380 EQUISETALES 
with their insertion directly on the axis, there can be no doubt of their 
true homology.! 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.*\ He has also noted 
in C. Brnneyana the abortion of certain spores of the tetrad:* 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 
A 
surrounded by the tapetum (¢). and sporangial wall. 4, 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.» 200. 
; apex of sporangium of Lgzisetum Mmosum, V.., showing the sporogenous cells, 
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 Ca/amostachys was in advance 
of Lgursetum. 
It has been shown above how completely the young sporangia are 
protected in the strobilus of Lywisetm 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. 
'The relation of the strobili of the type of Ca/amostachys 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, S/wdies, pp. 45, etc 
“Scott, Studies, Fig. 22. cia Gry sree Name TIE sae Gs OPA 
