22 Mr. D. Thoday. Experimental Researchen on [June 11, 



Table XVI. 



Changes of dimensions from stage to stage as percentages of dimensions at Stage A. 





a — b. 



c — d. 



<'-/■ 



f-ff- 



Decrease in 



area : 

 estimated. 



ZjOBS of 



weight. 



A— B 



2-9 



2 -8 



1-6 



0-4 



4 



H-7 



B— C 



0-9 



0-9 



1 1 



0-3 



1 -5 



V-8 



C— D 



0-6 



0-9 



3-2 



0-7 



2-5 



3-5 



A— D 



4-4 



4-6 



5-9 



1 -4 



8 



23-0 



As far as external appearance went, Stage B was not readily distinguishable 

 from Stage A, yet in passing from A to B the leaf had lost over 10 per cent, 

 of its weight by evaporation, and had diminished in area by about 4 per cent. 

 Passing on to C, a condition occurring quite commonly on the plant in the 

 open air, there was a further loss of water of 8 per cent., and a much smaller 

 change in area of about 1"5 per cent. From C to D the loss of water of only 

 3*5 per cent of the original fresh weight was accompanied hy a rather greater 

 loss of area of about 2-5 per cent., due to a greater shrinkage in length 

 at this stage, and this brought the leaf to what may l)e described as a 

 condition of complete flaccidity, such as occurs occasionally in hot sunny 

 weather. 



Summing up the changes, we find that between maximum turgidity and 

 complete flaccidity the leaf decreased in weight by 23 per cent, through loss 

 of water, and shrank by about 8 per cent, of its area. 



Change of Area under Natural Conditions. — Helianthus annuus. 



A large number of observations of a similar character have been made 

 which show that the liability to slirinkage varies fioni leaf to leaf, with the 

 character of the mesophyll itself, and with the nature of the epidermis and 

 of the supporting venation. Most interesting of all is a series of observations 

 on Heliantliits aiinvvs made (hiring tlu; period of tine weather at the end of 

 July and beginning of August of 1908. 



This plant is of especial interest, beciuisc it was witli its leaves that Snclis 

 obtained some of his highest values for the rate of assimilation. He com- 

 menced his experiment with atUtched leaves at 5 a.m., and continued it till 

 ',i r.M., and my original object was to determine by direct measurement 

 the change of dimensions which takes place between those hours on a bright 

 sunny day. 



Altliough the obsfi vatioiis were begun |Soon after the ground had I)een 



