32 



Mr. D. Thoday. Experimental Researches on [June 11, 



four coruers of a 100 sq. cm. templet laid upon the leaf. A decrease in the enclosed area 

 of 2"5 per cent, was observed between 8 a.m. and 1.30 p.m. Such a change during Sachs' 

 experiment would have meant an over-estimate of the gain per square metre per hour of 

 0"16 gramme, i.e. 33 per cent., the " corrected " result being 0'49 gramme. 



The relative magnitude of the error introduced by a given change of area is greater 

 the smaller the total photo.synthetic increase, the shorter the time of experiment, and 

 the greater the average dry weight of the jMirticular leaves used. On each of these 

 considerations Sachs' experiment with attached leaves of Cvmrhita pepo* is liable to have 

 been vitiated by shrinkage errors to a far greater extent than those with Helianthvf. and 

 Rheum. 



The average dry weight per unit of area of the Cumrhita leaves was greater than that 

 of his Helianthus leaves (56 grammes as compared with 44 grammes per square metre), 

 and as the experiment lasted only three hours, the total increase to be expected was 

 comparatively small, and any error was distributed over 3 hours instead of 10 as in 

 the Helianthus experiment. In addition to this, the second halves were taken at noon, 

 just the time when, as a rule, the shrinkage is most pronounced. 



Sachs commenced his experiment at 9 a.m. on August 21, the temperatui-e rising from 

 18° C. at that hour to 24° C. at noon, and sunshine continiiing throughout. He found an 

 increase of 0'68 gramme per square metre per hour. 



Under similar (not extreme) conditions I have observed a shrinkage in area of 3'6 per 

 cent, in a leaf of Cucurbita. A change of area by this amount would mean a corre- 

 sponding ap])arent increase of 0'7 gramme per square metre per hour, so that the uhole of 

 the ohseroed difference might have been due to shrinkage. 



From these considerations it is clear tliat the results of all Sachs' experi- 

 ments with leaves attached to the plant are open to doubt ; they may all be 

 too high to a greater or less degree. 



Sachs' Experimmt vnth iJetaclied. Leaves q/ Helianthus annuus. 



The high result which Sachs obtained for detached, leavesf suggests the 

 possibility that here, too, shriidvage may have introduced a considerable 

 error, especially as Brown and Morris's results for detached leaves^ were nruch 

 smaller. When, however, the details of the experiment are examined, this 

 seems less ]irol table. 



Sachs cut eight leaves of Jfclia)Uhm anmms at 5 A.M. and set them in 

 water in the laljoratory in dull light till 8 A.M. He exposed tlie experi- 

 mental halves in the garden with their stalks in water from 8 a.m. till 

 2.45 P.M. Finding then that they were flaccid, he stopped the experiment 

 and immersed them in water for half an hour to make them turgescent. 



The following experiment shows that this trciilinent proltably eliminated 

 shrinkage : — 



Three leaves were dctaclKid and li ft for an Ikuu- in a (bill light with [\w\v stiilks in 

 watei'. After nii'asnrcnicnt of (lie ilistanccs lietween nuii'ks on the leaves, in the way 



* Loc. eit., \). 23. 

 + Lnc. eit , p. 25. 

 X hoc. cit., p. (!28. 



