100 STERILISATION 
unalterable function.”! 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. 
Fic. 58. 
_Tmesipteris Tannensis, Berph. A, median section through synangium, showing the 
tissue where the septum normally is developing as sporogenous cells (s). ¢=tapetum. 
B, part of the contents of a similar synangium, rather older. x2 shows the line where 
ae ay should normally be, while a chain of fertile cells stretches continuously across 
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, p. 352. 
EN 
