34 



CHAPTER VIIL 



OF THE SAP-VESSELS, AND COURSE OF THE SAP; 

 WITH MR. knight's THEORY OF VEGETATION. 



iVluCH contrariety of opinion has existed among 

 physiologists concerning the vascular system of plants, 

 and the nature of the propulsion of the sap through 

 their stems and branches. Indeed it is a subject upon 

 '•^'hich, till lately, very erroneous ideas have prevailed. 



That the whole vegetable body is an assemblage of 

 tubes and vessels is evident to the most careless ob- 

 server ; and those who are conversant with the micro- 

 scope, and books relating to it, have frequent oppor- 

 tunities of observing how curiously these vessels are 

 arranged, and how different species of plants, especially 

 trees, differ from each other in the structure and dis- 

 position of them. Such observations, however, if pur- 

 sued no further, lead but a little way towards a know- 

 ledge of the wonderful physiology of vegetables. 



In our 2d chapter, mention is made of the general 

 cellular and vascular texture of plants ; we must now 

 be a little more particular in our inquiries. 



That plants contain various substances, as sugar, 

 gum, acids, odoriferous fluids and others, to which 

 their various flavours and qualities are owing, is fami- 



