1 8 SCIENCE PRIMERS. [iv. 



oxygen. Gluten, the most common oY them, occurs 

 in granules in the outer cells of wheat and other 

 grains. The viscid matter left in the mouth after 

 chewing wheat, especially hard wheats, is gluten. 



21. Alkaloids. These are very remarkable sub- 

 stances, which all contain nitrogen; many of them 

 are medicines, as quinine and morphia ; others are 

 poisons, as strychnine and nicotine ; still others have 

 stimulating properties, as theine and caffeine, to which 

 tea and coffee owe their refreshing qualities. 



22. Other substances, of a mineral character, 

 enter into the composition of the cell and cell- 

 contents ; as sulphur, which is a constituent of the 

 albuminoids ; iron which, as already stated, is indis- 

 pensable for the production of chlorophyll; silica, 

 which is found deposited in the insoluble state 

 within the cell-walls ; compounds of phosphoric acid, 

 which are associated in some way not understood 

 with the formation of albuminoids; and lastly, salts 

 of potash, which are concerned in a manner equally 

 unknown with the production of starch and sugar. 

 Other mineral constituents are found in the plant, 

 often in considerable quantity, such as salts of soda 

 in seashore plants. But this is due to their acci- 

 dental presence in the soil ; and marine plants 

 grown inland will usually flourish with very little 

 soda. Calcium, again, is very commonly present in 

 plants, being taken up from the roots as sulphate of 

 calcium. This, however, is decomposed by oxalic acid, 

 and oxalate of calcium, which is not readily soluble, is 

 formed, and this is deposited in the plant in the form 

 of crystals, while the sulphuric acid yields its sulphur 

 for the formation of albuminoids : these occur in 



