IRRITABILITY. 119 



downwards or beneath the earth, and the latter up- 

 wards or above it. Other instances of the disposition 

 of different parts of a plant to direct, turn, or twist 

 themselves in particular directions, are not wanting, and 

 presiding over which a ruling principle is seen which is 

 independent of any external exciting cause. 



173. The tendency of the stem towards light is well 

 known. Potatoes which have been lying in dark cel- 

 lars during the summer often bud out their stems, and 

 immediately direct them towards a hole or crevice 

 through which a little light may be entering, and will 

 continue to advance towards it until they attain the 

 particular spot. Potatoes have been seen with such 

 stems twenty feet long. 



174. Du Trochet remarks that there are some 

 plants like the common Hop and large Convolvolus 

 which endeavour to avoid the light. He placed stems 

 of these two plants in a glass vessel of water close 

 to a small window, the tops of the stems were placed 

 against the window in the morning, but during the 

 course of the day they turned away from it, but in 

 the night came back to their original position. The 

 disposition of plants to twine only in a certain course, 

 and the almost impossibility of preventing them doing 

 so, may be also remembered. 



175. It would require more space and time than can 

 be here devoted to enter into this interesting subject 

 further than we have done, and to endeavour to prove 

 from facts well known to exist amongst the phenomena 

 of vegetable organization, that the existence of some 

 power of volition, however low that may be, or if you 

 will, something super added to the material structure, 

 of which we have evidences totally distinct from those 

 generally received as proofs of anything else except a 

 nervous power, is not an opinion that should be passed 

 over with complete disregard, or that is incompatible 

 with the modes of our reasoning upon the phenomena of 

 other organized bodies. As it is we shall conclude with the 

 following remarks from Meyen's observations upon Mar- 



