1910.] 



Vegetable Assimilation atid Respiration. 



441 



Moreover, Brown and Morris' dry weight experiments with Helianthus 

 support Sachs rather than Brown and Escombe. That they should liave 

 appeared to harmonise so well with the Cataljxi experiments quoted above 

 was due to a misquotation of the result of one of Brown and Morris' 

 experiments, in which the attached leaf is quoted as showing an increase of 

 4"G milligrammes per square decimetre per hour,* but according to the original 

 only increased by 4"6 milligrammes in nine hours, an amount so small that it 

 was neglected as coming within the errors of experiment.f 



Thus while the detached leaves increased in weight at the rate of 10 milli- 

 grammes per hour, the attached leaves showed no increase at all. There is 

 no reason whatever for supposing that the stomata of all the eight attached 

 leaves were completely closed during the whole period of 12 hours covered 

 by the experiment.^ It therefore remains impossible to doubt that a 

 considerable quantity of substances was translocated from the leaves during 

 that period. In no other way can we adequately account for the lack of 

 accumulation of the products of photosynthesis. 



The relation between the stomatal apertures of the attached and detached 

 leaves in this experiment of Brown and Morris is another question, and one 

 which cannot be decided in the absence of the necessary data. It may not 

 be unprofitable, nevertheless, to consider how far the results of Brown and 

 Escombe's experiments with Catalpa are likely to be applicable to other 

 leaves. To do this it is necessary to inquire into the conditions which 

 produced the differences of stomatal aperture which they found. 



The Effect of Detachment from the Plant upon the Supply of Water to 

 Leaves. — The difference in the osmotic quality of the water supply has been 

 suggested as a possible factor ; attached leaves receive water charged with 

 mineral salts from the soil, whereas detached leaves are supplied with 

 distilled water.§ It seems, however, unlikely that the concentration of 

 mineral salts in the sap would ever be great enough to produce an 

 appreciable diminution of turgor in the leaf, or to induce any movement on 

 the part of the stomata. 



Of far greater significance is the change, accompanying detachment, in the 

 mechanical conditions which govern the water supply. During vigorous 

 transpiration a considerable negative pressure has been shown to exist in the 

 water channels of the stem, and this means, necessarily that considerable 



* Brown and Escombe, loc. cit., p. 50, Table IV. 

 t Brown and Morris, loc. cit.^ pp. 626 — 7. 



X The day was " dull " : it is therefore unlikely that transpiration was rapid, and so 

 any large difference of stomatal aperture between attached and detached leave cannot 

 reasonably be assumed. 



§ Brown and Escombe, loc. cit. 



