CHAPTER XV 

 THE MICROSPORANGIUM 



The microsporangium of the vascular plants is considered first > 

 because there can be no question that it is the primitive type of 

 sporangial structure for the long series of forms which are character- 

 ized by the possession of water-conducting tracheary tissues. In 

 the lower representatives of the Vasculares the microsporangium 

 is the only type present, and in the heterosporous cryptogams and 

 the seed plants it keeps its place, with little change of its original 

 condition of organization, side by side with the highly modified 

 megasporangium and seed. The relative constancy of microspo- 

 rangial structures makes them in many respects of the greatest 

 value from the evolutionary standpoint. 



If the liverworts are correctly regarded as the forms nearest 

 to the Pteridophyta in the series of the bryophytes, there can be 

 little doubt that the sporangium in its primitive form of sporogo- 

 nium is the forerunner of the sporophyte of the vascular series. 

 Professor Bower has brought forward an impressive aggregation 

 of evidence in favor of the hypothesis that the sporophyte is the 

 result of progressive sterilization of sporogenous tissue. Although 

 the definite mode by which the simple sporogonium of the thallose 

 liverworts gave rise to the sporophyte of the Pteridophyta, so 

 complicated in its internal structure and external organization, is 

 highly speculative, it will serve a useful purpose to indicate the 

 main probabilities in this connection based for the most part on 

 the investigations of Leitgeb. In certain liverworts, such as, for 

 example, Corsinia and Boschia, the spore sac gives rise to sterile 

 cells as well as spores. There is clear evidence in these and in 

 similar cases that the sterile cells are modified or, as Professor Bower 

 expresses it, sterilized potential sporogenous cells. In many 

 liverworts the sterile cells are useful in distributing the spores. 

 In this instance they are much elongated and have their walls 

 spirally thickened. The spirals recoil when the spores are ripe, 



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