44 THE VITAL FUNCTIONS. 



the appearances afford sufficient evidence of the 

 existence of a general circulation of nutrient juices 

 in the vegetable systems of those plants that exhibit 

 them, which is the case with all those that contain 

 milky juices. Are these moving fluids, it may be 

 asked, to be regarded as the proper nutrient juices 

 of the plant, or as the products of secretion ? And 

 again, it may be made a still further question, 

 wliether such circulation exists universally in vas- 

 cular plants. The difficulties of accurate obser- 

 vation are here so formidable, that a long time will 

 probably elapse before these problems can be 

 satisfactorily solved. We find that they cease the 

 moment that the plant has received an injury, and 

 that they are more active in proportion as the 

 temperature of the atmosphere is higher. But 

 we are perfectly in the dark with respect to the 

 cause of these motions ; and we must be con- 

 tent for the present to refer them, with Treviranus 

 and De Candolle, generally to vital contractions of 

 the vessels. 



Phenomena, in some respects analogous to these, 

 and of a nature equally curious, are exhibited by 

 cellular plants, in which no vessels can be traced, 

 and of which the juices are nearly transparent, and 

 contain only a few floating globules. Of this kind 

 are the Clicua, or Stone- wort, and the Caulinia 

 fragilis : in each of the enlarged cells of which, the 

 contained fluid may be seen by the aid of a power- 

 ful microscope, moving in a continued current, 

 ascending on one side and descending on the other, 

 so as to perform a complete circulation within the 

 space bounded by the transverse partitions which 

 close the cell at each end. In the stem of the 



