178 PHYSIOLOGICAL BOTANY. I'AHT II. 



which it is consumed. This view is further sup- 

 ported by the fact, that the facility with which dif- 

 ferent liquids are absorbed, appears to depend entirely 

 upon their degrees of fluidity ; and thus even the most 

 noxious materials will be more readily imbibed than such 

 as are nutritious, provided they are presented to the 

 spongioles in the more fluid state. Now if their ab- 

 sorption were the result of a vital action, we might have 

 expected that a greater degree of energy would have 

 been exerted in favour of the more nutritious matter, 

 and that the noxious ingredient would have been ab- 

 sorbed with difficulty. 



(162.) Stimulants to Absorption Whatever be the 



immediate cause of absorption, it does not depend upon 

 the action of light ; for plants absorb by night as well 

 as by day, and the absorbing organs are most frequently 

 placed under ground, and in the dark. In an indirect 

 manner, however, light does certainly exert a consider- 

 able effect upon the quantity of fluid absorbed ; because 

 it is the stimulant by which a large portion is con- 

 tinually removed by the function of exhalation ; and 

 we consequently find that when plants are placed in the 

 dark, although the absorption continues it is consider- 

 ably checked, so that ' the water imbibed accumulates 

 until they become dropsical, and their leaves fall off upon 

 the slightest touch. An increase of temperature aug- 

 ments the quantity of water absorbed ; but this again 

 may depend upon some local stimulus upon another 

 function. Thus if a branch from a plant growing 

 in the open air be introduced within a stove during 

 the winter, it will immediately begin to push its leaves, 

 and become the remote cause of accelerating the ab- 

 sorption of the sap, which had been going on very lan- 

 guidly. 



SECOND PERIOD OP NUTRITION. 



(lf)3.) Ascent of the Sap The fluid introduced by 

 the absorption of the spongioles bears the general name 



