00 PRESIDENTIAL ADDKESS SECTION C. 



In this climate the plants of the open veld are daily exposed 

 during the greater part of the year to the full blaze of the sun. 

 Now both Blackman and Willstatter have shown that dii*ect sun- 

 light is far more intense than is necessary for the assimilation by 

 normal green leaves of all the CO2 that can reach the chloroplasts 

 fi'om the atmosphere, and even for the maximal assimilation 

 possible, when the supply of CO.. is not limited, at a temperature 

 higlier than 25°G. 



Blackman and Matthaei's calculations of the photosynthetic 

 value of full sunlight, though they are based on the doubtful 

 assumption that the rate of assimilation is proportional to the 

 intensity of the light, when temperature and COo supply are in 

 excess, nevertheless gives an approximate basis for calculating 

 the proportion of direct sunlight which is used under the most 

 favourable conditions in nature. 



The highest figures for the rate of assimilation in the open 

 air have been obtained by Sachs* for Hcliantlius anmias, which 



1 confirmedi in 1910, and for the cotton plant in Egypt by Balls | 

 whose results are somewhat higher. These figures represent the 

 utilisation of about 30 per cent, to 35 per cent, of direct sunlight. 

 ]^Iost leaves utilise not more than 10 to 20 per cent, calculated 

 on the same basis. 



Willstatter and StoU have shown, moreover, that the normal 

 green leaf contains far more chlorophyll than is necessary to 

 enable the chloroplasts to assimilate up to their full capacity in 

 direct sunlight, for yellow varieties of leaves with far less chloro- 

 phyll assimilate nearly as rapidly. 



They suggest, therefore, chat the large chlorophyll content of 

 the normal leaf is an adaptation to lower intensities of light. 

 From this point of view it is interesting that shade leaves contain 

 more chlorophyll, bulk for bulk, than sim leaves of the same 

 species. 



Yellow varieties of leaves stand in marked contrast to green 

 leaves, for even direct sunlight is not intense enough to enable 

 the chloroplasts to assimilate at the maxinuim rate made possible 

 by the temperature, and even with ordinary atmospheric air the 

 intensity of light required to assimilate all the COo diffusing to 

 the chloroplasts will be very much greater for yellow than for 

 green leaves. We may, in fact, regard yellow leaves as light 

 demanders par excellence . 



Now, if we compare tlie generality of plants in the more 

 open types of vegetation, and even the relative light demanders of 

 forest fringes, with the vegetation typical of our forests, we can- 

 not bub be struck by the depth of the green colour of the forest 

 trees and the lighter colour of the light demanding plants. § 



* Arbeit, d. Bot. Inst in Wurzbuis iii, 1883, p. 19. 

 tProc. Rov. Soc. B. 82, 1910. p. 121. 

 X Nature. 89. 1912. p. 555. 

 § LpiHfidfurhon adtfrnidpnA and (UiJen'm (ijr'xnna may he cited as extreme 



examples. 



