THE GENERAL STRUCTURE OF PLANTS 9 
The simplest plants, as we have seen, are unicellular, 
and many remain in this condition throughout the whole of 
their existence. When they have attained a certain size 
the cell or protoplast divides into two. Sometimes these 
two become separated from each other, and we have two 
plants where but one existed before. Plants with this habit 
remain unicellular, and the 
division of the cell is equiva- 
lent to the reproduction of 
the plant. The unicellular 
condition in other cases is 
transitory, and the plant soon 
comes to consist of two, four, 
or more cells, in consequence 
of the products of each divi- a es ae 
sion remaining attached to- 
gether. We get in this way a small colony of cells, each 
like the others both in structure and in function. When 
the power of division is limited the resulting colony 
consists of a limited number of cells, and is often found 
surrounded by a common cell-wall or membrane. This 
condition is seen in such plants as Chroococcus, Proto- 
coccus, and other humble Algw (fig. 10). A colony of 
somewhat higher type, though still of microscopic size, is 
found in the form of a hollow sphere (fig. 11), the wall 
of which is one cell thick (fig. 11, a). This organism, 
known as Volvoz, shows a little higher differentiation than 
those last described, the cells being furnished with cilia by 
means of which the little sphere can propel itself through 
the water. 
In other cases the association of a number of protoplasts 
is not complicated by the formation of any cell-wall. Fig. 
4, ashows an aggregation of a number of naked protoplasts 
which have combined to form a plasmodium. These 
organisms are found creeping about upon moist surfaces ; 
they form the group known as the Myxomycetes or slime- 
fungi. One species, Mthalium, is found frequently among 
