THE GENERAL STRUCTURE OF PLANTS 9 
of these units of construction, a variety which finds its 
expression in the multiplicity of existing forms, and the 
differences of dimensions which various organisms exhibit. 
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 condi- 
tion 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 division re-  ™% 1°—Covontss or Protosoceus. 
maining attached together. 
Weget 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, Protococcus, and other humble Alge 
(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 differentia- 
tion 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, a shows an aggregation of a number of naked proto- 
plasts which have combined to form a plasmodium. These 
organisms are found creeping about upon moist surfaces ; 
