ae 
1922] BENSON—HETEROTHECA GRIEVII 133 
stoma in 1914 was to have been followed by the account of the 
microsporange Heterotheca, but other things have claimed atten- 
tion, and it has only been since November 1918 that the work has 
again been taken up. The investigation of a larger number of 
specimens has thrown new light on the structure of Heterotheca, 
especially on the character and differentiation of the non- 
sporogenous tissue. With the possible exception of Crossotheca 
(text fig. 8, II), it differs from all other described microsporangia, 
and approaches what must be assumed to be a synthetic type of 
seed and microsporange. It is a succulent, sclerotic, vascular 
synangium, with central as well as peripheral loculi, and exhibits 
in its own tissues most of those found in the seed 
If we may have even in sporangia coexistent in the same species 
of plant a large amount of vegetative tissue, it is obvious that there 
is ground for assuming that in the early phases of evolution of the 
seed this would have been available as an envelope to the central 
fertile loculus, if the peripheral ceased to form sporogenous tissue. 
A difficulty some have had in accepting the theory of the synan- 
gial origin of the seed has been expressed by the statement that 
“nothing useful has ever been produced by a sterilized sporange.” 
It is granted that abortive sporogenous tissue generally eventuates 
only in a mucilage cavity, as CoULTER and Lanp (7) and STARR 
(16) have shown in particular cases, but useful structures certainly 
can be formed by the elaboration of the vegetative tissue surround- 
ing masses of sporogenous tissue, as has recently been demonstrated 
(6) in the megasporange of Mazocarpon. It is not altogether 
sterilized potential sporogenous tissue of the peripheral loculi in 
Heterotheca which formed the inner integument of the ovule 
(canopy), but mainly an elaboration of the vegetative tissues which 
originally surrounded those masses of sporogenous tissue and 
finally supplanted them. For example, Azolla and all such lepto- 
sporangiate ferns were foredoomed to failure in the construction of a 
seed on these lines, as their peripheral sporangia had in all prob- 
ability lost their vascular supply and their skeletal tissue before 
heterospory was evolved. The parallel to the seed habit which 
may be noted in Azolla is limited to the fact that the megasporange 
is one-spored, and occupies a central position in the sorus. 
