136 DUTROCHET'S THEORY. 



discovery of the endosmose. This philosopher was one of 

 the first to perceive that two liquids, separated from one 

 another by a membrane, quickly effect or induce a current 

 which always carries the thinner liquid towards the denser, 

 and ends by mingling the two completely. " It is endos- 

 mose," he said, " which produces at one and the same time 

 the progression of the sap by impulsion, and its progression 

 by affluxion. The sap would receive its impulse in the 

 spongioles of the roots ; thence would be carried towards the 

 upper parts by the turgescence of the organs by the affluxion, 

 which would thus act as a forcible mode of suction." 



The basis of this theory is, that the sap contained in the 

 upper parts will be more concentrated or denser than that 

 in the lower portions of the same plant. But this is a mere 

 supposition. And even this supposition has been swept 

 away by the recent experiments of Hartig and others, which 

 show that the difference in density between the two saps 

 is not only almost null, but in many ligneous plants the 

 lower sap is, on the contrary, denser than the upper.* 



Finally, and more recently, M. Joseph Boehm has put 

 forth a theory which offers some points of analogy with 

 that of Grew. According to Boehm, the rise of the sap 

 is the effect of a suction, the cause of which must be sought 

 both in the atmospheric pressure and in the transpiration 

 which takes place through the organs, and notably through 



* See the Botanische Zeitung (" Botanical Gazette ") for the years 1853, 

 1856, 1859, and 1861. 



