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 Isoetes 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, 



FIG. 51. 



Sclagincllii spiii>tlo*a, A. Br. Section of 

 megasporangium showing the single fertile 

 tetrad still very small, and the rest of tlie 

 sporogenous cells arrested, x 100. 



FIG. 52. 



Iseetes lacuslris, L. Vertical section of a 

 young microsporangium. sp = fertile tissue. 

 tr= trabeculae. ^ = tapetum. 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 Lepidostrolnts J-irotcnii, and their origin by 

 sterilisation is highly probable, though naturally this is hardly susceptible 



