504 NOTICES OF BOOKS. ` 
being either simple or forked at the apex. In fact a sporangium 
may pass successively through all these stages, and hence may so 
change its appearance that its different states are liable to be taken 
for sporangia belonging to different species. In Tiresias also we 
sometimes meet with sporangia bearing spines, but in that genus 
they are arranged like the spokes of a wheel, and not seattered as 
in the Desmidiee. What is the nature of the sporangia and why so 
complicated a process is necessary, since the species is also propa- 
gated by means of the granules or zoospores which escape from 
the ruptured cell, are questions to which we cannot, in the pre- 
sent state of science, return a satisfactory answer. The sporangia 
I consider capsules ; and this view seems to be confirmed by the 
experience of Mr. Jenner, who informs me that the covering of 
the sporangium swells, and a mucus is secreted, in which minute 
fronds appear and, by their increase, at length rupture the atten- 
uated covering. That some purpose, distinct from that performed 
by the zoospores, is served by the coupling of the cells and forma- 
tion of the sporangium cannot be doubted; for where we can 
trace the operations of nature, we find that nothing is useless or 
in vain; nor is it reasonable to suppose that this complicated pro- 
cess should fulfill no other purpose than one already provided for 
without it. The sporangia are most abundant in spring before 
the pools dry up; and I would suggest, as no improbable conjec- 
ture, that the zoospores may be gemme, analogous to those present 
in Marchantia polymorpha and Lunularia vulgaris, and that they 
possess merely a limited vitality, which is destroyed unless they 
are at once placed in circumstances favourable to their growth, 
whilst on the other hand, in the conjugated cells, some important 
change takes place during the commingling of their contents and 
the formation of the sporangium, like what happens in the pro- 
duction of seeds in general, which renders the sporangia capable 
of retaining the vital principle uninjured throughout long periods 
of drought.” 
We quite agree with Mr. Ralfs, in considering the contents of 
the sporangium as the real reproductive matter of the species; we 
-re also, with him, much disposed to view the zoospores as probably 
