OF VEGETABLE TISSUE. 31 



vascular plants has been deduced. The imme- 

 diate cause of these phenomena appears to be 

 that the cells and vessels of the tissue are endued 

 with a contractile power, analogous to that of the 

 heart in animals, or rather, perhaps, to the con- 

 traction and dilatation observed in certain micro- 

 scopic infusoria : there are cases in which this 

 action (though ordinarily confined to parts so 

 minute as to escape observation) becomes visible : 

 for instance, if a branch of the Euphorbia, or 

 any other milky plant, be cut across, the milky 

 juice exudes from both surfaces. If it flowed 

 by an impulse given either from below or from 

 above, it would only appear on one half of the 

 severed plant ; if it issued forth by its own weight 

 by the law of gravity, it could only flow when 

 turned downwards, and if the lower half were 

 held upright,-the fluid would stand as in a cup ; 

 but it exudes let the branch be held in whatever 

 direction it may, and it must therefore be owing 

 to some contractile power within. 



The agents which occasion or modify vegetable 

 excitability are light, heat, and perhaps elec- 

 tricity ; and in addition to these, accidental causes 

 of excitement, such as blows, the action of certain 

 chemical substances, &c. will in some cases pro- 

 duce the phenomena by which it is manifested. 



