1. The life cycle shows distinct heteromorphic alternation of generation. 2. Plant body of Sporophyte is dominant phase. 3. Sporophyte is differentiated into true root, stem and leaves. 4. Vascular tissue i.e xylem and phloem are present. Xylem lacks vessels but tracheids are present. In phloem sieve tubes and companion cells are absent. 5. Asexual reproduction takes place by spores. 6. Most pteridophytes are homosporous i.e they produce one type of spores. A few show heterospory i.e they produce two types of spores microspores and megaspores. 7. Spores are produced from spore mother cells after meiosis in multi-cellular sporangia. 8. Sporangia bearing leaves are called sporophylls. 9. Spores on germination develop into gametophyte which is haploid, multicellular, green and an independent structure. 10. The gametophyte develops multicellular sex organs. The male sex organ is called antheridium and the female sex organ is called archegonium 11. Sex organs have a sterile jacket. 12. Antherozoids are spirally coiled and multiflagellate. 13. Fertilization takes place inside archegonium. 14. Opening of sex organs and transfer of male gametes to archegonium for fertilization are dependent on water. 15. Fertilized egg i.e zygote develops into embryo.
Ferns represent a more specialized group of higher pteridophytes with larger leaves (megaphyllous). They are world wide in distribution and grow luxuriantly in forests, mountains, valleys etc. Some common examples of ferns are Nephrolepis, Ophioglossum, Osmunda, Pteris, Adiantum, Marsilea, Azolla, Salvinia etc.
There are fossil records of blue green algae (Cyanobacteria) living 3000 million years ago and many eukaryotic organisms have existed for more than 1000 million years. However the first organisms to colonize the land, primitive plants did so only 420 millions years ago. The greatest simple problem to overcome in making the transition from water to land is that of desiccation. Any plant not protected in some way, for example, by a waxy cuticle, will tend to dry out and die very soon.
Salient features of Bryophyta Bryophyta are the simplest group of land plants. They are relatively poorly adapted to life on land, so they are mainly confined to damp,shady places. These are terrestrial non-vascular plants(no vascular tissue namely xylem and phloem) which still require moist environment to complete their life-cycle. Hence these are called amphibians of plant kingdom. They are more advanced than algae in that they develop special organs. The male sex organ is called antheridium and the female sex organ is called archegonium. Bryophytes show distinct alternation of generation in their life cycles. Bryophytes include mosses, liverworts and hornworts.
Distinguishing features of Bryophytes 1. They are small terrestrial plants. 2. They are without a distinct root system but are attached to the substratum by means of thin, filamentous outgrowth of the thallus called rhizoids. 3. Water and mineral salts can be absorbed by the whole surface of the plant body, including the rhizoids. So the main function of rhizoids is anchorage, unlike true roots (true roots also possess vascular tissues, as do true stems and leaves). Thus the “stems” and “leaves” found in some Bryophytes are not homologous with stems and leaves of vascular plants. The plant body is called thallus. 4. They do not possess true vascular tissues. 5. Male sex organ is called antheridium and female sex organ is called archegonium. 6. Sex organs are multi-cellular and they have a protective jacket layer of sterile cells. 7. Sexual reproduction is of oogamous type. 8. Bryophytes show distinct alternation of gametophytic generation with sporophytic generation. 9. Gametophyte generation is dominant and independent. 10. Sporophyte generation is very small, microscopic and dependent on the gametophyte phase.
Alternation of Generations In common with all land plants and some advanced algae such as Laminaria, bryophytes exhibit alternation of generations. Two types of organism, a haploid gametophyte generation and a diploid sporophyte generation, alternate in the life cycle.
The haploid generation is called the gametophyte because it undergoes sexual reproduction to produce gametes. Production of gametes involves mitosis, so the gametes are also haploid. The gametes fuse to form a diploid zygote which grows into the next generation, the diploid sporophyte generation. It is called sporophyte because it undergoesasexual reproduction to produce spores. Production of spores involves meiosis, so that there is a return to the haploid condition. The haploid spores give rise to the gametophyte generation. One of the two generations is always more conspicuous and occupies a greater proportion of the life cycle. This generation is called as the dominant generation. In all Bryophytes the gametophyte generation is dominant. In all other land plants the sporophyte generation is dominant. It is customary to place the dominant generation in the top half of the life cycle diagram. One point that must be remembered here is that gamete production involves mitosis and not meiosis as in animals. Meiosis occurs before the production of spores.
In Algae the plant body shows no differentiation into root, stem or leaf or true tissues. Such a plant body is called thallus. They do not have vascular tissues. The sex organs of this group of kingdom plantae are not surrounded by a layer of sterile cells.
Algae are autotrophic organisms and they have chlorophyll. They are O2 producing photosynthetic organisms that have evolved in and have exploited an aquatic environment. The study of Algae is known as Algology or phycology.
Occurrence and Distribution Most of the algae are aquatic either fresh water or marine. Very few are terrestrial. A few genera grow even in extreme condition like thermal springs, glaciers and snow. The free floating and free swimming minute algae are known as phytoplanktons. Species that are found attached to the bottom of shallow water along the edges of seas and lakes are called Benthic. Some of the algae exhibit symbiotic association with the higher plants. Some species of algae and fungi are found in association with each other and they are called Lichens. A few species of algae are epiphytes (i.e they live on another plant or another alga) and some of them are lithophytes (i.e they grow attached to rocks)
Thallus organization The thalli of algae exhibit a great range of variation in structure and organization. It ranges from microscopic unicellular forms to giant seaweeds like Macrocystis which measures up to 100 meters long. Some of them form colonies, or filaments. The unicellular form may be motile as in Chlamydomonas or non-motile as in Chlorella. Most algae have filamentous thallus. eg. Spirogyra. The filaments may be branched. These filamentous form may be free floating or attached to a substratum. Attachment of the filament is usually effected through a simple modification of the basal cell into a holdfast. Some of the Algae are macroscopic. eg. Caulerpa, Sargassum, Laminaria, Fucus etc.where the plant body is large. In Macrocystis it is differentiated into root, stem and leaf like structures. The chloroplasts of algae present a varied structure. For eg. they are cup shaped in Chlamydomonas, ribbon-like in Spirogyra and star shaped in Zygnema.