50 



THE VITAL FUNCTIONS. 



very limited. The cause of these motions is not 

 yet known ; but probably they are ultimately 

 referable to a vital contraction of the vessels ; for 

 they cease the moment that the plant has re- 

 ceived an injury, and are more active in pro- 

 portion as the temperature of the atmosphere is 

 higher. 



These phenomena are universally met with in 

 all plants that contain milky juices; but they 

 have also been observed in many plants, of which 

 the juices are nearly transparent, and contain 

 only a few floating globules, such as the Chara, 

 or stone- wort, the Caulinia fragilis, &c.,* where 

 the double currents are beautifully seen under 

 the microscope, performing a complete circulation 



within the spaces of the 

 stem that lie between two 

 adjacent knots or joints ; 

 and where, by the pro- 

 per adjustment of the 

 object, it is easy to see 

 at one view both the 

 ascending and descend- 

 ing streams passing on 

 opposite sides of the 

 stem. Fig. 239 shows 

 this circulation in the 

 cells of theCaulinia fragilis very highlymagnified, 



Amici, Annales des Sciences Naturelles, ii. p. 41, 



