42 



THE VITAL FUNCTIONS. 



540 



the extent of the circuit very limited. The cause of these 

 motions is not yet known; but probably they are ultimately 

 referrible to a vital contraction of the vessels; for they cease 

 the moment that the plant has received an injury, and are 

 more active in proportion as the temperature of the atmo- 

 sphere 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 Caulinafragilis, &c.,* where the double 

 currents are beautifully seen under the microscope, perform- 

 ing a complete circulation 

 within the spaces of the stem 

 that lie between two adja- 

 cent knots or joints; and 

 where, by the proper adjust- 

 ment of the object, it is easy 

 to see at one view both the 

 ascending and descending 

 streams passing on opposite 

 sides of the stem. Fig. 239 

 shows this circulation in the 

 cells of the Caulinia fragalis 

 very highly magnified, the 

 direction of the streams being indicated by the arrows. Fig. 

 240 represents the circulation in one of the jointed hairs, 

 projecting from the cuticle of the calyx of the Tradescantia 

 virginicaj in each cell of which the same circulatory motion 

 of the fluids is perceptible. 



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

 f Fig. 239 is taken from Amici, and Fig. 240 from that given by Mr. 

 Slack, Trans. Soc. Arts, vol. xlix. 



