1909.] 



Vegetable Assimilation and Respiration. 



19 



inverse ratio of the square root of the number of leaves used. In Brown and 

 Morris's example, for instance, the asymmetry of seven leaves of Helianthus 

 annmis taken together was I'l per cent. Some similar results for Helianthus 

 tuherosus and Cherry Laurel will be found in the tables, though with leaves 

 of the latter the error may still be high (e//. 2-7 per cent, for six leaves in 

 Experiment 10). Further discussion of these results will be postponed till 

 the errors as a whole are discussed.* 



Attention may be called here to the fact that all asymmetry determinations 

 are affected by errors classed above as errors of technique ; but, as will be 

 shown later,f these are as a rule relatively small, and account for only a 

 small part of the differences shown in the tables. 



Section V. — On Change of Area during Experiment. 



Brown and Escombe's conclusion, that the results given by the dry-weight 

 method are too high, was based not only upon their direct test with Catalpa, 

 but also on a consideration of the general high level of the results obtained 

 by themselves and others when using it. In particular, the value found by 

 Sachs for the rate of assimilation in detached leaves of Helianthus annuus 

 was far higher than any which they observed in their experimental chamber.t 



If the method itself is really responsible for these high values, some change, 

 related in a definite way to the conditions of experiment, must take place in 

 the experimental lialf-leaf. 



Of such possible changes, that of water fixation suggested by Brown and 

 Escombe has been disposed of in Section III, where it is shown that Sachs' 

 results would have been approximately the same had he measured carbon 

 content instead of dry weight, all other details of his method remaining the 

 same. 



On the other hand, shrinkage in area due to loss of water could produce 

 errors consistently in a positive direction, for the conditions under which a 

 leaf might be expected to assimilate most rapidly are just those conditions 

 which are favourable to increased evaporation. 



For instance, Broocks§ observed a much greater increase of dry weight in 



fully insolated leaves of the Sugar Beet than in leaves shaded from direct 



sunlight. If the more rapid evaporation to be expected in the sun resulted 



in appreciable shrinkage, the whole or part of this difference might be only 



* Section VIII, p. 48. 

 t Sections VI and VII. 



I Sachs' result was 16 milligramuies per square decimetre per hour {loc. cit., p. 25). The 

 highest rate found by Brown and Escombe for H. annuus was 5'o milligrammes {loc. cit., 

 .p. 44). 



§ Loc. cit., p. 17. 



c 2 



