356 



CALAMITES. 



[CH. 



The strobilus axis agrees in structure with that of C. 

 Binneyana, but in G. Casheana a band of secondary xylem 

 forms the peripheral portion of the triangular stele. Were 

 any further proof needed of the now well-established fact that 

 secondary growth in thickness is by no means unknown as an 

 attribute of Vascular Cryptogams, the co-existence in the same 

 cone of a cambium layer producing secondary wood and bark, 

 and cryptogamic macrospores and microspores, affords con- 

 clusive evidenced The dogma accepted by many writers for 

 a considerable number of years that the power of secondary 

 thickening is evidence against a cryptogamic affinity, has been 

 responsible for no little confusion in palaeobotanical nomen- 

 clature. 



On the axis of Calamostachys Casheana there are borne 

 alternate whorls of fertile and sterile appendages similar to 

 those in the homosporous C. Binneyana, but they are inclined 

 more obliquely to the axis of the cone. Macrospores and 

 microspores have been found in sporangia borne on the same 

 sporangiophore. 







GO 



a I 



Fig. 96. Calamostachys Casheana Will. 



A sporangium with macrospores and abortive spores, x 65. 



(After Williamson and Scott.) 



The spore-tetrads in the macrosporangia occasionally include 



^ An excellent figure illustrating the co-existence of heteroapory and 

 secondary thickening is given by Williamson and Scott, loc. cit., PI. lxxxii. 

 fig. 36. 



