64 OUR WOODLAND TREES. 
a minute portion of matter. This darker sub- 
stance is, in reality, merely a portion of the 
protoplasm; but it is distinguished from the 
other portions by being designated the nucleus. 
Growth in the young cell commences by the 
enlargement of the cell walls. The protoplasmic 
contents not growing at the same proportionate 
rate, holes or cavities are left in their substance, 
and these cavities are filled with a watery liquid 
which constitutes what is called cell-sap. The 
cell walls continue to be lined with layers of pro- 
toplasm ; but it is believed that by the nucleus 
separating into halves, the same kind of divi- 
sion is effected by a half of the surrounding 
protoplasm collecting round each portion of the 
divided nucleus. Between these separated por- 
tions of the original nucleus a wall of cellulose 
begins to form, and, when its formation is com- 
pleted, it will be seen that the original single 
cell has become two. 
By the repetition of this process of cell division 
the growth of young cellular tissue is effected, 
and the plant increases in size. The rate of in- 
crease varies with the varying nature, in different 
