Botany Bryophytes notes for 1st semester.pptx

Botany

1. Bryophytes

2. Bryophytes “ Amphibians Of The Plant Kingdom” Plants grow in two well defined habitats. These are the water and the land. The plants which grow are called the aquatics and the others terrestrial. The best examples of aquatic plants are the algae and of land dwellers the seed plants (spermatophytes). Between these two extremes of habitats is a transitional zone. It is represented by swamps and the areas where water and land meet. It may well be called the amphibious zone.

3. Bryophyta ( In Greek Bryon - moss ; phyton - plant), a division of kingdom Plantae   comprises of mosses, Hornworts and Liverworts. They are groups of green plants which occupy a position between the thallophytes (Algae) and the vascular cryptogams ( Pteridophytes ). Bryophytes produce embryos but lack seeds and vascular tissues. They are the most simple and primitive group of Embryophyta . They are said to be the first land plants or non-vascular land plants ( Atracheata ). Presence of swimming antherozoids is an evidence of their aquatic ancestory .

4. Origin of Bryophytes Nothing definite is known about the origin of Bryophytes because of the very little fossil record. There are two views regarding the origin of Bryophytes. These are ( i ) Algal hypothesis of the origin of Bryophytes. (ii) Pteridophytean hypothesis of the origin of Bryophytes.

5. ( i ) Algal Hypothesis of Origin There is no fossil evidence of origin of Bryophytes from algae but Bryophytes resemble with algae in characters like-amphibious nature, presence of flagellated antherozoids and necessity of water for fertilization. This hypothesis was supported by Lignier (1903), Bower (1908), Fritsch (1945) and Smith (1955) etc. According to Fritsch (1945) and Smith (1955) Bryophytes have been originated from the heterotrichous green algae belonging to the order Chaetophorales for e.g., Fritschiella , Coleochaete and Draparnaldiopsis .

6. (ii) Pteridophytean Hypothesis of Origin According to this hypothesis Bryophytes are descendent of Pteridophytes . They are evolved from Pteridophytes by progressive simplification or reduction. This hypothesis is based on certain characters like-presence of type of stomata on the sporogonium of Anthoceros and apophysis of mosses similar to the vascular land plants, similarly in the sporophytes of some Bryophytes (e.g., Anthoceros , Sphagnum, Andreaea ) with some members of Psilophytales of Pteridophytes (e.g., Rhynia , Hormophyton etc.)

7. Distribution of Bryophytes Bryophytes are represented by 960 genera and 24,000 species. They are cosmopolitan in distribution and are found growing both in the temperate and tropical regions of the world at an altitude of 4000-8000 feet. In India, Bryophytes are quite abundant in both Nilgiri hills and Himalayas; Kullu , Manali , Shimla , Darjeeling, Dalhousie and Garhwal are some of the hilly regions which also have a luxuriant growth of Bryophytes. Eastern Himalayas have the richest in bryophytic flora.

8. A few species of Riccia , Marchantia and Funaria occur in the plains of U.P., M.P. Rajasthan, Gujarat and South India. In hills they grow during the summer or rainy season. Winter is the rest period. In the plains the rest period is summer, whereas active growth takes place during the winter and the rainy season. Some Bryophytes have also been recorded from different geological eras e.g., Muscites yallourensis ( Coenozoic era), Intia vermicularies , Marchantia spp. ( Palaeozoic era) etc.

9. Habitat of Bryophytes Bryophytes grow densely in moist and shady places and form thick carpets or mats on damp soils, rocks, bark of trees especially during rainy season. Majority of the species are terrestrial but a few species grow in fresh water (aquatic) e.g., Riccia fluitans , Ricciocarpos natans , Riella etc. Bryophytes are not found in sea but some mosses are found growing in the crevices of rocks and are being regularly bathed by sea water e.g., Grimmia maritima .

10. Some Bryophytes also grow in diverse habitats e.g., Sphagnum-grows in bogs, Dendroceros -epiphytic, Radula protensa . Crossomitrium -epiphyllous, Polytrichum juniperinum-xerophytic , Tortula muralis -on old walls. Tortula desertorum in deserts, Porella platyphylla -on dry rocks, Buxbaumia aphylla (moss), Cryptothallus mirabilis (liverwort) are saprophytic.

11. Gametophytes

12. The gametophyte is a stage in the life cycle that is found in all plants and certain species of algae. A multicellular haploid generation known as Gametophyte. This process includes both multicellular diploid generation known as Sporophyte . The word Diploid refers to two sets of chromosomes in the cells, and normally written as ‘2n’. Haploid to only one set of chromosomes in the cells and written as ‘n’.

13. Gametophyte 1.Plant body The bryophytes are a small group of most primitive land dwellers . A few of them, however are strictly aquatic. The plant body is undoubtedly more differentiated than that of a complex alga. It is compact and better protected against desiccation. It grows prostrate on the ground and is thallus like . It is attached to the substratum by delicate, unicellular hair like organs called rhizoids.

14. The majority of the bryophytes however have the plant differentiated into stem and leaves. The leafy gametophyte of the liverworts is dorsiventral but in mosses it is erect.

