ENATION OF LEAF 143 



morphological experience. Therefore the validity of the strobiloid theory 

 is not open to a priori objection. The real question is whether those 

 processes which are seen to have been in operation elsewhere did actually 

 take part in the production of the Pteridophyte strobilus as it is now 

 seen? The applicability of the theory to the various known types of 

 Pteridophytes will be the true test of its validity. This will be carried 

 out in detail in the second part of this work, so far as the very imperfect 

 evidence will allow. In questions such as this of the origin of the shoot, 

 it is desirable to take the simplest possible reading of the facts as the 

 basis of an opinion : on this ground the theory of enation, as accounting 

 for the origin of the appendages of the strobilus, seems to be preferable 

 to any phytonic theory. It has been remarked that the strobiloid theory 

 involves " tremendous morphological assumptions in the way of the origin 

 of new organs" (Tansley, New Phytologist, 1907, p. 28, etc.): the only 

 assumption, however, which is apparent to the mind of its author, is that 

 the order and mode of origin of the appendicular parts in the course of 

 Descent has been that which is actually seen in their individual develop- 

 ment. They are formed by enation from the axis now, and it is held 

 that they originated in the first instance in the same way. 



