16 



THE SYNTHESIS OF CARBOHYDRATES 



Water Content per 



ioo sq. cm. of Leaf 



Area in gms. 



•043 

 •053 

 •058 



•no 



•122 



Real Assimilation per 



100 sq. cm. of Leaf 



Area in gms. of C0 2 . 



OOOI8 



00034 



00043 



OOI30 



OOI55 



Further, leaves with a lower water content, e.g. Abntilon 

 Darwini, have a greater rate of assimilation per unit area 

 than those of species with a higher water content, e.g. Spar- 

 mannia africana and Cineraria stellata. Also it was found 

 that the ratios between the decrease in carbon assimilation 

 and the decrease in the water content for different plants is 

 not the same ; in other words, there is a specificity in this 

 relationship. 



Carbon Dioxide. — In normal conditions, the carbon di- 

 oxide for carbon assimilation is derived from the atmos- 

 phere and to a lesser extent from the products of respiration.* 

 The amount of respiratory carbon dioxide is conditioned mainly 

 by the temperature and may be equal to half the possible inflow 

 from the atmosphere at the higher temperatures possible in 

 laboratory experiments. The entry of atmospheric carbon 

 dioxide into the plant is either through the intact epidermis, 

 as in those plants which like certain aquatics lack stomates, 

 or mainly through the stomates and, to a much lesser and 

 negligible extent, provided the amount of carbon dioxide is 

 not unduly increased, through the unbroken epidermis. This 

 cuticular path of gaseous interchange once was thought to be 

 the main route, but the work of F. F. Blackman f and of Brown 

 and Escombe \ has shown that the stomates are the important 

 paths. F. F. Blackman found by direct measurement that 

 the degree of gaseous interchange was proportionate to the 



* In abnormal ( conditions it appears that plants can make use of 

 carbon dioxide from the soil. Pollacci (" Atti. Inst. Bot. Univ. Pavia," 

 J 9i7. '7< 3°) found that plants grown in soil rich in humus or in water 

 culture enriched with carbon dioxide could form starch and increase in dry 

 weight notwithstanding the fact that their aerial parts were in an atmos- 

 phere freed from carbon dioxide. The assimilation, however, was not 

 sufficient for normal growth. 



t Blackman : " Phil. Trans. Roy. Soc," B, 1895, 186, 485, 503. 



\ Brown and Escombe: id., 1900, 193, 223. 



