37 °] TRANS. ST. LOUIS ACAD. SCIENCE. 1 $ 
4. With crests and ridges, distinct or anastomizing: I. Zacus- 
tris , Tuckermani , and riparia. 
5. The confluent crests form a regular net-work: I. Engel * 
manni . 
The microspores are minute bodies of an ash-gray or a dusky 
color (dark gray in I. pygmoea, Bolanderi and melanopoda ,. 
deep brown in I. melanospora, Butleri and Nuttallii) and of 
a somewhat triangular-oblong shape, nearly straight on one 
and curved on the two other edges, more than half as wide as 
they are long, between 0.020 and 0.040 millimeters in the long* 
est diameter. Their surface is smooth or minutely papillose or 
spinulose, the edges smooth or somewhat cristate. Their size 
furnishes good characters, but the condition of the surface much 
less so. They ought to be examined under water and with a 
power of about 400 diameters. 
§ 3 . Biological Characters. 
After the maturity of the spores the leaves wither or rot away,, 
the sporangia decay and set the spores free, which scatter near 
the base of the plant, often being retained between the matted 
roots.* The cellular mass of the macrospores developes into a 
prothallus , which bursts the spore-case through the opening of 
three valves which correspond to the three upper faces of the 
spore, and forms an archegonium, which is fertilized on coming 
in contact with the zoospores emitted from the microspores, and, 
thus gives rise to the young plantlet whenever moisture and tem¬ 
perature favor this process. 
The germination of the late-maturing water-species probably 
takes place in the succeeding spring, at least in the more north¬ 
ern localities; in our land- and marsh-species it may be observed 
soon after their maturity in summer or in early autumn. 
I have studied the whole process in T. Engelmanni, which, I 
kept in Cultivation for several years. At the end of July the 
spores were perfectly mature and the leaves were coming off. 
On the 28th of that month I spread out both kinds of spores on 
a muddy surface and kept them slightly covered with water, and 
found on the plant; one or the other may be discovered there and help out the diagnosis- 
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