LETTER X. 



99 



within the tree by the young spongioles, the fluid 

 has no help but to move on and pass upwards. Even 

 were there no force in advance, as we have seen there 

 is, sucking it upwards, the fluid must needs pass in 

 this direction, because driven on by the portions 

 subsequently absorbed. Physiologically considered, 

 then, the force acting at the spongioles is, I am 

 inclined to think, attractive only^ and only indi- 

 rectly and mechanically propulsive.* It is one, too, 

 I apprehend further, which is strictly subordinate to 

 that seated in the living and growing parts above, f — 

 the two acting in harmony together, in ways provided 

 for by the Creator, but which we can as little fathom 

 as we can the essential nature of the powers them- 

 selves. In anywise, the facts known to us regarding 



* Professor Henslow, indeed, regards it as a vis a tergo and pro- 

 pulsive in its mode of action, — while Dutrochet, regarding it as 

 simply phj^sical, resolves it into a principle, of a twofold natm*e, 

 designated by him the principle of Endosmose and Exosmose, I agree 

 with Professor Henslow that there is a difficulty in resolving the one 

 into the other, and feel inclined to reject Dutrochet 's theory 

 altogether as inadequate to the explanation of the phenomena 

 included under it, — but I differ from Professor Henslow in the view- 

 taken of it by him. To my mind, a vis a tergo must lie without and 

 beyond the roots, L e., in the soil itself, or the matters contained 

 in it. 



Energetic as his experiment vdth the vine shewed this power 

 to be, Dr Hales found that, cut off from the force acting above, " it 

 soon diminished, and after a time ceased altogether." — Dr Carpen- 

 ter, Principles of Physiology} Gen. and Comp,, p. 655. 



