104 DIFFUSION AND OSMOTIC PRESSURE 



phenomena occurring within the cells. 1 If this be the true 

 explanation, we have no exact knowledge of the processes by 

 which these phenomena occur. That osmotic pressure within 

 the vacuoles might cause movement of an exuded solution 

 under pressure, through intercellular spaces or through the 

 channels of the xylem, need not be questioned; this can be 

 simulated in the laboratory with the ordinary thistle tube 

 of molasses closed with animal membrane. But to explain 

 the phenomenon of exudation pressure it must be shown 

 how it comes about that this solution gets into the channels 

 at all. A small amount of solution might be exuded in a 

 manner similar to that supposed above in connection with 

 nectaries and water pores. But the absence of evaporation 

 within the plant body will deprive us of that source of 

 energy to which has been ascribed the maintenance of a 

 relatively high concentration in the exudate of nectaries. 

 However, it is probable that even in nectaries and water 

 pores this discovery of Wilson is not of fundamental impor- 

 tance. The main desideratum is to know how the original 

 exudate comes to get through the otherwise only semi- 

 permeable protoplasm ; if a little can be exuded there seems 

 to be no reason why more could not pass out in the same 

 way. In all the cases of exudation the exudate is known to 

 be a solution; the solutes of the vacuoles pass out and 

 appear in the exudate. This is an unquestionable indica- 

 tion that the protoplasmic membrane is permeable to them. 

 An explanation of this phenomenon was elaborated by 

 Pfeffer 2 and later, apparently independently, by Fuchs. 3 

 This depends upon the assumption of some sort of vital 

 activity within the cells. These authors point out that in 

 order to have a current of water through a cell, it is only 



1 W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 223. 



2 Ibid., pp. 222-5. 



3 K. FUCHS, "Zur Theorie der Bewegung des Wassers im lebenden PflanzenkOr- 

 per," Beih. Bot. CentralbL, Vol. X (1901), pp. 305-8. 



