SECT, III. CAUSES OF THE SAP's ASCENT. 127 



spurge is cut in two, a milky juice will exude from 

 both sections, in almost any season of the year. 

 Also if a plant is inverted, the stem will become a 

 root, and the root a stem and branches, the sap 

 ascending equally well in a contrary direction 

 through the same vessels ; as may readily be proved 

 by planting a willow twig in an inverted position. 

 But these facts are totally incompatible with the 

 existence of valves ; and the opinion of Malpighi 

 proved consequently to be groundless. 



The next hypothesis is that of M. De la Hire, of De la 

 who seems to have attempted to account for the 

 phenomenon by combining together the theories 

 of Grew and Malpighi. Believing that the ab- 

 sorption of the sap was occasioned by the spongy 

 parenchyma which envelopes the longitudinal tubes, 

 he tried to illustrate the subject by means of the 

 experiment of making water to ascend in coarse 

 paper, which it did readily to the height of six 

 inches, and by particular management even to the 

 height of eighteen inches. But in order to com- 

 plete the theory, valves were also found to be ne* 

 cessary, and were accordingly summoned to its aid. 

 The sap which was thus absorbed by the root was 

 supposed to ascend through the woody fibre, by 

 the force of suction, to a certain height, that is, till 

 it got above the first set of valves, which prevented 

 its return backwards ; when it was again supposed 

 to be attracted as before, till it got to the second 



