COURSE OF THE SAP. St 



num, and the central vessels, of Mr. Knight, may be 

 considered as analogous to the arteries of animals ; or 

 rather they are the stomach, lacteals and arteries all in 

 one, for I conceive it to be a great error in Dr. Darwin 

 to call by this name the vessels which contain the pecu- 

 liar secretions of the plant.* These sap-vessels, no 

 doubt, absorb the nutritious fluids afforded by the soil, 

 in which possibly, as they pass through the root, some 

 change analogous to digestion may take place ; for there 

 is evidently a great difference, in many cases, between 

 the fluids of the root^ at least the secreted ones, and 

 those of the rest of the plant ; and this leads us to pre- 

 sume that some considerable alteration may be wrought 

 in the sap in its course through that important organ. 

 The stem, which it next enters, is by no means an es- 

 sential part, for we see many plants whose leaves and 

 flowers grow 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 Mr. Knight'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 su- 

 perfluous matter passes off by perspiration. These ^ 

 secretions not only give peculiar flavours and qualities 



* Phytologia^ sect. 5 



