NECESSITY OF NITROGENOUS SUBSTANCES. 



have any effect upon their growth, or to replace 

 what they have lost. Sugar, gum, and starch, are 

 not food for plants, and the same must be said of 

 humic acid, which is so closely allied to them in 

 composition. 



If now we direct our attention to the particular 

 organs of a plant, we find every fibre and every 

 particle of wood surrounded by a juice containing 

 an azotised matter ; while the starch granules and 

 sugar are enclosed in cells formed of a substance 

 containing nitrogen. Indeed everywhere, in all 

 the juices of the fruits and blossoms, we find a 

 substance, destitute of nitrogen, accompanied by 

 one which contains that element. 



The wood of the stem cannot be formed, quasi 

 wood, in the leaves, but another substance must 

 be produced, which is capable of being transformed 

 into wood. This substance must be in a state of 

 solution, and accompanied by a compound contain- 

 ing nitrogen ; it is very probable, that the wood 

 and the vegetable gluten, the starch granules and 

 the cells containing them, are formed simultane- 

 ously, and in this case, a certain fixed proportion 

 between them would be a condition necessary for 

 their production. 



According to this view the assimilation of the 

 substances generated in the leaves will (cceteris 

 paribus) depend on the quantity of nitrogen con- 

 tained in the food. When a sufficient quantity of 

 nitrogen is not present to aid in the assimilation of 



K 2 



