440 Mr. D. Tlioday. Experimental Researches on [Mar. 1, 



It was assumed by Sachs that leaves were in the same condition, in so far 

 as their rate of assimilation was affected, whether on the plant or detached 

 from it. Thus he regarded the fact that attached leaves of H. annuus 

 showed a much smaller rate of increase of dry weight than detached leaves 

 as due entirely to the translocation of part of the products of photosynthesis 

 from the attached leaves into other parts of the plant. In support of this 

 view he adduces, from his own results, the concordance of the rate of 

 increase of detached leaves, 16'5 milligrammes per square decimetre per hour, 

 with the value obtained when the rate of translocation from attached leaves 

 at night, 9'6 milligrammes, is added to their rate of increase during the day, 

 9"! milligrammes. | 



Brown and Escombe* have, however, pointed out that it is not justifiable 

 in the absence of direct experimental evidence to assume that leaves on the 

 plant fix as much carbon dioxide as detached leaves. Even under identical 

 external conditions of temperature, illumination, humidity, and carbon 

 dioxide content of the air, the condition of the stomata may be different 

 after detachment. 



With the object of finding whether such differences of stomatal aperture 

 actually occur they made measurements of the amounts of carbon dioxide 

 diffusing into the leaf during the same time and under the same conditions 

 in two similar leaves of Catalpa, one still on the tree, the other detached. 

 Two such experiments both showed a considerable excess in favour of the 

 detached leaf. The inference is that the stomata of the latter were more 

 widely 0})ened. 



It is clear that these results establish their contention that the possibility 

 of such differences must be considered in comparing the results of 

 experiments witli attached and detached leaves ; but they went further than 

 this : they pointed out that, as they believed, the rate of increase of attached 

 leaves of H. annuun bears a relation to the rate of increase of detached 

 leaves comparable with that which they found in Catalpa. Hence they 

 maintained that in the case of Helianthus also the difference is to be 

 attributed to the wider opening of the stomata of detached leaves, and 

 .denied altogether Sachs' assumption that translocation and assimilation 

 proceed concurrently. 



On the other hand, it must l)e remembered that, although not fully 

 justified by tlie particular results to wliich lie apjdied it, Sachs' assumption 

 was not purely gratuitous, but was founded upon numerous observations 

 made upon leaves at different hours of tlie day under varying external 

 conditions by the iodine starch test. 



* Loc. cit., p. 51. 



