1910.] Vegetable Assimilation and Respiration. 433 



from the rate of increase found by Brown and Morris (10 milligrammes per 

 hour per square decimetre), as it appeared at the time to be a safer value than 

 that of Sachs. 



It now becomes of interest to recalculate this waste from the much higher 

 rate of increase which has now been established. 



Blackman and Matthaei estimated the total radiation available on a 

 representative sunny day as sufficient for the assimilation of 53 milligrammes 

 of CO2 per hour per 50 sq. cm. of leaf surface. This is equivalent to the 

 formation of 65 milligrammes of starch per square decimetre per hour. An 

 increase of dry weight of 18 milligrammes per hour is about 30 per cent, of 

 this possible 65 milligrammes, and there is thus a waste of about 70 2)er cent, of 

 the energy vjhich could he utilised ; whereas Brown and Morris' figure 

 represents a waste of over 80 per cent. Since there is some evidence for still 

 higher rates of assimilation the waste may be at times reduced to 65 per cent, 

 or less. 



(iii) Brown and JEscombe's Experiments on Leaves of H. annuus.* 



The results which Brown and Escombe obtained with leaves of H. annuus 

 enclosed in their experimental case are almost of a lower order of magnitude 

 than those described here. The highest rate of assimilation which they 

 observed corresponds to an increase in dry weight of 5*5 milligrammes per 

 square decimetre per hour. This must be regarded as demonstrating that the 

 conditions holding in their experiments were widely different from those 

 occurring in the open air. 



In the first place they found it impossible to carry out experiments 

 successfully in full sunshine as the temperature was injuriously high. The 

 resultant gaseous exchanges were reduced practically to nil, owing partly to 

 the increased intensity of respiration, partly perhaps to the reduction of 

 assimilation by the iojurious action of the high temperature. 



On the other hand, in their experiments in moderated illumination 

 Brown and Escombe considered that the leaves were at no disadvantage 

 compared with leaves under natural conditions ; for they found that the 

 intensity of the illumination under their canvas screen in sunlight was greatly 

 in excess of the capacity of leaves of Tropceolum majus for assimilation in 

 ordinary air. 



That they were justified in generalising, from experiments with a single 

 species, as to the sufficiency of the supply of energy for the rates of assimila- 

 tion with which they were dealing, follows from Blackman and Matthaei's 

 demonstration that experiments with leaves as different as those of Cherry 



* Loc. cit., Table I. 



