analogous to the Irritability of Animals, 167 



entirely when it has become dry and weathered (as geologists 

 would say) by exposure in our hedges through the winter. 

 But, in the latter case, it is most certainly much more elastic 

 than when alive and growing. 



3. Lastly, Poisons destroy the power of divergence, which 

 they would not do if it were dependent on a mere physical 

 cause such as elasticity. This fact I state on the authority 

 of very numerous experiments, which it seems needless to 

 relate circumstantially. 



I infer, then, from the preceding facts and arguments, that 

 elasticity is not the cause of divergence. I proceed, in the 

 next place, to state the experiments and observations which 

 lead me to conclude that it is a vital property. 



1. It is most active in those parts of plants which exhibit 

 other vital properties and functions in the greatest perfection. 

 For instance, whilst, as I have stated above, it does not exist 

 in dead wood, and ceases as a plant loses its moisture, it is 

 found in stems, and flower- and leaf-stalks when in their most 

 vigorous and healthy state. 



2. If the opinion, that this property is of a vital nature, 

 were correct, I thought it would be destroyed by poisons, and 

 this I find to be the case whether a plant be supplied with a 

 poisonous liquid instead of water, or a divergent stem totally 

 immersed in such a liquor. I shall give the following experi- 

 ments in proof of this. 



Exp, 3. A stem of Bryony {Bryonia dioica) was placed 

 in a solution of arsenite of potash *. In two days it became 

 so flaccid that the head and tendrils hung downwards. They 

 were not discoloured, and but little shrivelled. The divergent 

 power was completely destroyed. 



Exp, 4. I confined two stems of a Red Dead-Nettie {La- 

 mium purpureum) in an inverted jar filled with sulphuretted 

 hydrogen. In two days one of the stems was so perfectly 

 flaccid as to be incapable of holding up its head. They were 

 not withered, and the blossoms only looked a little paler. 

 Every part of the stems which was exposed to the influence 

 of the gas had completely lost its divergent power. 



3. The following experiments show that the divergent 

 power is capable of being excited or increased by stimulants. 



Many poisons, whose ultimate effect is to destroy this pro- 

 perty, do at first increase it. This has occurred with laurel- 

 water, dilute nitric acid, brandy, oil of turpentine, hot water, 

 and a mixture of aether with sal volatile. Cold water, also, 

 so augments the divergence of the segments of a divided stem, 



* Made by boiling together in I5 of water 8 grains of white arsenic 

 and the same quantity of subcarbonate of potash. 



