138 THE ART OF CULTURE. 



In whatever form, therefore, we supply plants with 

 those substances which are the products of their 

 own action, in no instance do they appear to 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 azotized 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 containing nitro- 

 gen ; it is very probable that the wood and the 

 vegetable gluten, the starch granules and the cells 

 containing them, are formed simultaneously, 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 wnll {ccBteris 

 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 

 the substances which do not contain it, these sub- 

 stances will be separated as excrements from the 

 bark, roots, leaves, and branches. The exudations 

 of mannite, gum, and sugar, in strong and healthy 

 plants cannot be ascribed to any other cause.* 



^ M. Trapp in Giessen possesses a Clerodendron fragrans, which 

 grows in the house, and exudes on the surface of its leaves in Sep- 

 tember large colorless drops of sugar-candy, which form regular crys- 

 tals upon drying; — I am not aware whether the juice of this plant 



