﻿242 RELATIONSHIPS. 



certain kind of economy. The plant, instead of employing segments of all its crown 

 of leaves, delegates the reproductive effort to a fugacious branch whose leaves have 

 in the course of time been reduced to scales, though here it is to be borne in 

 mind that in certain more or less closely related primitive forms there must have 

 been free branching, all branches being fertile, and perhaps bearing both kinds of 

 spores. Likewise in the Cycadeoidea; the reproductive branches and concomitant 

 changes of the sporophyte either originated early or proceeded at an accelerated rate. 



Always considering the especial case taken up, namely, the changes in 

 presumed Marattiacean ancestors of the cycadalean and ginkgoalean alliance, the 

 origin of heterospory must be considered as primarily beginning with the spores. 

 Changes in these were reflected in the prothallial structures and, when the primitive 

 stages of megaspory appeared, prothalli tended to dioecism and, what is of more 

 importance in the elimination process, to briefer and briefer stages of separate exist- 

 ence. That is to say, more and more of the vegetative process concerned in repro- 

 duction was accomplished in the spore stages, the hitherto asexual plant playing a 

 more and more conspicuous role in the nutrition of the reproductive cells. A 

 resultant change in the fertile fronds finding its expression in dimorphism and later 

 trimorphism was also corollary to these spore changes. We may hence consider 

 the life of the separate prothalli, after spore change sets in, as growing shorter and 

 shorter, with a prolonged retention and more active nutrition of the slowly differ- 

 entiating spores, accompanying prothallial dicecism and finally dimorphism or 

 trimorphism. The separate prothallial stage grew shorter until eliminated. When 

 this finally took place nutritive disturbance was at its height, the ultimate result 

 being marked change of form, and at last retention of the megaspore. The long 

 retention of the megaspore is not seen, therefore, to present any insuperable diffi- 

 culty. It is a method of reproduction which is the final outcome of spore sterili- 

 zation and the resulting changes in the sporophylls.* 



Accompanying these profound reproductive changes there was a constant 

 reduction of the microsporophyll. The most striking intermediate stage known is 

 that exhibited by the staminate fronds of the Cycadeoidere. In the case of the 

 megasporophyU there is, along with some reduction, a reorganization and evolution 

 of a more highly developed and persistent nutritive system. Correlated with evo- 

 lution of both mega- and microsporophylls, there may be increasing variance in 

 the time intervening between the maturing of the microspores and fertilization. 

 The long interval between these two stages in many gymnosperms is a striking 



* The answer to the question whether the true ferns of the Paleozoic are of the type strictly ancestral 

 to the seed-bearing forms, rather than devolute, awaits future paleontologic discovery. For bryophyte 

 ancestry of the ferns and polyphyletic origins lie on the remote borders of our present knowledge. How- 

 ever, should our more naive theory of an evolutionary sequence based throughout all its later phases on 

 increasing spore complexity coordinated with sporophyll reduction, which we preferably confine to a mono- 

 phyletic fern series, prove to fall short of the actual facts, it must yet afford a virtually complete outline of 

 early seed-fern and cycadofilicinean origin. At least, it appears evident that, so far as the cycads are con- 

 cerned, from the cycadofilices on, reduction of the sporophyte and evolution of the spore was the method 

 nature followed ; and that a partly reversed process with more or less evenly coordinated foliage and 

 sporophyte evolution from plants very different from the simpler fern types could have led up to the 

 cycadofilices appears to be the much more unlikely, involved, and difficultly conceivable hypothesis. 

 Conversely, it is an at-first-hand method to predicate the descent of the seed-ferns more directly from 

 an ancestry with well-developed monomorphic foliage, and distinctly filicinean stem organization. 



