Chapter - Progymnosperm lecture 2nd year

progymnosperm lecture 2nd year

1. University of Lucknow Faculty of Science Department of Botany Topic- Progymnosperm Present by- Madhesh kumar rai (011) (M.Sc, 2nd sem.) Plant science

2. Contents of the Topic Introduction of the Progymnosperms Characteristics of the Progymnosperms Origin of gymnosperm stele Homospory Anisospory Incipient Heterospory Heterospory Evolution of integument

3. Introduction The stratigraphically oldest known examples belong to the Middle Devonian order the Aneurophytales, with forms such as Protopteridium, in which the vegetative organs consisted of relatively loose clusters of axes. Tetraxylopteris is another example of a genus lacking leaves. In more advanced aneurophytaleans such as Aneurophyton these vegetative organs started to look rather more like fronds, and eventually during Late Devonian times the anuerophytaleans are presumed to have given rise to the pteridosperm order, the Lyginopteridales.

4. INTRODUCTION The gymnosperms are believed to have appeared during Upper Devonian and Lower Carboniferous in Palaeozoic era approximately 350 million year ago. The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the trimerophytes, and eventually gave rise to the gymnosperms . They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida (as opposite ).

5. Characteristics In Late Devonian times, another group of progymnosperms gave rise to the first really large trees known as Archaeopteris . Vascular cambium with unlimited growth potential is present as well as xylem and phloem. Ancestors of the earliest seed plants as well as the first true trees. Strong monopodial growth is exhibited. Some were heterosporous but others were homosporous

6. Origin of gymnosperm stele The Origin of gymnosperm stele was earlier believed (Jeffrey 1899,1902,1917) to be from a basic protostelic pattern which gave rise to siphonostele by enclosing extrastelar parenchymatous cortical tissue (as pith) and simultaneous appearance of leaf gaps. The presence of leaf gaps was considered by Jeffrey (1917) as one of the characteristic features of the siphonostele in Filicales, gymnosperms and angiosperms.

7. Homospory Homospory or isospory can be defined by the occurrence of equal sized spores within the same sporangium, which are morphologically similar but genetically they may be similar or different. Most of the bryophytes and pteridophytes show homospory of one kind or the other. All the steps towards heterospory are basically involved around the size and production of spores within the same or differents porangia .

8. Anisospory Anisospory is reported only in dioecious mosses and hepatics (e.g., Orthotrichaceae and Targioniaceae). Two size of spores (small and large) are produced in the same tetrad in a 1:1 ratio and they subsequently give rise to male and and female plants, respectively (Mogensen, 1983).

9. Incipient Heterospory Incipient Heterospory is aphenomenon distinguished usually in fossil plants where spores of different sizes were found within the same sporangium but size of spores dose not correspond to the sexes after their germination as in the case of living Queensland genus Platyzoma (Bower, 1926; Tryon, 1964 ). During the change described above the sporangia became sex determined and the gametophyte became from monoecious to dioecious.

10. Heterospory In geological time, the evolution from homospory to heterospory and then seed was accomplished in about 40 million year (within the Devonian). During Late Silurian and Early Devonian, only homosporous plants were present. Two theories are known which use fossil evidence to show how seeds and integument evolved.

11. Images of progymnosperm archaeopteris

12. Evolution of integument Long (1966) described and discussed the evolution of integument of pteridosperm ovules by the fusion of telome trusses. Genomosperma kidstoni represent the most primitive form where eight filaments ( tilome trusses) cover a centrally placed megasporangium .