15. The erect, leafy moss gametophyte has a stem like central axis which bears leaf like appendages. It is fixed to substratum by means of branched multicellular rhizoids apparently resembling the roots. The rhizoids arise from the older, basal part of the stem. The thallus like plant body of bryophytes bears the gametes , for this reason it is called the gametophytes plant . It is concerned with sexual reproduction amd consitutes the most conspicous nutritionally independent phase in the life cycle.

16. Reproduction Bryophytes show a marked advance over the thallophytes in the method of sexual reproduction . Without exception it is highly oogamous in the whole group. The gametes are produced in complex sex organs. They have attained a degree of complexity far above that of the thallophytes . In the thallophytes the sex organs are formed directly out of the protoplasts of the sex cells.

17. The bryophytes on the other hand have multicellular , jacketed sex organs. Each sex organs consists of outer, protective wall of sterile cells surrounding the cell or group of cell which produce the gametes. The male sex organ is still called the antheridium . the female is known as the archegonium . Both kinds of sex organs may be developed on the same individual ( monoecious ) or on distinct plants ( dioecious ).

18. Antheridium The antheridium is a multicellular object ellipsoidal or club-shaped sometimes spherical in form. It is borne on short stalk which attaches it to the gametophyte tissue . Often it is embbeded in the gametophyte tissue. The body of the antheridium has a wall of single layer of sterile cells .

19. It is surrounds a mass of small squarish or cubical cells called the androcytes . The androcytes produce the biflagellate male gametes called the sperms . They are motile structure . Each sperm usually consists of a minute slender, spirally curved body furnished with two long terminal, whiplash type flagella.

20. Archegonium The female sex organ of the bryophytes is a remarkable structure. It is called the archegoium It appears for the first time in the liverworts and mosses and continues in the pteridophytes . In this respect the liverworts and mosses seem to be akin to the pteridophytes than to the algae to which they resemble in their thallus like plant body.

21. The archegonium is a flask shaped organ . The slender, elongated upper portion is called the neck and the lower sac-like, swollen portion, the venter . The venter is attached to and often deeply embedded in the parent plant tissue. The neck has a wall of single layer of sterile cells called the neck canal cells.

22. The neck is usually projecting or freely exposed so as to be accessible to sperms. The venter also has a wall of sterile cells one or more cell layers in thickness The venter wall encloses two cells. they are the larger egg cell or the ovum and the smaller venter canal cell just above it .

23. Fertilization It occurs when the sex organs are mature. Moisture is essential for the maturing of the sex organs and also for the movements of the sperms to the archegonia. The mature antheridium ruptures at its apex liberating the sperms. At the same time the axial row of neck canal cells including the ventral cell in the mature archegonium disorganise . The tip of the archegonium also opens. the narrow canal opening to the exterior is formed . It acts as a passage way to the ovum in the venter .

24. The liberated sperms swimming in a thin film of water reach the archegonia. They enter through the open neck and swim down the canals of the archegonia. Reaching the venter one of them, probably the first one to reach there, penetrates the ovum. It fuses with the nucleus of the ovum to accomplish fertilization. with the act of fertilization the gametophyte generation end and the sporophyte generation starts. The gametes are the last structures of the gametophyte generation.

25. SPOROPHYTE With fertilization starts the second phase in the life cycle of the bryophytes. It is called the sporophyte . The pioneer structure of this phase is the zygote. Zygote The fertilized egg or ovum secretes a cellulose wall around it and is called the zygote. The zygote has a fusion or diploid nucleus which contains chromatin material of both the male and female gametes. The zygote marks the beginning of the sporophyte generation in the life cycle.

26. It is neither independent of the parent gametophytes plant nor passes into the resting condition. The development of the zygote into the embryo occurs within the venter of the archegonium which protects the egg, the zygote and the embryo against the varagies of the external environment. B. Embryo Within venter of the archegonium the zygote undegoes segmentation and develops without a resting period into a muticellular , undifferentiated structure called the embryo.

27. It obtains its nourishment directly from the thallus or the parent gametophyte to which it is organically attached. The zygote of the bryophyteshas thus a better chance for growth as compared with the zygote of the thallophytes which is always independent. c. Sporogonium The embryo by further segmentation and differentiation finally develops into a full fleged sporophyte individual. It is called the sporogonium . The sporogonium in bryophyte is leafless and rootless. Throughout its life it remains attached to the gametophyte plant.

28. It is thus spatially separated from the soil by the gametophyte tissues. Generally it has a limited life span. In some bryophytes it remains embedded in the gametophyte tissue. Generally it is projecting and consists of three parts The foot The seta The capsule The foot is embbeded in the tissue of the parent gametophyte. It absorbs nutrition for the sporogonium .

29. The rest of the sporogonium is free and projecting to promote easy dispersal of spores. The seta conducts the food absorbed by the foot to the capsule. The terminal capsule is where the development of the spores take place which are non-motile and wind disseminated. The spores of bryophytes are highly specialised cells. They are differentiated from the diploid spore mother cells by meiosis. They are thus haploid structures and are known as the meiospores or gonospores . Morphologically the spores produced in the capsule in all the bryophytes are homosporous .

30. https://www.youtube.com/watch?v=2kY7uzeYWFc https://www.youtube.com/watch?v=eN5ROvxpx8Q