The Carboniferous Arborescent Lepidodendra. 59 



As to the magnitude of the primary xylem strand, and 

 the enormous number of the tracheids which compose it, 

 these equally reached their largest proportions at the base 

 of each solitary aerial stem. How such numbers of 

 tracheids, varying in the type of L. Wunschianum from 

 4,000 to 15,000, could be produced in that position is 

 difficult to understand. The young sporophyte could 

 not possibly have contained them ; hence some pro- 

 cess of growth, of the nature of which we have as 

 yet no knowledge, but which was capable of producing 

 these marvellous results, must have succeeded, if not been 

 developed out of the sporophyte. Anyhow, it is obvious 

 that my original hypothesis of an enlargement of the 

 Primary Xylem proceeding from above downwards, is 

 incapable of explaining the facts recorded in the pre- 

 ceding pages. Hence, that explanation appears more 

 likely to be found in some modification of the views of 

 Graf. Solms. But even in that direction the difficulties 

 are almost insuperable in the present state of our knowledge. 

 A hiatus exists between the sporophytal conditions of each 

 plant and its enlarged state when it began -to develop its 

 aerial stem supported upon its four Stigmarian root-organs. 

 This hiatus we have at present no prospect of filling. 



