THE MECHANISM OF ACTIVE EXUDATION 271 



to the inactive condition when these are removed (Sect. 65). Similarly, 

 in Droscra and other carnivorous plants particular stimuli may excite or 

 accelerate the excretion of fluid. Even in these cases the mechanism of 

 the secretory process has not yet been precisely determined ; nevertheless, 

 it is certain that in Drosera and Dionaea we have to deal with an active 

 excretion of water. It must, however, be remembered that the escape of 

 water is in all cases due directly or indirectly to the activity of living cells, 

 for even in the passive or plasmolytic excretion of water the osmotic substances 

 concerned are produced by living cells. The functional activity thus excited 

 persists as long as a higher osmotic concentration is maintained externally 

 to a permeable cell or tissue. An active excretion of water may, however, 

 be stopped by the action of chloroform or by the absence of oxygen. 



When turgidity falls below a certain limit no perceptible excretion of 

 water is possible so long as the cell absorbs water with greater rapidity 

 than it excretes it. The fact that in flaccid plants the nectaries still 

 continue to excrete water shows that the process is not one of active 

 excretion under pressure, but is purely passive, the distribution of water 

 being determined by the osmotic concentrations of the fluids outside and 

 inside the cells. 



Dutrochet erroneously supposed that bleeding was due to osmotic action, 

 whereas Hofmeister's interpretation was correct, in so far as he recognized the active 

 filtration and exudation of water into the interior of the plant as the immediate cause 

 of bleeding 1 . The attempts of Hofmeister and other authors to refer the pressure 

 of exudation to the vital activity of special cells were not very successful, owing to 

 the fact that the osmotic relationships were not properly understood at the time they 

 wrote, and to their imagining that the explanation of the osmotic powers of the cell, 

 and of the transmission of water in a particular direction, was to be found in the pro- 

 perties of the cell-wall. Hence the apparatus 2 employed by Hofmeister and others 

 to demonstrate the process of ' bleeding ' does not represent at all accurately what 

 actually occurs in the plant. Nor was any clear distinction made between an intra- 

 cellular activity independent of exosmosis, and the action of an extracellular substance 

 causing a withdrawal of water from the cell. A full account of these problems is given 

 in Pfeffer's Osmotische Untersuchungen, 1877, p. 22 3> where the principal views 

 with regard to them have been brought into agreement with our present knowledge. 

 It has been shown more recently by Pfeffer that the character of the plasmatic 

 membrane has no influence upon the osmotic pressure, so long as no exosmosis is 

 possible, and hence a change in the osmotic properties of the plasmatic membrane 

 will hardly suffice to cause water to be driven from the cell in a definite direction 3 . 



1 Dutrochet, Mem. d. vegetaux et d. animaux, Bruxelles, 1837, p. 202; Hofmeister, Flora, 1858, 

 p. 8 ; 1862, p. 142. 



2 Hoffmann, Ann. d. Phys. u. Chem., Bd. CXVII, p. 264; Sachs, Experimentalphysiol., 1865, 

 p. 207; Detmer, Beitrage z. Theorie d. Wurzeldruckes, 1877, p. 21. Cf. the summary given by 

 Pfeffer, Energetik, 1892, p. 265. 



3 Pfeffer, Zur Kenntniss d. Plasmahaut u. Vacuolen, 1890, p. 302 (cf. Sect. 24). 



