ABSORPTION OF CARBON 37 



cells. These cells contain fewer chloroplasts, and are less active 

 in assimilation. The spongy tissue is regarded as being specially 

 adapted to the aeration of the leaf. The stomata are usually 

 arranged on both sides of the leaf, especially in herbaceous plants 

 that are adapted to exposed habitats. In trees and in shade plants 

 the stomata are situated most often on the lower side, the upper 

 epidermis being free of them. 



To secure a normal progress in assimilation, the same degree 

 of saturation of the cells with water must be maintained. But 

 the heat of the sun's rays induces a continuous evaporation of 

 water from the leaf. Therefore, the leaf must have a dense net- 

 work of bundles carrying water, which communicate through the 

 petiole directly with the xylem tissues of the stem and the roots. 

 The translocation of the products of assimilation takes place 

 through the phloem of the same bundles, which is of great value 

 for the unimpeded formation of new products by photosynthesis. 

 Experiments have shown that when leaves are separated from 

 the plant and, therefore, are unable to remove the products of 

 assimilation, upon continuous exposure to light they will become 

 filled with these products and as a consequence assimilation will be 

 checked. Repletion with assimilates, as well as water deficiency 

 in the leaves, is frequently observed during the midday hours on 

 bright summer days, especially in dry and hot localities, for 

 instance, in the steppe zone. During these hours, the assimila- 

 tion process is perceptibly delayed, and frequently stopped. 

 Assimilation is most energetic during the early morning hours, 

 when the leaf still shows an abundant water supply and is at the 

 same time free from assimilates which during the night have been 

 translocated to other plant organs. 



12. Dependence of Assimilation on Light. Light and Shade 

 Plants. Growing of Plants in Artificial Light. — Light being the 

 source of energy for photosynthesis, it is natural that with increased 

 intensity of light the process also goes on at a higher rate. No 

 direct relationship between the amount of light and the rate of 

 decomposition of carbon dioxide, however, has been observed. If, 

 beginning with low values, the intensity of light is augmented by 

 the same magnitudes, assimilation will increase by progressively 

 smaller steps till a certain intensity of light is reached. Further 

 increase of light induces no increase of assimilation. If the depen- 

 dence of assimilation on the amount of light is represented graphi- 



