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. 
