198 FIRST FORMS OF VEGETATION. 
are very diversified ; almost every family having 
a peculiar plan of its own. Some reproduce the 
species by the simple breaking up of the filaments 
into larger or shorter pieces, or into single joints. 
Others are reproduced by gonidia or zoospores 
developed from the contents of the filaments and 
covered with vibratile cilia, by means of which 
they swim actively in the water. But the most 
remarkable mode of propagation is that which is 
known as the process of conjugation. When two 
filaments approximate, each throws out from one 
side a small process, which unites with a corre- 
sponding process from the side of the other ; the 
two ends of the processes become absorbed, and 
the interval between the two plants is thus bridged 
over by a transverse tube. The endochrome of 
the one cell then passes through the communica- 
tion thus formed into the other, and the contents 
of both cells become intimately mixed and form a 
round mass, which ultimately becomes the seeds 
or resting-spores by which new plants of the same 
kind are destined to be produced. In consequence 
of some differences of structure, to our eyes inap- 
preciable, the filaments appear to exercise in one 
case the function of the male, and in another that 
of the female. But how is it, it may be asked, 
that process meets process in two contiguous fila- 
ments, and form between them a germinating 
