appendages called sporophylls. In some cases, sporophylls may form compact structures called strobili or cones.
   • Spores germinate to produce inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothalli.

4. Reproduction:

   • Gametophytes bear male and female sex organs called antheridia and archegonia, respectively.
   • Water is necessary for the transfer of male gametes (antherozoids) to the archegonium for fertilization, resulting in the formation of a zygote.
   • The zygote develops into a multicellular, well-differentiated sporophyte, which is the dominant phase of the pteridophytes.

5. Spore Types:

   • Most pteridophytes are homosporous, producing spores of similar kinds.
   • However, genera like Selaginella and Salvinia are heterosporous, producing two kinds of spores: macrospores (large) and microspores (small).

6. Classification:

   • Pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium); Sphenopsida (Equisetum); and Pteropsida (Dryopteris, Pteris, Adiantum).

3.4 GYMNOSPERMS

1. Definition:

   • Gymnosperms are plants in which the ovules are not enclosed by any ovary wall, remaining exposed both before and after fertilization. Hence, the seeds are naked.

2. Description:

   • Gymnosperms include medium-sized trees or tall trees and shrubs. For instance, the giant redwood tree Sequoia is one of the tallest tree species.
   • Their roots are generally taproots, and some genera have fungal associations with mycorrhiza or coralloid roots associated with N2-fixing cyanobacteria.
   • Stems may be unbranched (as in Cycas) or branched (as in Pinus and Cedrus).
   • Leaves may be simple or compound, with adaptations to withstand extremes of temperature, humidity, and wind. For example, conifers have needle-like leaves with a thick cuticle and sunken stomata to reduce water loss.

3. Reproduction:

   • Gymnosperms are heterosporous, producing haploid microspores and megaspores.
   • Microspores develop into pollen grains, which are highly reduced male gametophytes confined to a limited number of cells.
   • Macrospores develop into multicellular female gametophytes within the ovules, each bearing two or more archegonia or female sex organs.
   • Unlike bryophytes and pteridophytes, the male and female gametophytes in gymnosperms do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.

4. Pollination and Fertilization:

   • Pollen grains are released from the microsporangium and carried by air currents to the opening of ovules borne on megasporophylls.
   • The pollen tube carrying the male gametes grows towards the archegonia in the ovules and discharges its contents near the mouth of the archegonia.
   • Following fertilization, the zygote develops into an embryo, and the ovules develop into seeds, which remain uncovered.

3.5 ANGIOSPERMS

1. Definition:

   • Angiosperms, also known as flowering plants, are characterized by the development of specialized structures called flowers, within which pollen grains and ovules are produced.

2. Seed Enclosure:

   • Unlike gymnosperms where ovules are naked, seeds in angiosperms are enclosed within fruits, which develop from the fertilized ovary of the flower.

3. Habitat and Diversity:

   • Angiosperms are an exceptionally large group of plants found in a wide range of habitats.
   • They vary greatly in size, ranging from tiny plants like Wolffia to towering trees like Eucalyptus, which can grow over 100 meters tall.

4. Importance:

   • Angiosperms are of immense economic and ecological importance, providing food, fodder, fuel, medicines, and numerous other commercially significant products.

5. Classification:

   • Angiosperms are divided into two main classes: dicotyledons (dicots) and monocotyledons (monocots).
   • Dicots typically have seeds with two cotyledons, netted venation in leaves, and flower parts in multiples of four or five.
   • Monocots usually have seeds with a single cotyledon, parallel venation in leaves, and flower parts in multiples of three.