CARBOHYDRATES. 35'< 



The direction of transfer of the above compounrls is towards 

 the point of use, or of storing ; there is never anj' approach to a 

 true circulation througliout the plant, corresponding, as was for- 

 merly taught, to the circulation in animals. 



932. Classification of the principal organic prodacts. For the 

 present purpose these may be conveniently grouped into (1) 

 those which are free from nitrogen, and (2) those which con- 

 tain nitrogen. Some have been already treated of in earlier 

 pages of this volume ; of the rest, little more than a mere 

 enumeration can here be given. 



933. Products free from nitrogen. I. Carbohydrates. In general 

 these are solid bodies manj' of which are soluble in water. They 

 are conveniently divided into the cellulose group, having the 

 empirical formula, Cul-I,„Oj, and the sugars, — grape-sugar, fruit- 

 sugar, and cane-sugar. 



The cellulose group comprises the following isomeric 

 bodies : — 



934. Cellulose. This substance (see page 31) is regarded as a 

 product of the direct transformation of starch or its equivalent. 

 When once separated from the protoplasm as cell-wall, cellulose 

 is not again dissolved save in the exceptional cases of germi- 

 nation where it serves as a food. Sachs has shown that in 

 the germination of the date, the pitted tliickening masses of the 

 cell-walls of the endosperm are dissolved and utihzed by the 

 embryo. 



935. Starch (see pages 47-50). The occurrence of this sub- 

 stance in the chlorophyll granules under certain conditions has 

 already been described. Its occurrence in reservoirs of food, 

 and the relation of this to the starch-generators, have been dis- 

 cussed in 174. 



The following table gi^'es some idea of the amount of starch 

 found in the ordinary commercial sources : — 



Source. Amount of starcli present. 



Grains of wheat 64 per cent. 



Grains of corn 65 " " 



Grains of rice 76 " << 



Potato tubers 15-29 " " 



When starch is to be transferred from the places where it is 

 held in reserve to the points where it is to be consumed, it is 

 converted into a form of sugar by some one or more of the 

 unorganized ferments occurring in plants. Although the sugar 

 thus formed passes at once into soliition, it is a curious fact 



