OUTLINES OF THE PHYSIOLOGY OF NUTRITION 203 



Now, in a water culture of this kind, the young 

 plant will remain healthy and continue to grow; it 

 will even in time flower and ripen its seed. In fact, 

 under these artificial conditions, the plant may be 

 brought successfully through its entire life-history, 

 while the amount of its organic substance is increased, 

 it may be several hundredfold, as compared with the 

 quantity contained in the seed to start with. The 

 success of this experiment proves that the compounds 

 presented to the plant contain all that is necessary for 

 its nutrition. Yet, among the elements contained in 

 the water and salts of the food-solution, one is conspicu- 

 ous by its absence ; we have given the plant no carbon 

 whatever. We know, however, that the dry-weight 

 of a plant that is to say, its weight after evaporating 

 the water which it contains is half made up of carbon. 

 We know also that every organic substance cellulose, 

 starch, sugar, proteid, or anything else which goes to 

 build up an organism is a compound of carbon ; we 

 know, then, that as the organic substance of a plant 

 has increased some hundredfold, so also has its carbon 

 increased in like proportion. Our specimen, then, has 

 obtained abundant carbon, but not from the food- 

 solution, and therefore not by means of its roots. The 

 only remaining source is the atmosphere, and the 

 carbon can only have been obtained by means of 

 those organs which are exposed to the air, namely, the 

 leaves and stem. 



We will return to the question how the plant 

 obtains its carbon ; our immediate object is to 

 determine which of the other elements are essential 



