VEGETABLE ABSORPTION. 25 



velopment, which usually takes place when the genial 

 warmth of spring has penetrated heyond the surface, and 

 expanded the fibres and vessels of the plant, there arises an 

 urgent demand for nourishment, which the roots arc active- 

 ly employed in supplying. As the leaves are not yet com- 

 pleted, tl\p sap is at first applied to purposes somewhat dif- 

 ferent from those it is destined to fulfil at a more advanced 

 period, when it has to nourish the fully expanded orgp.ns: 

 this fluid has, accordingly, received a distinct appellation, 

 being termed the nii7\sling sap. Instead of rising through 

 the alburnum, the nursling sap ascends through the inner- 

 most circle of wood, or that which is immediately contigu- 

 ous to the pith, and is thence transmitted, by unknown 

 channels, through the several layers of wood, till it reaches 

 the buds, which it is to supply with nourishment. During 

 this circuitous passage, it probably undergoes a certain de- 

 gree of elaboration, fitting it for the ofilce which it has to 

 perform: it apparently combines with some nutriment, which 

 had been previously deposited in the plant, and which it 

 again dissolves; and thus becoming assimilated, is in a state 

 proper to be incorporated with the new organization that is 

 developing. This nursling sap, provided for the nourish- 

 ment of the young buds, has been compared to the milk of 

 animals, which is prepared for a similar purpose at those 

 times only when nutriment is required for the rearing of 

 their young. 



Several opinions have been entertained with regard to the 

 channels through which the sap is conveyed in its ascent 

 along the stem, and in its passage to its ultimate destination. 

 Many observations tend to show, that, in ordinary circum- 

 stances, it is not transmitted through any of the distinguisha- 

 ble vessels of the plant: for most of these, in their natural 

 state, are found to contain only air. The sap must, therefore, 

 either traverse the cells themselves, or pass along the inter- 

 cellular spaces. That the latter is the course it takes, is the 

 opinion of De Candollc, who adduces a variety of arguments 

 in its support. The sap, he observes, is found to rise equally 



Vol. II. 4 



