1909.] 



Vegetable Assimilation and Respiration. 



51 



following percentage differences between the areas of the same half-leaves 

 before and after exposure in the chamber: — 





Per cent. 



(1) 



-3 12 



(2) 



+ 0-98 



(3) 



+ 0-36 



(■i) 



+ 0-14 



Average . 



. ±1 -1 



The conditions under which these figures were obtained may have differed 

 from those under which the actual assimilation experiments were performed. 

 The leaves for the latter were covered, overnight, with tinfoil, in order to 

 render them starch free. On the other hand, no mention is made of the like 

 treatment of the leaves for this experiment on change of area. If this pre- 

 caution were really omitted, the results may require a negative correction 

 owing to the less turgid condition of the leaves when first enclosed. 



Apart from this, the fact that one leaf had shrunk by as much as 3 per 

 cent, shows that an average difference much greater than 1"1 per cent, might 

 have occurred in some of their experiments. It is interesting in this con- 

 nection that leaves of Catalpa hignonioidcs were found by Halsted* to shrink 

 to an exceptional extent when dried for the herbarium ; and I have observed 

 that leaves of this species show very little sign of flaccidity, even after a 

 shrinkage in area of 4 per cent. 



Calculating first from their average change of area of I'l per cent., we 

 obtain a possible positive error of 4-9 milligrammes per square decimetre in 

 the total gain of dry weight. An average shrinkage of 3 per cent, would 

 mean an error of 13'5 milligrammes. Adding the asymmetry error, assuming 

 it to be positive, the total positive error becomes 14'3 or 22'9 milligrammes, 

 according to the degree of shrinkage assumed. 



This has to be divided by the number of Lours, which is not stated 

 Considering, nevertheless, that one of their dry weight experimentsf lasted 

 only Z\ hours, we may perhaps calculate the error per hour on the basis 

 of a 3-hour experiment, as well as of the 5-hour experiment for which their 

 own calculation was made. 



* Halsted, " Shrinkage of Leaves in Drying," ' Bull. Torrey Bot. Club.,' 1894, p. 129. 

 t Loc. cit., p. 56, Table VII. 



E 2 



