5g CIRCULATION OF THE SAP. 



formed. The plants continued some time in health, and 

 during that period the tubers continued to grow, deriving 

 their nutriment, as I conclude, from the leaves, by an inverted 

 action of the alburnous vesseb. The tubers, however, by 

 no means attained their natural size, partly owing to the de- 

 clining health of the plant, and partly to the stagnation of a 

 portion of the true sap above the decorticated space. 

 Probability that The fluid contained in the leaf has not, however, been 



the sap de- proved, in any of the preceding experiments, to pass down- 



scends through ' , , , • , i , , 



the alburnum wards through the decorticated space, and to be subsequently 



where the discharged into the bark below it: but I have proved with 

 corticated.^ amputated branches of different species of trees, that the 

 water which their leaves absorb, when immersed in that 

 fluid, will be carried downwards by the alburnum, and con- 

 veyed into a portion of bark below the decorticated space; 

 and that the insulated bark will be preserved alive and moist 

 during several days ;* and if the moisture absorbed by a 

 leaf can be thus transferred, it appears extremely probable 

 that the true sap will pass through the same channel. This 

 power in the alburnum to carry fluids in different directions 

 probably answers very important purposes in hot climates, 

 where the dews are abundant and the soil very dry ; for 

 the moisture the dews afford may thus be conveyed to the 

 extremities of the roots : and Hales has proved that the 

 leaves absorb most when placed in humid air; and that 

 the sap descends, either through the bark or alburnum, dur- 

 ing the night. 

 Th" ■ "rted ^^ ^^^ inverted action of the alburnous vessels in the de- 

 action of the ' corticated space be admitted, it is not difficult to explain the 



alburnum ex- (^^use why some degree of growth takes place below such 



plains the tacts .-^ i n ,, 



in the experi- decorticated spaces on the stems of trees ; and why a small 



ments of Hales portion of bark and wood is generated on tlie lower lip of 



'the wound. A considerable portion of the descending true 



' sap certainly stagnates above the wound, and of that which 



escapes into the bark below it, the greater part is probably 



carried towards, and into, the roots ; where it preserves life, 



^ This experiment does not succeed till the leaf has attained 

 its fyll growth and maturity, and the alburnum of the annual 

 flioot Us perfect organization. 



