340 ^ century of progress in the natural sciences 



Bennettitales 



Very important progress was made in the study of the bennettites, primarily 

 by the epoch-making work of Wieland (1906, 1916). It gradually became evi- 

 dent that the great majority of ey cadlike plants of mesozoic age belonged to 

 this group. Nathorst (1902) proposed the name Cycadophyta as a noncommittal 

 designation of all cycadlike gymnosperms. Two main groups of bennettites were 

 distinguished, the Williamsoniaceae, of a higher average geological age, and the 

 Bennettitaceae (Cycadeoidaceae). These groups agree closely in the structure 

 of their reproductive organs, but differ in vegetative features. While the former 

 had slender, often branched stems, the latter were characterized by short, stout 

 trunks, and their strobili or flowers were embedded in a thick mantle of per- 

 sistent leaf bases and ramenta. 



Williamsojiia has short, stalked, probably unisexual fertile shoots covered 

 with spirally arranged scale leaves. Seward (1912) found the conical upper 

 part of the axis densely covered by both stalked ovules and interseminal scales. 

 Nathorst (1909, 1911, 1912) was the first to discover the male flowers of WiJ- 

 liamsonia. These consist of a whorl of leaflike microsporophylls fused in their 

 lower parts and placed on a short peduncle. The microsporophylls are either 

 pinnate or simple, and bear synangia. WilUamsofiiella (Thomas, 1915) has in- 

 stead bisexual flowers. Each consists of a whorl of microsporophylls bearing 

 synangia, and inside them a central column extending upwards into a sterile 

 summit, but for the greater part covered with sessile ovules and interseminal 

 scales. WielandieUa (Nathorst, 1902, 1909) has branched stems like William- 

 soniella, but its flowers are sessile instead of pedunculate. The microsporophylls 

 were probably simple; the gynoeceum agreed with conditions in WilUmJisonia. 



The stems of Cycadeoidea (Wieland, I.e.) resemble in their external features 

 those of the living cycads, and their leaves are pinnately compound, but the 

 leaf traces are simple and of a less complicated course. The flowers are bi- 

 sexual and borne terminally on short axillary shoots, covered with spirally ar- 

 ranged pinnate bracts. The microsporophylls are whorled and form a disc by 

 fusion of their bases. They are pinnately compound and exhibit two rows of 

 complex synangia on each pinna. Just above the microsporophylls the apex of 

 the fertile shoot forms a broadly cone-shaped receptacle which bears stalked, 

 orthotropous ovules and interseminal scales. The tips of the latter are fused be- 

 tween the ovules to form a continuous surface layer. 



In the cycadeoideas the scalariform-pitted traeheids dominate the centrifugal 

 xylem, but are sometimes succeeded by traeheids with bordered pits. The struc- 

 ture of the stomata in the bennettites (Thomas and Bancroft, 1913; Thomas 

 1930) is quite distinct from that of the cycads. 



Wieland (1906, 1916, 1919) regarded the true cycads and the bennettites 

 as having in late paleozoic time separated from a common hypothetic pterido- 

 sperm ancestor. Scott (1923) stated, however, that there is no clue to the origin 

 of the Bennettitales beyond the general pteridospermous hypothesis. 



CORDAITALES 



Advances in this group relate less to the reproductive than to the vegetative 

 organs. C. E. Bertrand's (1911) interpretation of the morphology of their fe- 



