112 MINUTE STRUCTURE. [chap. 



consisting respectively of three or of four elementary or 

 simple substances. The cell-wall consists of carbon, oxygen, 

 and hydrogen, forming a ternary compound {cellulose). The 

 essential part of the cell-contents consists of the same 

 elements combined with nitrogen and sulphur, constituting 

 protoplasm. Wherever we have growth going forward, 

 there w-e have protoplasm, .in varied association with water, 

 in activity. 



8. The way in which growth in plants takes place is 

 simply this. The contents of the cells of the growing part 

 divide into two, and between the halved contents there 

 forms a thin layer of the ternary cell-wall, which divides 

 each cell into two distinct cells. The new cells then in- 

 crease in size until they become as large as their parent 

 cell, when they each divide again, and the process is 

 repeated. As a general rule, long and tapering cells 

 acquire very thick walls, while short cells remain per- 

 manently thin, or become thickened according to circum 

 stances. 



9. In observing the tissue of a Yam or Potato, referred to 

 above as well adapted to show thin-walled, closely packed 

 cells, or of any similar farinaceous tuber or rhizome, you may 

 notice that the cells which are not cut into (and thus 

 emptied) in making a very thin slice are filled with very 

 minute oblong granules. If the slice be too thick, the 

 granules are so numerous that they entirely conceal the 

 delicate cell- walls. These granules, which are stored up in 

 nearly all the cells, are called sta7-ch granules. To compare 

 with potato-starch, you may take the smallest possible pinch 

 of dry arrowroot and dust it upon your slide, and you will 

 find that the granules of which arrowroot consists, though 

 they differ a litde in form, are, in other respects, like those 

 of potato-starck You may make quite sure of it if you 



