2l8 CHAP. XXIV. CARBOHYDRATE-FORMATION 



value. The difference between the calculated and the 

 observed value can only be accounted for by attributing 

 it to the formation of some organic substance or substances 

 other than glucose having a lower carbohydrate-factor ; 

 hence the variation in the value of the oxygen-carbohydrate 

 factor indicates the varying proportions in which glucose 

 and the other substance or substances are produced. 



One of the carbohydrates produced in the assimilating 

 leaf of Hydrilla after exposure to light is starch. The plant 

 kept for about two days in the dark becomes, as tested by 

 the iodine method, practically de-starched. After exposure 

 to light, a profuse deposit of starch-grains is found in the 

 leaves, and in very sensitive specimens after a period as 

 short as ten minutes. 



The observed increase of weight in the leaf is thus partly 

 due to the production of starch of which the empirical 

 formula is C 6 H 10 O 5 . There is a general consensus of opinion 

 that starch is formed as a secondary product from sugar. 



Since the starch contributes its quota to the observed 

 increase of weight found by direct weighing, we may try to 

 obtain an approximate idea of the proportion of glucose and 

 starch produced in normal specimens of Hydrilla. Let us 

 imagine a particular case where the glucose and starch were 

 formed in equal proportions. The ratio of the carbohydrate- 

 factor in such a case to that where glucose alone was 

 produced is 



180+162 



: 180 



2 



171 : 180 



that is to say, the carbohydrate-factor in the case of the 

 mixed product would be 5 per cent, lower. Now this 

 lower value of the carbohydrate-factor is what was actually 

 found in the experimental determinations. This would 

 seem to indicate that in normal specimens the glucose and 

 starch were formed (the latter as a secondary product) 

 in about equal proportions. 



