Pteridophytes are found in cool, damp and shady places. Some of them may also proliferate in sandy soil. The main plant body is a sporophyte. It is differentiated into true root, stem and leaves. Well-differentiated vascular tissues are present.
There are two types of leaves. Small leaves are called microphylls and they are found in some pteridophytes, e.g. Selaginella. Large leaves are called megaphylls and they are found in some others, e.g. ferns. A leaf with a single vein is called microphyll, while a leaf with more than one vein is called megaphyll.
Sporophyte bears sporangia which are subtended by leaf-like appendages called sporophylls. In some cases, the sporophylls may form distinct compact structures called strobili or cones, e.g. Selaginella, Equisetum.
Spores are produced by mother cells in sporangia, through meiosis. The spores germinate and produce thalloid gametophyte called prothallus. The prothallus is inconspicuous, small but multicellular. The prothallus is free-living and usually photosynthetic.
The gametophytes need cool, damp and shady places to grow. Due to this specific requirement, pteridophytes are spread to limited areas to narrow geographical regions.
The gametophyte bears antheridia and archegonia. Transfer of antheroizoids is facilitated by water. The zygote produces a multicellular well-differentiated sporophyte.
Most of the pteridophytes are homosporous, i.e. they produce spores of similar type. Some pteriophytes are heterosporous, e.g. Selaginella and Salvinia. In heterospory, the megaspore germinates to produce female gametophyte, while the microspore germinates to produce the male gametophyte. This event is a precursor of the seed habit in higher plants.
The pteridophytes are divided into four classes, viz. Psilopsida, Lycopsida, Sphenopsida and Pteropsida.
Alternation of Generation in Pteridophytes
Pteridophytes show haplo-diplontic type of alternation of generation. Both sporophyte and gametophyte phases are independent living. Sporophyte represents the diploid phase, while gametophyte represents haploid phase.
Characteristics of Gymnosperms
The ovules in gymnosperms are not enclosed by an ovary wall. The seeds too are not covered, i.e. naked. Gymnosperms include medium-sized or tall trees and shrubs.
These plants usually have tap roots. Roots of some plants have fungal association in the form of mycorrhiza, e.g. pinus. In some other plants, small specialized roots; called coralloid roots; are associated with nitrogen-fixing cyanobacteria, e.g. cycas.
The stems are unbranched (Cycas) or branched (Pinus, cedrus). Leaves can be simple or compound. The pinnate leaves in cycas persist for a few years. The leaves are well adapted to withstand extremes of temperature, humidity and wind. Needle-like leaves of conifer reduce surface area and thus reduce transpiration. Thick cuticle and sunken stomata also help in preventing water loss.
Reproduction In Gymnosperms
Gymnosperms are heterosporous. Haploid microspores and megaspores are produced. Spores are produced within sporangia. Sporangia are borne on sporophylls which are arranged spirally along an axis. The Sporophylls form lax or compact strobili or cones.
The male strobili are called microsporangiate. Microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This small-sized gametophyte is called pollen grain.
The female strobili are called megasporangiate. The male or female cones can be found on the same tree (pinus) or on different trees (cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. Nucellus is protected by envelopes and the composite structure is called an ovule. Meiotic division in the megaspore mother cell produces four megaspores. One of the megaspores develops into a multicellular female gametophyte. The female gametophyte bears two or more archegonia or female sex organs. The female gametophyte is retained within the megasporangium.
The pollen grains are released from the microsporangium and carried by air currents. They come in contact with the opening of the ovules on the female cone. A pollen tube develops in the pollen grain. The male gamete travels through the pollen tube to reach near the mouth of archegonia. After fertilization, zygote develops into an embryo. The ovule develops into seed.
Alternation of Generation in Gymnosperm
Gymnosperms show diplontic type of alternation of generation. Sporophyte phase is dominant. Gametrophyte phase is unicellular and grows on sporophyte.