OF VITAL MOTION. 31 



in the plant are not always definitely and absolutely 

 the same; and from what is known, it may be argued 

 that the changes which take place are due to the 

 operation of causes analogous to those which deter- 

 mine changes in the centres of the laticiferous vessel. 

 In particular, the statement of Biot is very clear as 

 to the existence of general expansion under the 

 influence of heat, and of contraction under the oppo- 

 site condition of cold; and hence we may infer that 

 these eflects will always follow when the plant is sub- 

 jected to these agents. 



2. Of intra-organic force as an agent in the general 

 movement of the sap. 



The evidence which appertains to this division of 

 our subject, above what has been cited in connexion 

 with the cell and latex vessel, is very obscure and 

 imperfect; and to enter upon it would involve the 

 discussion of questions which will be more advan- 

 tageously considered when we speak of vascular move- 

 ments in animal bodies. As, however, the general 

 movement of the sap is in great part the aggregate 

 expression of the cellular and vascular movements, 

 we may argue that the influence of intra-organic force 

 will be the same as in the cell and latex- vessel, and 

 as to the rest, we must be content to reason from 

 analogy, from what we shall find hereafter in animal 

 bodies. 



