CHAP. I. VITAL PROPERTIES AND 8TJMTJ1ANTS. 159 



When these are turned in any particular direction, they 

 retain the position in which they are placed, without 

 any effort to return again to that in which they were 

 previously disposed. 



(143.) Hygroscopicity of Tissue. The hygroscopic 

 properties of some tissues are very great, and are the 

 cause of certain motions, which might be mistaken for 

 the direct effects produced by the vital force. If the 

 awn or bristle of the wild oat be moistened, it imme- 

 diately untwists ; the teeth of mosses suddenly collapse 

 when moistened by the breath, and readily expand upon 

 drying again. In estimating the hygroscopic properties 

 of the tissue, we must distinguish between the action of 

 the whole mass, and the property of the membrane 

 which forms the separate vesicles and tubes of which 

 the tissue is composed. It seems easy to account for 

 the hygroscopicity of the mass of the tissue, when we 

 remember that it is penetrated in all directions by inter- 

 cellular passages, and thus resembles a sponge, which 

 absorbs moisture by the common properties of capillary 

 attraction. This action is found to be much more 

 powerful in proportion as the vegetable tissue is but 

 slightly charged with foreign matter. Some plants, as 

 the mosses, readily imbibe water, however long they 

 may have been dried ; and reassume an appearance of 

 freshness nearly equal to that which they possessed in a 

 living state ; but, in these cases, the effect is most pro- 

 bably due to the hygroscopic action of the elementary 

 membrane composing the vesicles, and not to the capil- 

 larity of the tissue itself. The immediate result of any 

 hygroscopic action upon a portion of the tissue is to 

 enlarge it; and consequently, where two portions 

 are in contact, one of which is more hygroscopic than 

 the other, there exists a tendency to separation. 

 When, however, they do not separate, the portion 

 which is the least hygroscopic, becoming less dis- 

 tended than the other, necessarily produces an incurv- 

 ation of the mass upon that side on which it is placed. 



(144.) Endosmose. Connected with the hygro- 



