42 THE SYNTHESIS OF CARBOHYDRATES 



In this connection it may be mentioned that Henrici * 

 concluded that ionized air accelerates carbon assimilation of 

 lowland plants, when the illumination is low, but not when the 

 light is of medium intensity ; in strong light the ionized air 

 retards the process. In alpine plants, an acceleration was 

 observed both in weak and in medium light, whilst in strong 

 illumination neither increase nor decrease obtained. In some 

 examples the ratio of the rate of carbon assimilation in ionized 

 air to the rate in de-ionized air was as 5 is to I. This work is 

 criticized by Whimster f who points out that the results are 

 not based on the means of a number of experiments, wherefore 

 it is impossible to calculate the probable error. Further, the 

 figures given by Henrici represent apparent assimilation, no 

 allowance being made for respiration which, as will be seen 

 later, is markedly affected by an ionized atmosphere. Using 

 the leaf of Pelargonium zonule, Whimster found no further 

 effect on the rate of carbon assimilation under the action of 

 ionized air than a 3 per cent, depression, a value of no sig- 

 nificance ; the real assimilation, in which the increased rate of 

 respiration is taken into consideration, showed a 5 per cent, 

 increase which again is below the significant level. 



INTERNAL FACTORS. 



Chlorophyll. J — Plant physiologists for long have recog- 

 nized that the intensity of carbon assimilation must be de- 

 pendent on the chlorophyll and its amount ; it is, however, 

 but recently that the problems involved have been critically 

 examined. Irving, § who used the leaf's carbon dioxide of 

 respiration in her experiments, found by gasometric methods 

 that etiolated leaves, either when they are orange-yellow or 

 when they have attained a considerable degree of greenness, 

 do not possess any appreciable power of synthesizing carbon 

 dioxide. If there be any photosynthetic activity, it cannot 

 be greater than one-tenth part of respiration nor come within 



* Henrici : " Arch. Sci. Phys. Nat.," 1921, 3, 276. 

 t Whimster : " Ann. Bot.," 1927, 41, 357. 



X A general account of chlorophyll, its chemistry and constitution, will 

 be found in Vol. I. 



§ Irving : " Ann. Bot.," 1910, 24,, 805. 



