64 



BOTANY. 



[chap. II. 



being composed of alternating dark and light bands that 

 are arranged, depending on the particular kind of starch 

 examined, either concentrically or eccentrically round a 

 hilum or starting point. This layered appearance is due 

 to the alternation of dense layers with more watery layers, 

 the hilum being the most watery portion of the whole 

 grain. Starch is readily recognized by becoming blue 

 when mixed with a weak, cold, watery solution of iodine. 

 If the mixture is heated, the blue colour disappears but 

 returns on cooling. The great majority of edible pro- 

 ducts furnished by the vegetable kingdom consist of 



Fig. 20. /, Potato starch, 

 ■with an eccentric hilum; 

 //, Tapioca starch, with a 

 central hilum. (Highly 

 magnified.) 



starch which is obtained from those parts where it has 

 been stored up by the plant for its own future use, as in 

 seeds or fruits, as wheat, barley, rice, indian corn, sago, 

 etc., the meal consisting of starch along with the cells in 

 which it was contained ; starch is sometimes replaced by 

 sugar, as in ripe fruits, sugar-cane, beet, etc. 



The above process of assimilation or conversion of 

 carbonic dioxide into an organic compound is the only 

 one known, hence all carbon present in the tissues of 

 either plants or animals is in the first instance derived 

 from the carbonic dioxide decomposed in the chlorophyll- 

 grains. 



