106 A TEXT-BOOK OF BOTANY 
drical chloroplast appears as a thick green mass on each 
side of the central nucleus (Fig. 97, A). Each cell is able 
to divide, and so the filament grows in length; or frag- 
ments of old filaments may develop new ones, resulting in 
vegetative multiplication. 
Aithough each cell of the filament is an ordinary nutri- 
tive cell, under certain conditions one or more of these cells 
contain other cells, that have been formed by what is called 
the internal division of the older one (Fig. 97, B). In 
ordinary cell-division the wall of the old cell forms a part 
of the walls of the two new cells; but in internal division 
the wall of the old cell is only a case which encloses the new 
ones, and from which they escape. When these cells formed 
by internal division escape from the mother-cell into the 
water, it is discovered that they are able to swim about by 
the lashing movements of four cilia that appear in a cluster 
at the pointed end (Fig. 97, C, 6). After a time these 
swimming cells settle down, lose their cilia, and by division 
begin the development of new filaments like those from 
which they came (Fig. 97, D). It is evident that the 
swimming cells have introduced a new method of reproduc- 
tion—a method that involves the formation of a special cell 
for reproduction, quite distinct from the ordinary nutritive 
cells. A special cell thus set apart for reproduction is called 
a spore, and spores that swim are distinguished as swim- 
ming spores. A very important fact about Ulothriz, there- 
fore, is that it reproduces not only by vegetative multipli- 
cation, but also by swimming spores. 
In other cells of the same filaments, or in cells of fila- 
ments under different conditions, the same formation of 
cells by internal division may be observed; but the con- 
tained cells are smaller and more numerous (Fig. 97, C, c). 
When they escape, it is discovered that they also are ciliated 
swimming cells; but since they do not produce new fila- 
ments, it is evident that they are not swimming spores. 
