How Plants Grow 39 
tude of these little units. The food is made within 
the cells, and oxygen is used within the cells. It is the 
cells that need water, and it is the cells that are alive and 
grow. 
How a plant grows. Plants grow in two ways: the 
cells multiply, and they increase in size. 
When a cell divides, the nucleus first separates into 
two parts. Then a wall or partition growsacross the cell. 
Thus two cells are formed where there was but one 
before. In the tips of growing stems and roots, where the 
growth is active, the cells are dividing very rapidly, 
and this multiplication of cells causes growth in these 
parts. 
In older parts of the plant, growth is largely due to 
the increase in the size of the cells. This enlargement of 
the cells is caused chiefly by the taking in of water, 
which collects within the cell. After a warm rain in 
early summer, the young corn plants take in water and 
the cells expand so rapidly that sometimes the amount of 
growth in a single night is noticeable. In the older re- 
gions of the root or stem a cell often has a volume one 
thousand times as great as the volume of one of the young 
cells in the growing tips. As the cell enlarges, its wall 
stretches, and new materials are formed in it, so it in- 
creases in size with the rest of the cell. 
How new parts are formed. If we could see the plant 
at the very beginning of its life, we would find that it 
consists of just one cell within the young seed. This cell 
divides and multiplies and soon develops into the embryo 
or tiny plantlet which we find within a seed. The root 
which pushes out when the seed germinates is formed 
