280 VEGETABLE 



nature supplies are always such as are suitable to the organs 

 that are to receive them. 



The fluid which is taken up by the roots, and which, as we 

 have seen, consists chiefly of water, holding in solution at- 

 mospheric air, together with various saline and earthy ingre- 

 dients necessary for the nourishment of the plant, is in a per- 

 fectly crude state. It rises in the stem of the plant, under- 

 going scarcely any perceptible change in its ascent ; and is 

 in this state conducted to the leaves, where it is to experience 

 various and important modifications. By causing the roots to 

 imbibe colored liquids, the general course of the sap has been 

 traced with tolerable accuracy, and it is found to traverse 

 principally the ligneous substance of the stem ; in trees, its 

 passage is chiefly through the alburnum, or more recently 

 formed wood, and not through the bark, as was at one time 

 believed. 



The course of the sap, however, varies under different cir- 

 cumstances, and at different epochs of vegetation. At the 

 period when the young buds are preparing for their develop- 

 ment, 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 

 destined to fulfil at a more advanced period, when it has to 

 nourish the fully expanded organs : this fluid has, accord- 

 ingly, received a distinct appellation, being termed the nurs- 

 ling sap. Instead of rising through the alburnum, the nurs- 

 ling 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 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 degree of elaboration, fitting it for the 

 office 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 as- 

 similated, 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 



