ASSIMILATION OF CARBON BY PLANTS 211 



tion between these two processes determines the position of the 

 compensation point, i.e., of the intensity of ilhimination when 

 both of these processes are in equihbrium. The accumulation 

 of substance by a plant, otherwise its production per definite 

 time period or during its entire life, must be regarded therefore 

 as the difference between the total amount of organic substance 

 produced by chloroplasts in the light and the amount that, 

 during this same time period, is consumed in the process of 

 respiration and is dispersed in the atmosphere in the form of 

 water vapor and carbon dioxide. It is also essential to consider 

 the fact that during the metabolism of the products of photo- 

 synthesis inside the plant in the vegetative period, changes 

 may occur in the weight of the initial organic substance. Thus 

 cellulose weighs 10 per cent less than glucose, from which it is 

 formed. Other losses of organic substances must be considered, 

 such as the abscission of leaves, twigs, flowers, and fruits, the 

 dying of root hairs and fine rootlets, etc. These losses are of a 

 regular character in deciduous trees in respect to leaves and in 

 forest trees in general in respect to the drying and abscission of 

 the lower branches. In grass plants, a desiccation of the lower 

 leaves toward the end of the vegetative period may likewise be 

 observed, and also the shedding of a part of the flower buds 

 and flowers and of the excess of fruits set. 



Thus the total production of the plant is determined not only 

 by its photosynthetic work but also by the amount of the losses 

 mentioned above. Of great significance is the relation between 

 the producing and the consuming parts of the plant. The 

 greater the development of the total working leaf surface of the 

 plant and the less the development of stems, roots, flowers, and 

 fruits, the greater will be the total accumulation of dry substance. 

 Conversely, plants having a high intensity of assimilation of the 

 leaves, but possessing a small leaf surface as compared with the 

 stems and roots, will be very slow in accumulating dry substance. 

 Such are many desert plants, e.g., wild alfalfa and cameFs-thorn, 

 Alhagi camelorum. According to Kostytchev's observations, the 

 leaves of these plants photosynthesize intensely; but the enor- 

 mous root system, reaching many meters deep down to the water 

 table, consumes almost all the substance produced by the leaves, 

 and the total accumulation of dry material aboveground is 

 consequently very slow. 



