308 Scientific Proceedings, Royal Dublin Society. 



viz. starch. The mean molecular weight, 160, observed in experiment 37, is 

 to be explained in this way. The leaves under observation in this experi- 

 ment grew in a naturally overshadowed position, so that they had little 

 opportunity of storing starch. Screening them completely from light pre- 

 vented them from forming saccharose, and consequently the mean molecular 

 weight of their sap was run down in three days to the low figure of 160. 

 In marked contrast to this is the behaviour of most of the other screened 

 samples which, before they were artificially screened from light, were in an 

 exposed position. In them presumably', as has been pointed out, the stored 

 carbohydrates present were able by their conversion into maltose to keep up 

 the mean molecular weight. The low mean molecular weight of experiment 

 53, viz. 172, was likewise found in leaves which were naturally in an over- 

 shadowed position, after a continuance of cold and wet. In this case, too, 

 there is reason to assume an absence of starch and consequently, no source 

 of maltose, while the dark weather and shaded position prevented an active 

 formation of saccharose. In experiment 55, we have the record of the 

 lowest molecular weight for Syringa. The leaves had been cut off from 

 light for twelve days, so that their supply of starch must have been 

 approaching exhaustion. That it was not quite exhausted is shown by the 

 fact that a rise in osmotic pressure and molecular weight was observed when 

 some of the same sample of leaves were kept in the dark after gathering for 

 twenty-four hours. This observation rendered it probable that the low mean 

 molecular weight was due to retardation of the hydrolysis of starch in the 

 dark weather, and not to its exhaustion. 



This conclusion was rendered all the more probable by a subsequent 

 observation on the remaining leaves of the darkened shoots. From this it 

 appeared, although cut ofE from light as before, the mean molecular weight 

 rose subsequently on the return of the favourable conditions. Thus in 

 experiment 58, made nine days later on these leaves — that is, after they had 

 been in almost complete darkness for twenty-one days — the mean molecular 

 weight had risen to 249. The general supply of carbohydrate, however, being 

 much reduced by the long sojourn in the dark, the osmotic pressure had 

 fallen to 11-58 atmospheres. 



Leaves from the same branch, as was shown before, seem in each case 

 investigated to have approximately the same osmotic pressure when exposed 

 to the same conditions. The experiments on Eucalyptus globulus (Nos. 89 

 and 90, 91 and 92) and Pinus Laricio (Nos. 85 and 86) further illustrate this. 

 In both these cases, however, the leaves of the previous year showed a dis- 

 tinctly higher figure for the osmotic pressure of the sap than those of the 

 current year. 



