When we treated gametophytic blades of P. From this it is clear that there is a range among the spores in the oxidation reduction potential. Panigrahi and Chowdhury 1962 reported eight species of Selaginella from the eastern parts of our country. The homologous theory of Pringscheim 1878 maintains that the sporophyte is not a new structure but is a modification of the gametophyte. Homosporous gametophytcs are filamentous, cordate or tuberous in shape. Since a long time pteridophytes have remained the exclusive domain of academicians, rarely heard outside the academic world.
Peterson 1967 in Ophioglossum petiolatum and Bristow 1962 in Pteris critical have reported callusing. Scott 1924 , supports Church 1919 and states the discovery of Rhynie plants goes to show that they evolved from a fairly highly evolved group of algae. Although agamospermy could theoretically occur in , it appears to be absent in that group. Complete gametophytes can be cultured from practically a single cell. Most prolific spore-producing species did not necessarily d. The division and multiplication of free nuclei represents an early and latent form of classical cleavage polyembryony in Norway spruce.
In the asexual pathways, termed apospory and apogamy, a gametophyte is generated from sporophytic cells without meiosis and a sporophyte is generated from gametophytic cells without fertilization, respectively. By experimental induction of apospory it is possible to produce polyploid gamctophytes. According to the cortical intrusion theory, the cortical parenchyma cells intruded into the stele and produced the parenchymatous pith Jeffrey 1902. They share the characters of both these groups in addition to their own unique features. As has already been pointed out, the seed combines three generations and is a perfect example of parental care in land plants wherein the parent sporophyte assures the survival and growth of the embryo.
The spores are usually wind disseminated. Culture of the stem and rhizophore segments in Selaginella produces callus. The adult gametophytes were observed at 44 days in plant litter and 54 days in maquique; the gametangia were not appreciated. Apoptosis was biochemically characterized by the activation of endonuclease activity that contributed to massive morphological changes in nuclei and ultimately to cell death. In a siphonostele the vasculature is like a tube or siphon. A detailed analysis of all these theories is beyond the scope of this book. Key results: Asexual expression rather than sexual onset was limited by a minimal colony size.
Any of the living or fossil plants exhibiting the above mentioned steps may be said to be the forerunner of seed plants. A shift in morphology, driven by meiosis, could therefore constitute a mechanism for E. According to majority of workers the bryophytes have been originated from Chlorophyceae which are commonly found in fresh waters and rarely in sea waters. Compared to gemmae, sporophytes and protonemata or buds were more likely to be found on longer shoots than on shorter ones, but this relationship was only applicable to forest sites. By culturing these in the laboratory Alsopp 1955 has shown that at different levels of concentration, carbohydrates determine the differentiation of sporophyte into either land or water forms. I suggest that, both here and in other instances of stress-induced phase change, molecular chaperones may play a mediating role. Reversion to the juvenile condition occasionally occurs in buds on adult-type shoots, and invariably takes place in sexual reproduction.
An antheridium is a globose structure partially or completely embedded in the gamatophytic tissue. Favourable culture conditions tend to produce archegonia, while only antheridia are formed under starved conditions. According to Eames 1964 , in the face of overwhelming evidence in support of the axial nature of the sporophyte, the phyton theory seems to be irrelevant. In the presence of 0. Nayar and Kaur 1972 studied the detailed development of fern gametophytes. According to Bristow, the callus remains undifferentiated when the medium has 2. Origin of Equisetaceous Leaf: It is difficult to account for the origin of the equisetaceous leaves because of their whorled condition.
In megaphylls there is a branched venation. Most of the botanists however do not favour an anthocerotean origin for land plants. A high rate of metabolic activity also has been noticed during germination. Psilopsida gave rise to three groups viz. Bryophytic Origin: Even among the proponents of bryophytic ancestry there is no unanimity as to the precise group which constituted the ancestral stock or the mode of origin. When the auxin concentration in the medium was decreased, or when auxin was omitted altogether, differentiation occurred. In general, apomictic angiosperms are believed to occur preferentially at higher latitudes and elevations compared to their sexual relatives.
Independently, a genetic screen allows the identification of female sterile mutants defective in megasporogenesis. This type of development is said to be exosporic. Late microspores underwent miotosis during the first day of culture which resulted in a mixed population of bicellular pollen grains and uninucleate microspores, both embryogenic. These observations contradict reports for other taxa. Smith 1938 extended his observations to the sex organs and opined that the sex organs of pteridophytic gametophytes have much in common with those of Anthoceroies. According to modification or homologous theory the sporophyte is a direct modification of the gametophyte. Homospory describes the condition in which the asexual spores are all identical in structure and development.