22 PLANT LIFE. 



{b) Enclosed within the cell-wall is tha protoplasm, which in 

 the young condition almost entirely fills the cavity, {c) In 

 the protoplasm of all young cells of the higher plants, and 

 in most of the simpler forms also, there lies imbedded a 

 more or less spherical body called the nucleus. 



When quite young all cells agree in general structure and 

 appearance ; but owing to the division of labour already 

 alluded to, the mature cells differ very much from each 

 other both in structure and function. 



In young cells the wall is very thin, but owing to the 

 continued secretion by the protoplasm of the substance of 

 which the cell-wall is formed, the latter continues to increase 

 in area and thickness. The growth of the cell-wall is entirely 

 dependent on the contact of the contained protoplasm with 

 its inner surface, consequently cells from which the proto- 

 plasm has disappeared cease to grow, and as all vital 

 functions emanate from protoplasm, such cells can only 

 perform mechanical functions, as giving strength or rigidity. 

 This is clearly seen in the case of hollow trunks, the heart- 

 wood or central portion is the oldest, and in old trees the 

 protoplasm has almost entirely disappeared from the thick- 

 walled cells of this part, and their decay and total disappear- 

 ance does not at once cause the death of the tree, the 

 necessary communication between the root and the leaves 

 being carried on by the young wood lying just below the 

 bark. 



In young cells the thin wall consists mostly of cellulose, 

 a substance having the same chemical composition as starch 

 (CuHioOs), in addition there is always a considerable 

 quantity of water, and a varying but always very small pro- 

 portion of mineral matter which remains as ash when the 

 cell-wall is burnt. 



