192 PLANT PHYSIOLOGY 



the aeration of the leaf. The stomata are usually arranged on 

 both sides of the leaf 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 bein^ 

 without them. 



Having reached the surface of the chlorophyll-bearing cells, 

 carbon dioxide dissolves in the water with which the cell walls 

 are impregnated and then diffuses in the form of H2CO3 or, more 

 correctly speaking, in the form of bicarbonate, HCOa" into the 

 protoplasm toward the green plastids, which absorb it. 



The absorption and assimilation of carbon dioxide are favored 

 by the fact that the chlorophyll, which according to Willstatter's 

 scheme combines chemically with carbon dioxide, is situated on 

 the surface of the chloroplast. 



The total surface area of the chloroplasts is very great indeed. 

 According to Schroder's calculations (1926), the total surface 

 area of the chloroplasts of a beech tree 100 years old is about 

 200 times the area of the leaves. Such an enormous surface for 

 absorption is of great significance for photosynthesis; for the 

 plants are compelled to obtain the carbon dioxide required from 

 the atmosphere, which contains it in quantities less than 0.5 mg. 

 per liter, or 3 parts in 10,000 of air. To secure the normal 

 progress of assimilation, the cell walls must be sufficiently 

 saturated with water. Since the sun's rays induce a continuous 

 evaporation of water from the leaf, to supplement these losses 

 the leaf possesses a dense network of bundles that carry water. 

 They communicate through the petiole directly with the xylem 

 tissues of the stem and the roots. The phloem of these bundles 

 serves for the translocation of the products of assimilation, whicli 

 is likewise of major importance for uninterrupted photosynthesis. 

 Experiments show that leaves separated from the plant and thus 

 deprived of the possibiHty of translocating the products of 

 assimilation, with continuous illumination are finally so filled 

 with them that further photosynthesis is impeded. 



The translocation of assimilates from their place of orighi, 

 the chlorophyll-bearing cells of the leaf, is favored by the shape 

 of the cells. They are elongated perpendicularly to the surface 

 of the leaf in the palisade tissue. At their lower ends, the 

 pahsade cells join special collecting cells, which in their turn 

 oommunicate with those of the phloem. Thus a continuous 



