C 9 3 



constitution of plants. In all plants there exists a 

 system of tubes or vessels, which in one extremity 

 terminate in the roots, and at the other in leaves. It 

 is by the capillary action of the roots that fluid mat- 

 ter is taken up from the soil. The sap in passing up- 

 wards becomes denser, and more fitted to deposit 

 solid matter ; it is modified by exposure to heat, light, 

 and air in the leaves ; descends through the bark, 

 in its progress < produces new organized matter ; and 

 is thus in its vernal and autumnal flow, the cause of 

 the fermentation of new parts, and of the more per- 

 fect evolution of parts already formed. 



In this part of the enquiry I shall endeavour to 

 connect together into a general view, the observations 

 of the most enlighted philosophers who have studied 

 the physiology of vegetation. Those of Grew, Mal- 

 pighi, Sennebier, Darwin, and, above all, of Mr. 

 Knight. He is the latest enquirer into these interest- 

 ing subjects, and his labours have tended most to il- 

 lustrate this part of the economy of nature* 



The chemical composition of plants has within 

 the last ten years, been elucidated by the experiments 

 of a number of chemical philosophers, both in this, 

 and in other countries ; and it forms a beautiful part of 

 general chemistry ; it is too extensive to be treated of 

 minutely ; but it will be necessary to dwell upon 

 such parts of it, as afford practical inferences. 



If the organs of plants be submitted to chemical 

 analysis, it is found that their almost infinite diversity 

 of form, depends upon different arrangements and 

 combinations of a very few of the elements j seldom 



