3 8o 



EOUISETALES 



with their insertion directly on the axis, there can be no doubt of their 

 true homology. 1 This is illustrated by Scott's figure of the sporangiophore 

 of Calamostachys Casheana (Fig. 210), which shows the position and 

 structure of the sporangia; but the number of the sporangia on each was, 

 as a rule, only four. In some species there was heterospory, megasporangia 

 and microsporangia being found even upon the same sporangiophore : 

 this is illustrated by Scott in Calamostachys Casheana^ 1 He has also noted 

 in C. Binneyana the abortion of certain spores of the tetrad : 3 this, taken 

 with the condition as seen in C. Casheana, indicates that in the palaeozoic 

 genus "we are able to trace how heterospory originated. The facts suggest 

 that in the first instance a certain number of spores became abortive, and 



FIG. 208. 



A, apex of sporangium of Eqtiisetum limosum, L., showing the sporogenous cells, 

 surrounded by the tapetum (/). and sporangial wall. B, shows part of an older spor- 

 angium with its tapetum (/) still clearly defined, though the individuality of the cells is 

 lost : within this the sporogenous tissue, of which certain cells (a) are abortive. X 200. 



so allowed of better nutrition for the remainder : this process, going on 

 more freely in some sporangia than in others, may ultimately have rendered 

 possible the excessive development of those spores that survived at the 

 expense of the others, and may thus have led to the development of 

 specialised megaspores." 4 In this respect Calamostachys was in advance 

 of Equisetum. 



It has been shown above how completely the young sporangia are 

 protected in the strobilus of Equisetum by the close aggregation of the 

 sporangiophores, together with the covering afforded by the basal annulus 

 and terminal cap. In the more lax strobili of the Calamarians the pro- 

 tection must have been chiefly carried out by the intermediate whorls of 

 bracts, which overtopped the sporangiophores, a condition more nearly 

 comparable with what is seen in other strobiloid types. 



1 The relation of the strobili of the type of Calamostachys as regards their anatomy to 

 the Calamitean stem has been pointed out by Scott ; it will be unnecessary here to enter 

 into the evidence on such questions ; it suffices to refer to Scott, Studies, pp. 45, etc. 



2 Scott, Studies, Fig. 22. * L.c., p. 51. 4 /,.r., p. 53. 



