DEVELOPMENT. 65 
plants, of the power of growth; in some cases 
being marvellously rapid, in others much slower, 
When the cells have ceased to divide, they begin 
within thémselves—each within each—a process 
of development—a process, in some cases, shown 
by a continued secretion of cellulose by the pro- 
toplasm, ending in a considerable thickening of 
the cell walls; in other cases by the manufacture 
from the protoplasmic fluid of various sub- 
stances, which, in the wise economy of Nature, 
minister to the necessities and enjoyments of man, 
How beautiful, indeed, yet how wonderfully 
mysterious is the influence of this power of de- 
velopment in plants! There is sweet co-operation 
between air, earth, and sunshine in order not only 
to perfect the beauty but to promote the useful- 
ness of plant forms; for from the simple ele- 
mental materials which have been already enume- 
rated are formed in the cells, various substances, 
including chromophyll, oil, resin, starch, sugar, 
and wax. 
The development of a Tree naturally presents 
itself to us as being of six different kinds. There 
is development of the root, of the stem, of the 
