FRESH-WATER ALG. 273 
their growth, the frustules, or fragments of which 
they are composed, separate in some species into 
two portions, each of which forms around itself a 
cell-wall, possessing a form and character precisely 
similar to those of the original one; and thus a 
very material increase in the number of frustules 
is, through course of time, effected. This process 
is called fissiparous or merismatic division. It is 
nothing more than what Professor Huxley calls 
‘a process of discontinuous growth.’ What in the 
higher plants is a process of growth, an enlarge- 
ment and development of the individual, is in these 
simple organisms a process of multiplication of 
separate individuals. In some cases the process 
of reproduction is performed by the conjugation 
of two approximated frustules, as was seen in the 
case of the larger conferve, the result being the 
union of their contents by means of interposed 
tubes, and the subsequent production of a ger- 
minating spore ;—thus leaving their vegetable 
origin no longer a doubtful question. Professor 
Weiss regards the large cavity between the two 
frustules as analogous to the embryo sac of higher 
plants; and he has succeeded in observing the de- 
velopment of new individuals in it. The product 
of this new individual indicates the alternation of 
generations in the Diatoms. By these various 
methods they propagate themselves with incon- 
s 
