8 EVOLUTION OF PLANTS 
we are still far from understanding the ultimate struct- 
ure of the cell, although many ingenious hypotheses 
have been formed to explain the structure of the proto- 
plasm. 
We have seen that cells multiply by division. In 
the lowest organisms the cells thus produced usually 
separate at once, resulting in the formation of two or 
more individuals exactly like the parent. In case this 
division is repeated at short intervals, as happens, for 
example, in the Bacteria and many infusorians, the 
result is the production of an enormous number of in- 
dividuals in a surprisingly short time. In all but the 
lower forms of life the cells do not usually separate 
after division, the result being a multicellular organism. 
The cell-aggregates, of which these higher plants and 
animals are composed, are known as tissues, and these 
may be combined to form special organs. The cells of 
growing parts of the higher plants resemble the simple 
unicellular forms in structure, but as they grow older 
they may become extremely modified to fit them for 
special functions. 
If we examine one of the lower vegetable forms, such 
as a desmid (Fig. 1, B), we find that the single cell of 
which the plant is composed is at once vegetative and 
reproductive. Such a green cell is capable of perform- 
ing all the life-functions. It can absorb water con- 
taining certain food elements in solution, including the 
oxygen necessary for respiration, and, by virtue of the- 
chromatophore containing chlorophyll, is able in the pres- 
ence of light to decompose carbon dioxide and water, 
and from the oxygen, hydrogen, and carbon so separated, 
to manufacture the primitive carbo-hydrates necessary 
