COURSE OF THE SAP. 43 



forded by the soil, in which possibly, as they pass 

 through the root, some change analogou-i to digestion 

 may take place ; for there is evidently a great ditference, 

 in many cases, between the fluids of the root, at least 

 the secreted ones, and those of tlie rest of the y)lant ; 

 and this leads us to presume that some considerable al- 

 teration may be wrought in the sap in its course through 

 that important organ. The stem, which it next enters, 

 is by no means an essential part, for we see many plants 

 "whose leaves and flowers i2;row directly from the root. 



Part of the sap is conveyed into the flowers and fruit, 

 where various fine and essential secretions are made 

 from it, of which we shall speak hereafter. By far the 

 greater portion of the sap is carried into the leaves, of 

 the great importance and utility of which to the plant 

 itself Mv. Knigl)t's theory is the only one that gives us 

 any adequate or satisfactory notion. In those organs 

 the sap is exposed to the action of light, air and mois- 

 ture, three powerful agents, by which it is enabled to 

 form various secretions, at the same time that much 

 superfluous nmtter passes ofl' by perspiration. These 

 secretions not only give peculiar flavours and qualities 

 to the leaf itself, but are returned by another set of 

 vessels, as Mr. Knight has demonstrated, into the new 

 layer of bark, which they nourish and bring to perfec- 

 tion, and which thev enable in its turn to secrete matter 

 for a new layer of alburnum the ensuing year. It is 

 presumed that one set of the returning vessels of trees 

 may probably be more particularly destined to this lat- 



