CARBOHYDRATES. 35T 



The direction of transfer of the above compounds is towards 

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

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

 meiiv taught, to the circulation in animals. 



c* O 



932. Classification of the principal organic products. 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 many of which are soluble in water. The}' 

 are conveniently divided into the cellulose group, having the 

 empirical formula, C ti H 10 O 5 . and the sugars, - - grape-sugar, fruit- 

 sugar, and cane-sugar. 



THE CELLULOSE GROUP comprises the following isorneric 

 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 thickening masses of the 

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

 embrvo. 



*j 



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 gives some idea of the amount of starch 

 found in the ordinary commercial sources : 



Source. Amount of starch 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 bv some one or more of the 



C-? / 



unorganized ferments occurring in plants. Although the sugar 

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



