Strobilus in A rchegoniate Plants . 349 
while that of the Pteridophyta has discrete archesporia, and 
possesses appendicular organs. The origin of the Pteridophyta 
was certainly in the very remote past ; it is therefore not 
surprising that traces of the line of their advance should be 
few and uncertain, and the hypotheses based upon them 
divergent. Before discussing these matters, however, I shall 
show that sterilization of potentially sporogenous cells, which 
is so easily recognized in the Bryophyta, is also common 
among Vascular Plants. In some cases the function of the 
arrested cells appears to be simply nutritive to the developing 
spores : in other cases they form permanent tissue-masses. 
In the sporangia of Equisetum only about 60 per cent, of the 
cells of the sporogenous tissue undergo the tetrad division : 
the rest— in addition to the tapetum — become disorganized, and 
their substance absorbed into the developing spores. In 
Psilotum and Tmesipteris , of the ill-defined sporogenous mass 
which is surrounded by no definite tapetum, only a small 
proportion of the cells produce spores, the rest being dis- 
organized as in Equisetum. In Ophioglossum again, as has 
been recently shown by M. Rostowzew, and confirmed by 
myself, a broad peripheral band of the sporogenous tissue is 
disorganized, and in the tetrad stage numerous nuclei, which 
are ultimately absorbed, are seen in the protoplasmic matrix 
in which the tetrads float : these are derived partly from cells 
of the periphery of the sporogenous mass, partly from others 
distributed through it (see Rostowzew, Rech. sur l’Ophioglossum 
vulgatum, p. 28). These examples will serve to show that 
a partial sterilization is of common occurrence in the sporo- 
genous masses of homosporous Pteridophyta. 
Among the heterosporous plants it is also seen, but 
most obviously in connexion with the maturing of the mega • 
spores, though it also occurs in the microsporangia. The case 
of Isoetes is sufficiently well known : here bands of tissue, 
differentiated from the sporogenous mass, develop as the 
trabeculae : in this case their origin by sterilization of potential 
sporogenous tissue has been generally accepted. I have 
recently shown that somewhat similar rods of sterile tissue 
