1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 327 
it has no definite relation to the primary tapetum, and that in 
fact the term “primary tapetum”’ is a misnomer. The true 
tapetum, in many cases at least, is not represented by any mor- 
phological structure in the young sporangium. 
Is it not possible that the cells ss of figs. 72, 74 represent the 
wall layer of fig. 70, and that the ‘primary tapetum,” in addition 
to the protective and sometimes nutritive purpose which its 
derivatives subserve, has also a phylogenetic meaning as a sur- 
vival of the pteridophyte sporangium wall which has been in 
great part replaced by the true epidermis? Such at least is the 
view suggested by a comparison of embryonic organs in general, 
and of the relations of the primary sporogenous cells, 
RELATIONS OF THE VELUM. 
On the question whether the velum has any homologue 
among other plant structures my observations do not furnish 
any information. It has been compared on the one hand 
with the indusium of ferns, and on the other hand with the 
integument of an ovule. The possibility of the latter relation 
certainly has not been disproved, but the evidence for it is so 
Scant that it must remain merely an interesting suggestion. As 
to the other relationship, it ought to be borne in mind that the 
only ferns which can be at all closely related to Isoetes are the 
Susporangiate families, and all of these bear naked sporangia. 
The indusium appears in fact to be a special organ of the higher 
leptosporangiate ferns, without representation in the lower fami- 
lies, such as the Osmundacee or in the eusporangiates. .This 
absence of an indusium in the intermediate orders, and the 
doubtfulness of the homology of the various outgrowths known 
4S Indusia, make it impossible to regard the velum and indusium 
4S more than homoplastic structures. 
THE AFFINITIES: OF ISOETES. 
The systematic position of Isoetes has been discussed again 
and again, By Linnaeus it was placed among the vascular cry au 
a, Where most later taxonomists have been content to leave It. 
During the first half of the present century it was most frequently 
