260 Prof. F. 0. Bower. Studies in the [Jan. 30, 



the condition of any polysporangiate sporophyte as we see it is 

 the resultant of modifications such as these, operative during its 

 descent. 



The problem will, therefore, be in each case to assign its proper 

 place in the history to any or each of these factors. 



It is pointed out that in homosporous types, which are certainly 

 the more primitive, the larger the number of spores the better the 

 chance of survival, and hence, other things being equal, increasing 

 numbers of spores and of sporangia may be anticipated ; but in the 

 heterosporous ty pes reduction in number both of spores and of 

 sporangia is frequent. The former will accordingly illustrate more 

 faithfully than the heterosporous forms the story of the increase of 

 complexity of spore-producing parts. The general method put in 

 practice here is to regard homosporous forms as in the upgrade of 

 their evolution, as regards their spore-producing organs, unless there 

 is clear evidence to the contrary. The onus prooamdi lies rather 

 with those who assume reduction to have taken jriace in them. 



A summary of evidence of variation in number of sporangia by any 

 of these methods is then given for t the Lycopodinere, Psilotacese, 

 Sphenophyllea?, Ophioglossacea^, Equisetinea?, and Filieineae ; followed 

 in each case by a theoretical discussion of the bearing of that 

 evidence on the morphology of the spore-producing members. The 

 general result is that all of them, including even the dorsiventral and 

 megaphyllous types, are referable to modifications of a radial strobiloid 

 type ; progressive elaboration of spore-producing parts, followed by 

 progressive sterilisation, and especially by abortion of sporangia in 

 them, of which there is frequent evidence, together with the acquire- 

 ment of a dorsiventral structure, may be held to account for the 

 origin of even the most complex forms. But the vegetative organs 

 once formed may also undergo elaboration, and differentiation pari 

 passu with the spore-producing organs, a point which has greatly com- 

 plicated the problem, especially in the higher forms; all roots are 

 probably of secondary origin ; facts of interpolation of additional 

 sporangia, especially in Ferns, and of apogamy and apospory, are 

 also disturbing influences, which have probably been of relatively 

 recent acquisition. 



A comparison is drawn as regards position, physiological and 

 evolutionary, in the sporophyte between the fertile zone in certain 

 Bryophytes and the fertile region of certain simple Pteridophytes,. 

 e.g., the Lycopods ; though no community of descent is assumed,, 

 the relation of the reproductive to the vegetative regions is the- 

 same. In the Bryophytes that region is regarded as a residuum from 

 progressive sterilisation ; it is suggested that the same is the case for 

 a strobiloid Pteridophyte, such as Lycopodium. The theory of the 

 strobilus, based on this comparison, is that similiar causes would lead to 



