ASCENT OF THE SA1». 21 



recently formed wood, and not througli the bark, as 

 was at one time believed. 



The course of the sap, however, varies under 

 different circumstances, and at different epochs of 

 vegetation. At the period when the young buds are 

 preparing for their developement, which usually 

 takes place when the genial warmth of spring has 

 penetrated beyond the surface, and expanded the 

 fibres and vessels of the plant, there arises an 

 urgent demand for nourishment, which the roots 

 are actively employed in supplying. As the leaves 

 are not yet completed, the sap is at first applied to 

 purposes somewhat different from those it is des- 

 tined to fulfil at a more advanced period, when it 

 has to nourish the fully expanded organs : this fluid 

 has, accordingly, received a distinct appellation, 

 being termed the tiursUuo- sap. Instead of rising 

 through the alburnum, the nursling sap ascends 

 through the innermost circle of wood, or that which 

 is immediately contiguous to the pith, and is thence 

 transmitted, by unknown channels, through the se- 

 veral layers of wood, till it reaches the buds, which 

 it is to supply with nourishment. During this cir- 

 cuitous passage, it probably undergoes a certain 

 degree of elaboration, fitting it for the olhce which 

 it has to perform : it apparently combines with 

 some nutriment, which had been previously de- 

 posited 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 

 nourishment 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 re(iuired for the rearing of tiitir young. 



