302 LECTURE XIV. 



compounds, e.g., certain decomposition products, as bridges from one 

 food-stuff to another. 



If we follow the formation of the most varied carbon compounds in 

 plants, which are the prime source of all carbon combinations in living 

 organisms, we are forced to the assumption that the first product arising 

 from the assimilation of carbon dioxide under the influence of chlorophyll 

 and the sun's energy, is a carbohydrate, or at least some compound very 

 closely related to this class of substances. 1 The carbohydrates undoubt- 

 edly assume a central position in plant metabolism, while the albumins 

 and fats are of minor importance. But it is not impossible that the 

 assimilation of carbon dioxide may proceed in various ways, and form 

 different primary assimilation products. It is indeed possible that the car- 

 bohydrates, i.e., the large amount of starch present, may conceal other 

 compounds. 



Up to the present time, because starch is detected so easily and so 

 positively, this and analogous substances have been of chief interest to 

 us. On the other hand, the carbohydrates impart to the plant organism 

 their individuality. We find them in every direction; and when we 

 consider the numerous polysaccharides (the more simple ones like 

 starch, which arise from the condensation of a single kind of sugar, and 

 the other innumerable, more complicated sugars, consisting of unlike 

 components, such as arabinose, xylose, dextrose, etc.), we will recognize 

 the fact, immediately, that the carbohydrates occupy the same pre- 

 dominating position in plants that the albumins do in animal organisms. 

 At all events, each of the three groups of food-stuffs fats, carbo- 

 hydrates, and proteins is ultimately derived from the carbonic acid 

 of the air, for that is the only source worth mentioning of the carbon 

 in plants. 



In considering the assimilation of carbon dioxide by the plants, we 

 referred to Baeyer's hypothesis, that formaldehyde is to be looked upon 

 as the first condensation product. We stated then that we could easily 

 imagine the various different kinds of sugars to result from the condensa- 

 tion of several molecules of formaldehyde. On the other hand, we also 

 mentioned Emil Fischer's hypothesis, which includes the possibility that 

 the glycerose, discovered by him, may occupy the primary stage in the 

 whole process of carbonic acid assimilation. This assumption has much 

 in its favor; and if glycerose, perhaps, does not actually appear at this 

 first stage, we must remember the possibility that it may result from the 

 disintegration of carbohydrates. At any rate it is interesting to think 

 that glycerose may perhaps occupy an intermediate position in the further 

 syntheses of the plant organism. From glycerose on, it is easy to build 

 bridges to the group of proteins as well as to that of the fats. 



1 Cf. Lecture IV. 



