IOO 



STERILISATION 



unalterable function." 1 This thesis should be extended so as to include 

 also sporogenous cells : for, as we have seen, many cases can be cited of 

 the conversion of cells which are normally sporogenous to a vegetative 

 condition, and occasionally the converse. The facts before us show that 

 vegetative and sporogenous cells are not things apart or essentially 

 different, but that they are on occasions mutually convertible. The 

 influences, external or internal, which act upon the embryonic cell, and 

 determine whether it shall be vegetative or sporogenous, are still obscure : 

 but clearly they act within restricted limits, for in Vascular Plants neither 

 superficial cells of the plant-body nor deeply seated cells have ever been 

 found to develop as spore-mother-cells. 



FIG. 58. 



Tmesipteris Tannensts, Bernh. A, median section through aynsngnlw*, showing the 

 tissue where the septum normally is developing as sporogenous cells (.$). / = tapetum. 

 B, part of the contents of a similar synangium, rather older, xx shows the line where 

 the septum should normally be, while a chain of fertile cells stretches continuously across 

 it. X loo. 



The conversion of potentially fertile cells into vegetative cells was 

 recognised by Naegeli, and embodied by him in his fundamental law of 

 organic development, as follows : " The phenomenon of reproduction of 

 one stage becomes at a higher stage that of vegetation. The cells which 

 in the simpler plant are set free as germs, and constitute the initials of 

 new individuals, become in the next higher plant part of the individual 

 organism, and lengthen the ontogeny to a corresponding extent." 2 The 

 sterilisation seen in the sporophyte of the Archegoniatae and Seed-Plants 

 is only one special case of that included under Naegeli's general law. 

 He points out that the law is realised in three different ways, and the 

 case for the sporophyte generation, with which alone we are at present 

 concerned, falls under the first head, expressed by him as follows : " The 

 propagative cells which arise by division are converted into tissue cells." 

 1 Organbildung, p. 241. * Abstammungslehre t p. 352. 



