EMBRYOGENY OF THE PTERIDOPHYTES 677 



stratum of constant fact, which gives no uncertain support to the strobiloid 

 theory of the shoot. For it appears that from the very first segmentation 

 of the zygote the polarity of the embryo is defined, and the position of 

 its axis may thereafter be recognised with certainty. The embryo is in 

 fact from the first a shoot with pre-existent axis, not a congeries of parts 

 which are ultimately related to an axis of later origin. While we recognise 

 thus the importance of the facts of development in indicating the shoot- 

 character as initiated at, once, it is necessary always to bear in mind the 

 critical position of the young embryo until it is self-supporting: the 

 urgent need of nutrition is the chief influence which has contributed 

 to its biological specialisation, and to the assumption of those aberrant 

 forms which tend so strongly to disguise its real nature as a simple 

 and primitive, but from the earliest stages a leafy shoot. 



