PALEOBOTANY BERRY. 355 



of a pedicel surmounted by three or four or more vertically elongated 

 pollen sacs which split longitudinally, and contain large pollen grains, 

 which often show a group of prothallial cells. In appearance the ovu- 

 late cones are much like the staminate. The axis is thick and clothed 

 with spirally arranged bracts, many of which are sterile. In the axis of 

 others are found short-stalked ovules with bracteoles. The ovules had 

 two integuments — an outer fleshy one, and an innner one which be- 

 came crustaceous. The ovule shows a nucellus with a large pollen 

 chamber and a beak-like micropylar canal. The pollen grains found 

 in the canal and pollen chamber are larger than those in the pollen 

 sacs, with their mass of prothallial or antheridial cells more exten- 

 sively developed, and it is hence inferred that the habit of forming 

 pollen tubes had not yet been developed and motile sperms were 

 formed directly. The seeds of numerous species are common as im- 

 pressions and have frequently been found in a petrified condition. 

 They exhibit considerable variation in size and form and commonly 

 go under the name of Cordaicarpus. 



Several additional types of Cordaitean wood anatomy are known. 

 These include Mesoxylon, of which several species have been de- 

 scribed from the English Carboniferous, and which help bridge the 

 gap between Cordaites and the family Poroxvlaceae of the Carbonif- 

 erous and Permian. Both Mesoxylon and Poroxylon develop cen- 

 tripetal xylem in the primary wood. A third family, the Pityeae, 

 based on large trunks from the Lower Carboniferous of Scotland, is 

 chiefly distinguished by the wide rays. Other anatomical genera in 

 the Cordaitean plexus include Callixylon of the Upper Devonian of 

 Europe and America, Caenoxylon of the Russian Permian, Mesopitys 

 of the Permian, Parapitys of the middle Carboniferous and Archal- 

 aceopitys of the Lower Carboniferous of Kentucky. Evidently much 

 remains to be learned before these and other as yet unknown Cor- 

 daitean forms can be property allocated. The Cordaitales show 

 clearly their origin from the same ancient plexus that gave rise to the 

 Pteridosperms. They likewise show points of contact with the later 

 coniferophyte orders, especially with the Ginkgoales, Taxales and 

 Araucariales. 



The Ginkgoales as a group can be characterized only by the fea- 

 tures of the single existing species, and these may be misleading when 

 applied to remote ancestral forms. This obvious conclusion must be 

 borne in mind in the attempts to relate these forms to the ancient 

 Cordaitales on the one hand and the modern Abietineae on the other. 

 Thus the formation of pits on the tangential walls of the tracheids 

 at the end of the annual ring is a purely physiological response to 

 an alternation of growth periods and resting periods and is due to 

 climatic changes, as is the deciduous habit, and it is very probable 

 that neither of these characters was present in early Mesozoic forms 



