METABOLISM. 41 



tannin (C34H,80„) ; inulin (CsH^Oj) ; glucosides, colouring 

 matters, etc. In addition to the above certain mineral salts 

 taken up in solution in water remain in that condition in the 

 cell-sap. 



Crystals, consisting of oxalate of lime (CaC^O^) are 

 common in cells, and sometimes also in the substance of 

 cell-walls in Coniferce and Gnetacece. The crystals occur 

 in the form of regular octohedra or needle-like crystals, 

 sometimes called raphides. Crystals of phosphate of lime 

 (CajSPO^) and carbonate of lime (CaC03) are rare. 



Metabolism. — The process of Assimilation or the elabor- 

 ation of the elements contained in carbonic dioxide and 

 water into a carbohydrate, usually starch, rarely cane-sugar 

 or fat, has already been explained. These substances do not 

 remain where they are first formed, but change their position 

 to other parts of the plant, and the change of locality is 

 usually preceded by a change in chemical composition. 

 This process is called Metabolism. 



Substances contributing to form cell-walls and protoplasm 

 are called plastic substances. The plastic substances forming 

 the cell-walls are mostly starch, sometimes cane-sugar; 

 inuliri, or fatty matters. These are always changed in part 

 into grape-sugar, which passes in solution through parenchy- 

 matous cells to those points where cell-formation is going 

 on, for the purpose of supplying the cellulose. The plastic 

 substances going to form protoplasm consist of albumin- 

 ous bodies containing nitrogen and sulphur. Assimilated 

 material is usually produced in excess of the quantity re- 

 quired at the time of its formation, the surplus being stored 

 up for future use and known as reserve-material. Thus the 

 starch formed in leaves is removed and re-formed as starch 

 in other parts of the plant. Seeds and tubers contain 



