404 ON THE FLUIDS OF PLANTS. 



rays of the wood, by which the whole of the tissues of the plant were successively 

 contracted and dilated during the periods of night and day. 



Grew gave a compound theory, and ascribed the motion of the sap to its 

 volatility and magnetic tendency, aided by fermentation. 



But the most recent and generally-received theory is that which explains the 

 motion of the vegetable fluids by electrical action. Amici long since attributed 

 the intercellular motion of the sap in Chara to electricity, but it was left for 

 Dutrochet to develop a theory of its action. The construction of his endosmo- 

 meter, the effects produced by it, and the application of his theory to this 

 department of science, are so well known, that I need not detail them here. I 

 shall, therefore, only offer some objections which may be urged against it. First,, 

 there is not a strict analogy between the animal or vegetable membrane and the 

 tubes used in the endosmometer of Dutrochet, and the spongioles and fibres of 

 the root and trunk of a tree. Again, is it a fact that the fluid existing in the tree 

 is denser than the external fluid surrounding the spongioles of the roots ? This 

 is a point necessary to be determined before we can subscribe to the above 

 theory. In the next place, I am not aware that, in any instance, the applica- 

 tion of the most delicate galvanometers has detected in the processes of exosmose 

 or endosmose the slightest indication of a galvanic current. Lastly, if this prin- 

 ciple is admitted as a sufficient cause for the ascent of the sap, it cannot be 

 applied to the descent and appropriation of the proper juices of the plant, as a 

 contrary phenomenon takes places in this case to that observed in endosmose. 



From the imperfect sketch I have attempted to give, it will be perceived that 

 the subject of the vital motions of plants is one of considerable interest, and that 

 it has received the attention of the most eminent botanists. There are many 

 points connected with it, at present unexplained, which require for their elucidation 

 an accumulation of well-observed facts. On this subject, as well as many others, 

 there is too much to justify the observation of De Cakdolle, that " les idees 

 theoriques sur la structure generate des vegetaux, ont plus influence que 

 I'observation directe, sur les idees qu'on s'esi faites de I 'usage des vaisseaux." 

 — [Theoretical views on the general structure of plants have more influence than 

 actual observation, upon the opinions regarding the use of vessels.— Ed.] 



Campsall, near Doncaster, 

 Oct. 1, 1837. 



