X] CALAMOSTACHYS. 355 



mother-cell. The spores of a tetrad are in some cases of 

 unequal size, some having developed more vigorously than 

 others. This unequal growth and nourishment of spores is 

 clearly shown in fig. 96, which represents a sporangium of a 

 heteroporous Calamitean strobilus, C. Casheana. Williamson 

 and Scott ^ have described striking examples of spores in 

 different stages of abortion, and these authors draw attention 

 to the importance of the phenomenon from the point of view 

 of the origin of a heterosporous form of cone. The abortion 

 of some of the members of a spore-tetrad and the consequent 

 increased nutrition of the more favoured daughter-cells, might 

 well be the starting-point of a process, which would ultimately 

 lead to the production of well defined macrospores and micro- 

 spores. The young microsporangia and macrosporangia of 

 recent Vascular Cryptogams such as Selaginella, Salvinia and 

 other heterosporous genera are identical in appearance^ ; it is 

 not until the spore -producing tissue begins to differentiate into 

 groups of spores, that the sporangia assume the form of macro- 

 sporangia and microsporangia. During the evolution of the 

 various known types of pteridophytic plants heterospory gradu- 

 ally succeeded isospory, and this no doubt occurred several 

 times and in different phyla of the plant kingdom. In the 

 mature sporangia of some of the Calamitean strobili we have 

 in the inequality of the spores in one sporangium an indication 

 of the steps by which heterospory arose ; and in the immature 

 sporangia of some recent genera we are carried back to a stage 

 still nearer the starting-point of the substitution of the hetero- 

 sporous for the isosporous condition. 



Galamostachys Casheana Will. Fig. 96. 



To Williamson^ again is largely due the information we 

 possess as to the structure of this type of Calamitean strobilus. 

 Its special interest lies in the occurrence of macrospores and 

 microspores in the same cone. 



' Williamson and Scott (94), p. 911, Pis. lxxxi. and Lznn. 



'^ Vide Heinricher (82) ; Bower (94), p. 495; Campbell (95), pp. 806, 508. 



3 Williamson (81), PI. liv. 



23—2 



