396 EQUISETALES 



in point of fact bract-leaves and sporangiophores are associated together 

 in Calamarian strobili: these may typify the primitive shoot as the strobilus 

 of Lye. se/ago does that of the Lycopods. A separation of these appendages 

 might be effected in ways which are here suggested by analogy with other 

 phyla, rather than by direct observation in this. In the Lycopodiales it 

 has been seen that abortion of sporangia occurred in certain regions, 

 which thus became more effectively vegetative : such abortion of sporangio- 

 phores would produce a vegetative region in place of a Calamarian strobilus. 

 On the other hand, abortion of the bracts in the strobilus would produce 

 the condition seen in Archaeocalamites or in Etjiiisetitm : moreover, in the 

 Equisetales, where the sporangia are borne upon sporangiophores with 

 enlarged distal ends, such protective structures are not required in cases 

 where the sporangiophores are crowded : in fact the abortion of the whole 

 bract-leaf in the specialised strobilus would bring with it no biological diffi- 

 culty. It seems probable that both of these factors may have been effective 

 in producing the conditions shown among the Equisetales. In the Calamites 

 the chief distinction between the strobilus and the vegetative shoot is in 

 the absence of the sporangiophores in the latter. It is true that no 

 observations of vestigial sporangiophores have been recorded, but it is to 

 be remembered that where abortion is complete no record remains of 

 what has happened, and that this is the case in many Locopods where 

 there is good reason to hold that abortion of sporangia has occurred. 

 It seems probable, then, from comparison of strobili and vegetative shoots, 

 as well as from analogy with the Lycopods, that abortion of sporangio- 

 phores will account for the distinction of the strobili from the vegetative 

 region in Calamostachys. 



But in other cases the segregation of leaves and sporangiophores was 

 more fully carried out. In PhyHotheca successive fertile zones appear, inter- 

 rupted by whorls of sterile bracts. On the other hand, the strobilus of 

 Equisetum is without sterile bracts at all : this condition, which may be 

 held as the more advanced, is shared by Archaeocalamites. It is, 

 however, uncertain how the Equisetoid type of strobilus arose : possibly 

 it was without bracts from the first : but more probably it originated 

 by the complete disappearance, from the fertile head, of bracts origi- 

 nally present : in this case the annulus, which survives as having a 

 biological value for protective purposes, may be held to represent the last 

 remnant of the series of abortive bract-whorls. The evidence for such 

 progressive separation of the vegetative and reproductive functions is not 

 so conclusive in the Equisetales as it is in the Lycopodiales ; but the facts, 

 so far as they go, are at least in accord with a theory of such a process 

 acting on a shoot in which the two functions were originally combined in 

 a manner similar to that seen in other primitive Pteridophytes. 



As in many other phyla, terminal bifurcation of the axis is seen, 

 but here it appears only as a rare abnormality. The normal branchings 

 are accessory in their origin, and are effective as reduplications of the 



