AS SEEN IN PTERIDOPHYTA 95 
partly to the fact that owing to sterilisation the definitive fertile cells do. 
not form a continuous mass. 
Among heterosporous forms, sterile cells are commonly present in 
the female sporangium (Fig. 51): there is good reason to think that 
arrest of potential sporogenous cells has greatly favoured the advance in 
size of the relatively few remaining megaspores. But apart from this, 
the case of Jsvefs is interesting, since there is evidence of sterilisation 
both in the mega- and micro-sporangia, and in both it has resulted in 
permanent tissue-masses. In both types of sporangium an_ extensive 
potential sporogenous tissue is formed, which is at first uniform in 
structure, as it was also in origin. In the microsporangium considerable 
tracts of this tissue differentiate later as vegetative trabeculae and tapetum, 
Fic. 51. Fic. 52. 
Selaginella spinulosa, A. Br. Section of fsoetes lacustris, 1. Vertical section of a 
megasporangium showing the single fertile young microsporangium. sf=fertile tissue. 
tetrad still very small, and the rest of the ¢v=trabeculae. ¢=tapetum. — x 100. 
sporogenous cells arrested. x 100. 
while the remainder forms microspores. From the history of development, 
and from comparison, the conclusion seems justified that the trabeculae and 
tapetum in this case represent sporogenous tissue which has been converted 
into sterile tissue, serving nutritive and mechanical purposes in the very large 
sporangium (Fig. 52). Similarly, in the megasporangium there is_ sterilisa- 
tion, but it has been carried much further, and it has been possible to 
show that the megaspore-mother-cells are not morphologically predetermined, 
but are physiologically selected from among a large number of potentially 
sporogenous cells: also that each archesporial cell gives rise to several 
megaspore-mother-cells, as well as to trabeculae and tapetum (Fig. 53) 
(Wilson Smith). Thus there has been a differentiation of tissues of uniform 
origin, and a large part has been diverted to functions played by sterile 
vegetative tissue. Very similar sterile tracts of tissue have been seen in 
the large sporangia of Lepidostrobus Brownii, and their origin by 
sterilisation is highly probable, though naturally this is hardly susceptible 
