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1903] BRIEFER ARTICLES 28 7 



ant 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 

 types reduction in number both of spores and of sporangia is frequent. 

 The former will accordingly illustrate more faithfully than the hetero- 

 sporous 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 probandi lies rather with those who assume reduc- 

 tion to have taken place in them. 



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

 of these methods is then given for the Lycopodineae, Psilotaceae, 

 Sphenophylleae, Ophioglossaceae, Equisetineae, and Filicineae ; fol- 

 lowed 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 sterilization, 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 /^r/ /ax^w 

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

 cated the problem, especially in the higher forms; all roots are prob- 

 ably 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 rela- 

 tion of the reproductive to the vegetative regions is the same. In the 

 bryophytes that region is regarded as a residuum from progressive 

 sterilization; it is suggested that the same is the case for a strobiloid 



