218 Quantitative Analysis of Plant Growth 



loss by respiration and for the uptake of salts, which for this period shows 

 an average of 6-5 per cent., the real assimilation per 50 cm. 2 per hour is 

 found to be 4-5 mgs. C0 2 per cm. 2 per hour when making the assumption 

 that all light is equally efficient, and when light below one-fifth total 

 sunlight is limiting the value becomes 5-3. 



Muller(i8) has shown by the "half-leaf" method that the apparent 

 assimilation 1 of the leaves of monocotyledons as a class is definitely lower 

 than that of dicotyledons. The values obtained for monocotyledons 

 average about 9 mgs. C0 2 per 50 cm. 2 per hour 2 , the highest being 

 14 mgs. for Musa and the lowest 6-1 gms. for Cypripedium. Maize was 

 not used in Miiller's experiments. It should be noted that the values 

 given by Mliller are average ones determined under varying conditions 

 of illumination, the leaf being left on the plant and translocation being 

 allowed for. It is clear therefore that the value obtained for maize from 

 Ivreusler's results, that is, the value obtained for what we propose to 

 call the "growth" method, is distinctly lower than that for mono- 

 cotyledons as a class determined by the "half-leaf" method. Miiller's 

 figures, however, show that in monocotyledons most of the assimilation 

 takes place during the first few hours of illumination and that the rate 

 falls off considerably later. Since his figures are obtained from experi- 

 ments of only six hours' duration one would expect the value 9 to be 

 much reduced if the experiment had lasted for the whole day. 



As Boysen-Jensen(i) came to the conclusion that the values obtained 

 for assimilation by the "gasometric" method give a more accurate 

 value of assimilation under natural conditions than does the " half-leaf " 

 method and since he quotes the results of growth experiments by 

 Weber (21) as confirmatory of this view we think it useful to reconsider 

 this question in the light of other results from growth experiments. 



Unfortunately we have no figures for the assimilation of maize 

 determined by the "gasometric" method under natural conditions of 

 C0 2 -supply to compare with the results from growth experiments. We 

 can, however, use the results of some growth experiments carried out 

 by ourselves on Helianthus annuus. Further, Helianthus is a plant which 

 has received a good deal of attention, both "gasometric" and "half- 

 leaf." The results are given in Table X. 



It will be seen that the results obtained for assimilation with Helian- 

 thus by the "gasometric" method are of the same order as the results 



1 Miiller's figures do not take into account the respiration of the leaf; this, however 

 would make very little difference. 



- The value for C0 2 is calculated as 8/5 of the increase in dry-weight. 



