24 THE VITAL FUNCTIONS. 



to the leaves, where it is to experience various 

 important modifications. By causing the roots 

 to imbibe coloured 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 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 sup- 

 plying. 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, ac- 

 cordingly, received a distinct appellation, being 

 termed the nursling 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 several layers of wood, till it reaches 



