1909.] 



Vegetable Assimilation and Respiration. 



13 



He was led in consequence to take particular notice of apparent symmetry, 

 and concluded that it was impossible to find completely symmetrical leaves. 

 Not only was dissimilar distribution of veins sometimes impossible to avoid, 

 but differences in thickness were observed ; and these could not be taken 

 into account in selecting leaves for experiment.* Nevertheless he made no 

 measurements of the extent to which such differences could affect his 

 results. 



When Brown and Morrisf made the first actual determination of the 

 effect of such asymmetry, the difference was found to be considerable, and of 

 a higher order of magnitude than the errors of weighing and of measuring 

 area, the only errors mentioned and estimated by Sachs. They aimed at 

 repeating Sachs' procedure as closely as possible, and, using seven leaves of 

 Heliantlius annuus, found a difference of 0"43 gramme per square metre 

 (i.e. 11 per cent.) between the portions from right and left halves respec- 

 tively. Great care was taken in this experiment to choose pieces as similarly 

 veined as possible, which was made more easy by the use of glass templets. 



If fewer leaves and smaller areas were employed the error could be 

 considerably greater than that found by Brown and Morris using seven 

 leaves and 800 square centimetres. Brown and Escombe's results for single 

 leaves illustrate this. One of their Catalpa leaves showed a difference of 

 nearly 6 per cent. Their results may be quoted here for comparison with 

 others to be given later. 



Brown and Escombe's Asymmetry Determinations.^ 





Percentage difference. 





-3-9 









-4-3 









+ 2-3 









-5-7 









-0-7 









±3-4 









+ 2-5 



[misprinted 



+ 2-0] 





+ 1-4 



c 







-^•3 



-1-7] 





+ 0-4 









-1 -1 



-0-1] 



Mean of all (lie result? 



±2 -4 



c 



±2-2] 



* Loc. cit., p. 12. 



t Loc. cit., p. 625. 



1 Loc. cit., p. 60, Table IX. 



