n.] PLANT STRUCTURE AND GROWTH. 31 



roots) to the constituent parts of starch, protoplasm 

 has the power of forming albuminoids, 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 liquid 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- 



