CHAP. I. VITAL PROPERTIES AND STIMULANTS. 157 



stances which are favourable, or in avoiding those 

 which are hurtful to its existence, we may con. 

 sider it to be more elevated in the scale of nature, and 

 further removed from the condition of mere brute 

 matter. Most animals, by the faculty which they pos- 

 sess of locomotion, have a great advantage in this 

 respect over plants ; and even those among the very 

 lowest tribes of animals which are permanently fixed to 

 one spot during the whole period of their existence, still 

 possess a certain power of selecting their food, and re- 

 jecting what is noxious to them, which vegetables have 

 not. The consequence is, that the continued influ- 

 ence of external agents, is found to be far greater 

 in modifying the characters of plants than of animals. 

 As a sort of compensation however, the vital prin- 

 ciple in plants is so much less energetic than in 

 animals, that they are not so readily affected as these 

 latter, under any merely casual or temporary altera- 

 tion in the external conditions under which they may 

 be placed. 



(141.) Properties of Tissue. Before we describe 

 the functions performed by the vegetable tissues, it will 

 be necessary to remark upon a few of the properties 

 which these tissues themselves possess. In the com- 

 plex phenomena which vegetation furnishes, it is very 

 difficult to separate so much of each result as may be 

 strictly ascribed to the operation of the vital principle, 

 from such as may be due to the action of purely physical 

 causes, the chemical effects of affinity, and the mere 

 mechanical properties of the tissue. The most obvious 

 means which we can employ, for ascertaining the precise 

 properties of the tissue, is to perform experiments upon 

 it in the dead vegetable, and as nearly as possible 

 before any chemical change may have taken place in it. 

 It will not be necessary for us here to notice all the 

 properties which the vegetable tissues possess in com- 

 mon with other substances ; but there are two on 

 which we shall make a few remarks, as the pheno- 

 mena to which they give rise might in some cases 



