C 49 j 



CHAPTER Till. 



OF THE SAP-VESSELS, AND COURSE OF THE SAP ; WITH MR. 

 KNIGHT'S THEORY OF VEGETATION, 



Much contrariety of opinion has existed among phys- 

 iologists 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 which, 

 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 observ- 

 er ; and those who are conversant with the microscope, 

 and books relating to it, have frequent opportunities of 

 observing how curiously these vessels are arranged, and 

 how different species of plants, especially trees, differ 

 from each other in the structure and disposition of them. 

 Such observations, however, if pursued no further, lead 

 but a little way towards a knowledge of the wonderful 

 physiology of vegetables. 



In our 2d chapter, mention is made of the genera! 

 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 familiar to 

 every one ; and a little reflection will satisfy us that such 

 substances must each be lodged in proper cells and ves- 

 sels to be kept distinct from each other. They are ex- 

 tracted, or secreted, from the common juice of the plant, 

 and called its peculiar or secreted fluids. Various ex- 



