Evolution of Plants 355 



sic period. They in turn were being pushed aside by the aUied 

 Cycadese of common derivative stock with the Bennetitese. 

 These cycads evidently became a highly abundant factor 

 alongside the Bennetitese during the Jurassic and early creta- 

 ceous period. But, while the latter group developed striking 

 and highly modified lateral hermaphrodite flowers — as the 

 researches of Wieland {122: 139) have abundantly and beauti- 

 fully demonstrated — and then gradually died out, the cycads 

 have evolved larger and terminal dioecious flowers Tvdth corre- 

 spondingly large fruits. Whether the huge polyciliate spermat- 

 ozoids that now characterize cycads were equally typical for 

 all of the above is a point that has not yet been determined. 



As regards the flowers and fruiting organs of these groups, 

 sufficient naked eye and microscopic studies have been made 

 to prove that they had evolved to the stage of naked-seed pro- 

 duction, or the gymnospermic habit. Thus the staminate 

 flowers and ''Lagenostoma" fruits of Lyginodendron, the stam- 

 inate flowers and "Neuropteris" fruits of the MeduUosese, 

 and Grand d'Eury's "Pecopteris" fruits from the coal measures, 

 the Bennetitese mth their "Cycadoidea" flowers and fruits 

 of Jurassic and cretaceous age, as well as fossil and recent 

 cycadaceous flowers and fruits, all verify the true gymno- 

 spermic character of these families. 



It is therefore of considerable morphological interest that 

 these have all evolved staminate and ovulate structures in 

 diverging from a more primitive spore-bearing state, just as 

 Lepidocarpon of the biciliate lycopodineous alliance has at- 

 tained to the same dignity, though from a totally different 

 ancestry. This added example of parallelism in evolution is 

 therefore of value, as suggesting that, when similar en\dronal 

 agencies act as stimuli, they give rise to closely parallel lines 

 of growth and resulting moiphological details, at the same 

 time that other and diverse environal agencies or stimulation 

 intensities act, so as to originate divergent types, such as the 

 Lepidocaipese and Lyginodendrese. 



The primitively monostelic stem-bundles that sometimes 

 became broken up in higher types, the capacity for secondary 

 increase of the stem by cambial activity, the formation of 

 barred tracheids in the secondary wood, the relatively anrple 

 size of the vegetative leaves, the gradual evolution of a hetero- 

 sporous condition, the differentiation of the heterosporophylls 

 into stamens and carpels, the predominant developmerrt of 

 one megasporangium as an ovule of considerable size, the 

 anemophilous pollination of tliis, the ultimate fornratioir of 

 the gynrnospermous seed that enclosed an embryo with two 



