By Pant, D.D. and Osborne, R. and Birbal Sahni Institute of Palaeobotany
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Extra resources for An introduction to gymnosperms, cycas, and cycadales
ORIGIN AND EVOLUTION OF SEEDS AND CUPULES 27 A E B c Fig. 8. Cordaitanthus williamsonis. A-C, diagrams of longisections of abortive ovules in the female flower showing median abortive nucelli flanked by two sterile lobes. D, diagrammatic transections through dichotomised apex of a sterile scale. E, diagrammatic transections through the apex of a megasporophyIl from a telomous system with a fertile telome in the miodle flanked by tW<1lateral sterile telomes (all after Florin, 1951 but"~ G redrawn modified).
In these there is no indication of the cup-like structure having been fonned by aggregation of branches and the flat lobes of the cupule are fused to a greater extent. Thus a repetition of the same process of fusion in a second whorl of sterile telomes around one or more integumented megasporangia seems to have led to the fonnation ofcupules. As would be obvious from the discussion given in the f~regoing pages, palaeobotanical discoveries during the recent past seem to support only the Telomic Theory of origin of seed integuments and cupules out of the seven theories mentioned at the end of this chapter.
Formation of cupules by steps similar to proximal megaspore. 5. Megaspore starts germination in situ but megaspore wall is dehiscent and gametophyte is partially exposed. 6. Gametophyte becomes completely intrasporic and megaspore indehiscent. 7. Apical or terminal dehiscence of megasporanglUm. 8. Indehiscent megasporangium and permanent retention of megaspore in megasporangium, accompanied by shedding of megasporangium as a whole. 9. Change from tetrahedral to linear tetrad. 10. Arrangements for incipient pollination.