58 Irritability observed in the Stems of Plants, 



vergence to the vital irritability of the plants, in consequence 

 of its absence in some which are very elastic, as in the rattan 

 cane, and the dry and very elastic ligneous portions of the 

 stems of plants. He appears to have been led to this con- 

 clusion from the apparent necessity of attributing this phe- 

 nomenon either to the physical elasticity, or to the vital 

 " contractility " (? irritability) of the plant itself; and, cer- 

 tainly, the fact of poisons preventing the developement of this 

 divergent property appears to countenance this conclusion. 

 But I think that the observations I am about to offer will 

 be sufficient to show that this peculiarity may be explained 

 on purely physical principles, independent of the necessary 

 presence of vital agency ; for I apprehend that, in vegetable 

 as well as in animal physiology, it too often happens, that 

 calling in the aid of vitality to explain secondary effects 

 (independent of ultimate causes) is little else than, in other 

 words, a confession of our ignorance. 



I conceive that the separation of the divided portions of a 

 stem may be explained on the same physical principles which 

 Dutrochet has had recourse to, in his explanation of the irri- 

 tability evinced by the valves of the seed-vessel of the Impa- 

 tiens Balsamina, and the pericarps of Momordica Elaterium. 

 And here I may be excused a momentary digression, for the 

 purpose of giving a very brief outline of Dutrochet's dis- 

 covery. This philosopher has shown that, when a fluid, as 

 water, or a weak saline solution, is enclosed in an organised 

 membrane, as a piece of bladder, or placed in a glass tube 

 over which a piece of membrane is firmly tied, and immersed 

 in a solution of sugar, the bladder or glass tube becomes 

 rapidly emptied ; but if, on the contrary, the bladder or 

 tube, be filled with syrup, and immersed in distilled water, 

 the reverse takes place, the bladder becoming completely 

 injected and turgid, and the tube filled with fluid; which, at 

 last (if the tube be not too long), runs over. From these facts, 

 amongst many others, M. Dutrochet was inclined to draw the 

 following deductions: — 1. That, when a fluid of low specific 

 gravity, enclosed in an organic membrane, is immersed in 

 one of greater density, the membrane becomes rapidly emp- 

 tied, in consequence of a current being set up from the lighter 

 to the denser fluid (from within and without) ; and, 2., that, 

 when a dense fluid is enclosed in a membranous reservoir, 

 and immersed in a fluid of a lower specific gravity, a current 

 is set up, whereby the membrane becomes distended by a sup- 

 ply of fluid from without. The exceptions to these two rules 

 are very few, and may at present be set out of the question. 

 For the sake of convenience and conciseness of expression, 



