1910.] Vegetable Assimilation and Respiration. 



445 



which would have been observed, apart from true photosynthetic increase, 

 if the leaves had changed in area as much as leaves of Heliajitlmis annuus 

 have been observed to do.* This correction for shrinkage would be extreme, 

 so far as my observations go, as I have not found that leaves of the sugar 

 beet shrink so much as those of Heliantlius.] Even after this large correction 

 for shrinkage the change in the curve is but slight, either in form or in the 

 magnitude of the quantities it represents.^ 



The chief peculiarity common to both Broocks' curves strikes the attention 

 at once. Until noon they indicate a rapid increase of dry weight, but at 

 noon their direction is suddenly reversed, and from that time they represent 

 instead a decrease. 



Another interesting feature is the continuity on both days between the 

 curves of diminishing weight in the afternoon and from 6 p.m. to midnight. 

 The obvious inference is that the same interpretation holds for the entire 

 period of 12 hours and we may conclude that translocation took place at a 

 nearly uniform rate throughout the period in question. 



The change in the direction of the curve at noon shows that assimilation 

 practically ceased at that time ; a conclusion which also follows from the 

 interpretation just given of the continuity of the latter part of the curve. It 

 appears probable that this cessation was due to the closing of the stomata, for 

 no other explanation would account for its suddenness. 



This closure might have been brought about by loss of turgor in full 

 insolation. It appears more probable, however, that it followed upon 

 repletion of the leaf tissues with the products of photosynthesis ; for it will 

 be observed that in neither experiment had the leaves reached their original 

 weight by the next morning, so that by the time they checked their assimila- 

 tion they had already formed more products than could be translocated before 

 assimilation recommended. 



An interesting difference is to be noticed between the falling portions of 

 the two curves. In the experiment of August 18-19, the translocation was 

 rapid and uniform till midnight, but by 6 A.M. the weight had increased once 

 more : the morning was bright, the sky clear, and assimilation had begun. 



The morning of September 5, on the other hand, was dull and cold, and the 

 curve shows no sign of rising. It is also to be observed that in this 

 experiment translocation is less rapid than it had been three weeks earlier, 



* Thoday, loc. cit., p. 24. 

 t Cf. ibid., p. 34. 



J Similar results are to be found among those described by Miiller in ' Jahrb. f. wiss. 

 Bot.,' 1904, vol. 40. See Expt. 10, p. 454, Rwnex ; Expt. 14, p. 456, Rumex ; and 

 Expt. 21, p. 462, Verbascum. 



