88 MORPHOLOGY OF GYMNOSPERMS 



However, even if this is really a difference (pp. 46, 47), it deals with 

 the common condition of seed plants during the Paleozoic, and would 

 belong to the development of seed plants in general. 



The resemblance of Bennettitales to Cycadales is so evident that 

 reasons for keeping the two groups separate are more necessary than 

 reasons for claiming that they are genetically related. The vegetative 

 body in external appearance, and in the anatomical structure of both 

 stem and leaf, seems to be so exact a reproduction of that of Cycadales 

 that an intimate relationship of the two groups is an unavoidable 

 conclusion. If this were the only testimony, no one would suggest 

 a separation of Bennettitales and Cycadales; and, as it is, this testi- 

 mony justifies the belief that the two groups are more intimately 

 related to one another than to any other gymnosperm group. 



The divergent characters of Bennettitales, however, are very 

 striking, and force the conclusion that the two groups, although 

 undoubtedly of common origin, are divergent groups, and that "transi- 

 tions" from the Bennettitales to the Cycadales, in the important differ- 

 ences between the two groups, need not be expected. In such a 

 feature as its femlike ramentum, the Bennettitales might be regarded 

 as on the way to Cycadales. The direct leaf traces, also, may be 

 associated with the monopodial habit, and this might be conceived 

 of as passing into the slow-growing sympodial habit of Cycadales, 

 which may hold some causal relation to the formation of "girdles." 

 Even the lateral branching, represented by numerous dwarf fertile 

 shoots, may be conceived of as being more and more restricted to 

 the apical region until the condition in Cycadales is attained. In 

 fact, this restriction has apparently reached the cycad level in William- 

 sonia gigas, whose strobili are borne on "peduncles" arising from 

 within the crown of Zamia-like leaves. 



In the strobili, however, extreme specialization is met in almost 

 every feature. The bisporangiate character, the closely enveloping 

 bracts, the monadelphous and pinnate microsporophylls bearing 

 synangia, the admixture of sterile and fertile megasporophylls, the 

 reduction of the fertile, megasporophyll to a single pedicel bearing a 

 terminal erect ovule, the organization of both kinds of megasporophylls 

 into a compact fruiting body, the development of an embryo which 

 destroys completely the endosperm during its intraseminal develop- 



