ORGANS OF CIRCULATION. 93 



that the sap rises in peculiar vessels by the combined 

 influence of what is termed capillary attraction and of 

 heat, in the same way as water will rise to a certain 

 height in a glass tube with a capillary bore, that is of 

 the diameter of a hair ; but it is fatal to this explana- 

 tion, that fluids will not rise in the capillary vessels of 

 dead plants. 



Plants seem to be equally indifferent as to the quality 

 of the fluids which they suck up from the soil ; and 

 will as readily take a poisonous as a nutritious one, 

 provided it be equally thin and not viscous. 



Grew explained the rise of the sap by magnetic 

 attraction. 



Malpighi and Borelli referred it to the contraction 

 and expansion of internal air, and of the juices by heat. 



Du Hamel imagined the whole to be referable to 

 the influence of heat. 



Saussure, Mirbel and T. A. Knight refer to the 

 irritability of the vessels, and to what they term the 

 vital force. 



De La Hire referred it to organisation, and sup- 

 posed the return of the fluid was prevented by valves, 

 which, however, have not been seen. 



Perrault imagined it to proceed from some sort of 

 fermentation. 



De Candolle supposes that the nutritive fluids are 

 taken up from the soil by the spongelets on the same 

 principle 1 that water runs through blotting paper, or 

 rises up into a piece of loaf sugar, from getting between 



(1) Technically, Hygroscopicity. 



