362 CONSERVATIVE ORGANS. [LBCT. VII. 



downwards, is the greater owing to the pressure 

 of the ascending sap, it must necessarily advance ; 

 and this contraction being repeated in every suc- 

 cessive coil, the fluid is moved forward with a suf- 

 ficient impetus ; while the new quantity of sap 

 which supplies the place of that carried forward, 

 and which rushes into the coil at the instant of its 

 relaxation, forming the basis of resistance to the 

 return of the portion before it, and at the same 

 time exciting a renewal of the contraction, its pro- 

 gression must be uninterrupted. I should be an- 

 ticipating what I have to say on the general ascent 

 of the sap (or, as it has been erroneously termed, 

 its circulation), were I now to explain the means 

 by which this fluid is raised through the ligneous 

 parts of a tree, until it arrives at the succulent 

 twigs, in which alone the spiral vessels are active ; 

 and it is, therefore, here merely necessary to add, 

 that I believe these to be the only vegetable vessels 

 endowed with contractility, or which act in any 

 manner analogous to the arteries of animals. If 

 this hypothesis be maintainable, the spiral vessels 

 are the sap vessels of the succulent stem and the an- 

 nual shoot of dicotyledonous ligneous plants ; and 

 their spiral structure is essential for the performance 

 of their conducting function, in the spongy medul- 

 lary sheath, or cellular parenchyma in which they 

 are imbedded. How long they continue to act as 

 sap vessels it is impossible to conjecture ; but they 



