38 BOTANICAL GAZETTE [JANUARY 
No attempt was made to determine the amount of incom- 
bustible matter in the exosporium, as it was found rather difficult 
to collect it in sufficient quantity. It is evident from the fore- 
going reactions that the residue is very largely silica. 
We also find by calculation that the incombustible residue 
forms 78.14 per cent. of the spores dried at 100° C. The macro- 
spores when divested of their exosporium are found to be very 
combustible, leaving when pressed on white paper a transparent 
stain, and can hardly be supposed to contain less organic matter 
than the exosporium itself. If we adopt this supposition, we 
find that the integument contains about go per cent. of silica, 
which is a very large amount when we consider that the ashes of 
oat straw contain less than 5 per cent. 
It is quite possible, moreover, that the blackening of the 
macrospores when they are first subjected to heat is caused by 
discoloration from the gases produced by the ignition of the 
endospore; indeed it is questionable whether the exosporium 
contains organic matter. This naturally could only be decided 
by the collection and examination of a considerable amount of 
the integument, an undertaking that requires a larger opportu- 
nity and better facilities than the author has been able to bring 
to bear upon it. 
It is perhaps worthy of remark that J Eatoni, from its polyg- 
amous character and the number and size of its macrosporangia, 
furnishes, with but little labor, a large amount of macrospores 
free from microspores, and it was upon the macrospores of this 
plant that the previously mentioned experiments were made. __ 
An examination of p/. V, fig. g will show that a macrospore 
of I. Eatoni when divested of its integument is marked by faint 
ridges. In the case of Z. Engelmanni these ridges are reticulated, 
and the endospores of I. echinospora are dotted with small low | 
| tubercles. It is at. these little elevations that the spore secretes > 
ae the Saeed amount of silica, and it is. this extra | secretion ieee a 
