BENNETTITALES 87 



strobili and a restriction of the function of the branches. Instead 

 of remaining elongated, foliage-bearing branches, with sporangia 

 borne on more or less modified leaves, they became short branches, 

 without foliage, but with bracts and si)orophylls arranged in a stro- 

 bilus by the shortening of the axis. That the short, lateral, strobilus- 

 bearing branches of Bennettitales have been derived from elongated 

 branches with foliage more or less modified to bear sporangia appears 

 to be a safe conclusion. The bisporangiate character of the strobilus 

 is probably to be explained by the bisporangiate character of the 

 fronds of those Cycadofilicales which gave rise to Bennettitales. 



In addition to ramentum, vascular anatomy, direct leaf trace, and 

 lateral branching, a conspicuous feature which the Bennettitales 

 have carried forward from the Cycadofilicales is the microsporophyll, 

 both in form and in the structure of its sporangia. In fact, the modi- 

 fication of the ordinary Marattia-like frond is much more extreme 

 among certain of the Cycadofilicales. So clear is the resemblance 

 of these microsporophylls to "fertile fronds" of ferns and of Cycado- 

 filicales, that it is not disguised by their monadelphous character, 

 which is probably related to the development of such large organs 

 upon a shortened axis. The so-called "disk" or "basal union" of 

 the stamens, it must be remembered, is due to a final simultaneous 

 growth of the whole zone of tissue from which the stamens began to 

 arise as separate members. 



The change in the megasporophylls was more extreme, but it 

 does not differ in kind from that observ^ed in. the riiicrosporophylls. It 

 is simply a still greater reduction, which in this case is carried to an 

 extreme; a naked branching rachis bearing terminal ovules upon its 

 ultimate branches, common among the Cycadofilicales, has become 

 a single ovule-bearing stalk. The connection is still more evident 

 when it is noted that the cupule investing the seeds of certain 

 Cycadofilicales has become a basal cupule in the well-protected seeds 

 of Bennettitales. The extreme modification has been attained in 

 the differentiation of sterile and fertile megasporophylls, and their 

 organization into a single fruitlike body. 



The most inexplicable feature in this supposed connection of the 

 two groups is the transition from a seed with no ex'ident embryo to 

 one containing relatively the largest embryo among gymnosperms. 



