62 Irritability in Stems of Plants on Division. 



organic connexion. But when this bond of union is severed, 

 by an incision or otherwise, the segments instantly separate, 

 and curve in such a manner, that the internal portions, 

 or those made up of the looser vesicular tissue (i. e. cells 

 of larger diameter), form the convex part of the curve, and 

 thus have more room for their distention ; whilst the external 

 portions, formed of more compact tissue, occupy the concave 

 parts of the curve, in consequence of their not becoming dis- 

 tended so early or readily as the tissue nearest the axis of the 

 plant. 



This explanation is in perfect accordance with the result of 

 experiment ; for, on immersing the piece of stem, when the 

 segments have separated to a certain extent, in water, which is 

 a fluid of lower specific gravity than the sap in the cells, 

 endosmosis ensues, whereby a quantity of water is forced into 

 the already turgid cells, which, consequently, become more 

 distended, and the curvature increases ; and, on removing the 

 piece of stem into syrup, a much denser fluid than sap, exos- 

 mosis, for reasons already explained, ensues; the cells become 

 emptied, and the separated portions recover their former 

 rectitude ; the elasticity of the woody fibre present, also, pro- 

 bably assisting. Why, then, it may be asked, should poisons 

 prevent the developement of divergence, if it depends upon 

 purely physical principles P To this question a very ready 

 answer may be given ; for, as poisons diminish or destroy the 

 vital energy of the plant, they prevent the cells becoming dis- 

 tended with sap to an extent sufficient to cause their sepa- 

 ration (and, consequently, that of the segments) on making a 

 longitudinal incision ; but, on placing the piece of stem in 

 water, endosmosis occurs ; the cells become completely in- 

 jected, and the divergent property, consequently, developed* 

 But when (as in experiment 7.) the piece of stem used had 

 been previously desiccated, the cells, by evaporation, were 

 nearly, if not completely, emptied of their sap; and it was 

 necessary to fill them artificially with a tolerably dense fluid, 

 as syrup, before they became sufficiently injected to cause the 

 separation of the segments, on making a longitudinal incision. 



From a careful review of the phenomena connected with 

 divergence, as well as of the experiments related by Dr. John- 

 son (op. sup. cit.) and myself, I think that the conclusion I 

 'nave arrived at, in considering divergence to be the result of 

 a purely physical action, independent of vitality, is fully justi- 

 fied and borne out by the results of reasoning and experi- 

 ment. 



