ON THE ACTION OF DETACHED LEAVES OF PLANTS. 171 



to a slight degree of pressure at this period, I found that a considerable 

 quantity of liquid, being apparently the true sap of the tree, issued out 

 laterally through the medullary processes, as well as longitudinally 

 through the cellular substance of the alburnum; but the tubes of it 

 continued empty, and their position was marked by depressions of the 

 surface of the extravasated fluid. I endeavoured to ascertain what 

 proportion of water a given quantity of the alburnum of such oak trees 

 contained at this period, and I found that 1000 parts lost by drying only 

 371 parts ; which is not more than the weight of the water that the 

 cellular substance appears capable of containing, entirely independent of 

 the tubes. That the tubes, nevertheless, are not always empty, but that 

 they act at other periods of the year as reservoirs for the sap, I have given 

 an opinion in a former communication ; and I am now in possession of 

 facts which prove them to perform this office, even in the heart wood, 

 to a much greater extent than 1 had ever at any former period suspected ; 

 and which incline me to believe that the durability of the heart wood, as 

 well as of the alburnum of the oak, will be found to depend to a great 

 extent upon the period in which the tree is felled. 



