298 Scientific Proceedings^ Royal Dublin Society. 



A similar series is exhibited in tlie following table : — 



Table XI. 



Syringa vulgaris : leaves. 



In the first case the shading reduced the osmotic pressure by 2*5 

 atmospheres, while the mean molecular weight remained sensibly the same at 

 the end of six days. If monosaccharides were used principally in respiration, 

 no fall in the mean molecular weight should be expected to accompany the 

 fall in osmotic pressure. The second pair of observations offer in this respect 

 a sharp contrast. During two days of continuous rain the osmotic pressure of 

 the darkened sample has been greatly reduced, probably in a small degree 

 owing to dilution, and principally owing to the fact that little or no carbo- 

 hydrates liave been brought into solution, or have been translocated from the 

 exposed leaves. Tlie failure of the supply from the exposed leaves is to be 

 attributed to the unfavourable conditions for carbon assimilation on the two 

 days previous to the determination, which probably did not permit more 

 carbohj'drates to be formed than were utilized in fclie leaves wliere they were 

 produced. The startling reduction in the mean molecular weight of the solutes 

 in the shaded leaves is also to be explained by the exhaustion of the dissolved 

 carbohydrates in those leaves, and their consequent percentage reduction in 

 the solutes. The (/3) sample, on the other hand, has a high osmotic pressure 

 and mean molecular weight, because the assimilation on the bright morning 

 when the leaves were gathered has more than compensated for the reduction 

 of assimilation of the two previous days. 



