

VEGETABLE HISTOLOGY AND PHYSIOLOGY. 



145 



The chemical substances that have been detected in Protoplasm are Oxygen, 

 Hydrogen, Carbon, Nitrogen, Sulphur, Potassium, Calcium, Magnesium, Iron, Phosphorus, 

 Chlorine, and frequently Silicon and Sodium. 



The relative proportions of these substances differ in different orders, and 

 are not constant in the same plant. 



370. The wall of the cell 



(Fig. 466) is produced by 

 some action of the proto- 

 plasm. When first formed 

 it is very thin, soft, and uni- 

 form in thickness ; but as it 

 grows older, it is thickened 

 by additional coatings, or 

 strata, upon the inner sur- 

 face ; sometimes of uniform 

 thickness, but more fre- 



quently in VeinS, ringS, SpOtS, 



468, Forms of Protoplasm ; A and B, cells 



_ . 1 _ from the stalk of Indian Corn : C, from a tuber 



Or riQgeS, lOrming tile lOUn- of Jerusalem Artichoke after action of iodine and 

 . . sulphuric acid ; h, cell-wall ; k, nucleus ; b, nu- 



dation for the tissues and cieoius; P , protoplasm. 

 vessels of plants hereafter to be considered. 



371. Cellulose is the substance of which the cell- 

 wall is formed, It yields to the chemist the same ele- 

 ments that are found in starch, whose formula is 

 C 6 H, O 5 ; besides these, several other mineral sub- 

 stances are present in minute quantities. 



372. Woody material, called lignin, is deposited or 

 formed upon the walls of some cells, by which they 

 are hardened and strengthened. The component parts 

 of this substance are not accurately known ; there is 

 reason to believe they vary in different plants, and 

 even in different parts of the same plant. Mineral 

 substances, principally silica and lime compounds, also 

 thicken the cell-walls and increase their induration 

 and strength. 



