STRUCTURE OF CELLS. 237 



comes self-supporting; exercising a power to organize the 

 elements of plant growth which it finds in the soil and in 

 the air, and which contribute to its nourishment. Hereto- 

 fore it has merely assimilated the already organized sugar 

 which has been derived from the starch of the seed; just as 

 the chick in the egg subsists upon the organized matter of 

 the yolk which is absorbed and converted by a mere change 

 of form into animal tissue. Now it begins to exercise a true 

 vital function, viz: the change of inorganic matter into or- 

 ganized substance, the tissue of plants, which transformation 

 is effected by the aid of the air which is inspired through 

 the leaves. 



The mechanism of plant growth consists of very minute 

 bodies called cells, and in these the vital function is concen- 

 trated and performed. These vary in size from a twenty- 

 fifth, to one fifteen hundredth part of an inch in diameter, 

 and are usually round or oval in form. When compressed 

 in the act of growth, these cells take on various other shapes 

 as the pressure may cause them; and become flat or disc 

 like; hexagonal; elongated; and angular. The cells are 

 easily perceived by the aid of a microscope in the pith of a 

 corn stalk, or of the elder, or in the pulp of fruit, or the 

 flesh of a potato. 



The cells consist of an outer wall or membrane, a lining 

 sac, and within this a nucleus or small body which is the 

 truly active principle of vitality. The outer wall consists 

 of cellulose or woody fiber, which is identical in composition 

 with starch, and is insoluble in water or alcohol. It is near- 

 ly pure in elder pith, and in the cotton and linen fiber. Its 

 chemical formula is Ci2 H2o Oio, which is precisely the same 

 as that of starch. Gum is represented by the formula Ci2 

 H22 On which is that of starch increased by one atom of 

 water, viz : H2 O; while the addition of one more atom of 

 water changes the gum into sugar, represented by Ci2 H*4 

 Oi2 Thus it is seen that the change of the starch of the 

 seed into sugar in the process of germination, is simply pro- 

 duced by the combination of 2 atoms of water (H* 02) with 

 the atom of starch Ci2 H>o Oio. 



