II.] PLANT STRUCTURE AND GROWTH. 31 
roots) to the constituent parts of starch, protoplasm 
has the power of forming albuminotds, of which pro- 
toplasm itself is formed, and dependent upon for 
growth and increase. 
This process of manufacturing various substances 
out of these simple materials is termed assimilation. 
The substances formed by assimilation are stored up 
in the cells for future use in nourishing the plant. 
Thus in the potato, which is a part of the stem, 
the protoplasm of the cells is thickly dotted with 
grains of starch, it being laid up in the tuber to 
provide nourishment for the new shoots (eyes). In 
the wheat, oat, rice, pea, and bean we find it stored 
up in the seeds for the nourishment of the young 
plant or embryo, whilst it is developing its roots and 
leaves. The presence of starch in a plant may always 
be detected by the application of a slight quantity of 
iodine to the cell, when, if it contains starch, it will 
be stained blue. Oils and fats are also stored up as 
food for the plant in the same manner as starch; they 
are specially abundant in such plants as the Flax 
(from the seeds of which linseed-oil is obtained), 
Cocoa-nut, Olive, and the Castor-oil plant. Sugar, 
unlike starch, exists in a “guid state, and abounds 
chiefly in the stem of the sugar-cane and the tap- 
roots of parsnip and beet. It is manufactured by 
the plant from starch, A variety of substances, also 
formed in the cells, are known under the general 
term of alkaloids. Many of them have exceedingly 
valuable properties, and form important objects of 
commerce. Some of them are used in medicine, as 
morphia from the opium poppy, aconite from monks- 
