98 DIFFUSION AND OSMOTIC PRESSURE 



within. Experiments are needed to determine whether or 

 not this sort of extraction of water does really take place in 

 water pores. At any rate, the important point lies here, that 

 there is an original exudation of cell sap through the proto- 

 plasm. That in some cases at least the exudation is truly a 

 portion of the sap, and not pure water, has been shown by 

 Bonnier 1 in the case of honey-dew, and by Dandeno 2 in the 

 case of guttation drops. Also Moll 3 showed that when shoots 

 whose leaves bore water pores were injected from below with 

 the juice of Phytolacca decandra, the exudation always con- 

 tained the color. Since water cannot pass from the xylem 

 to the outside without traversing the cells bordering upon the 

 pore, the exuded water bearing the coloring matter must 

 have pased through these cells. 



The only serious difficulty with this hypothesis as an 

 explanation of the action of water pores lies in the assumed 

 continually decreasing concentration of the cell sap. But 

 there is good evidence that the protoplasm of the plant cell 

 is continually discharging substances into the vacuole, which 

 increase the osmotic pressure therein. There is apparently 

 no reason why we may not postulate this same property, per- 

 haps in an unusually marked degree, in the case of the cells 

 just discussed. 



In the case of nectaries there is exhibited an apparently 

 similar set of phenomena. Wilson's work * shows that after 

 the dissolved substances of the nectar (mainly sugar) have 

 once passed out of the cells and into the cup of the nectary, 



1G. BONNIEE, " Recherches experimentelles sur la miellee," Rev. gen. bot.. Vol. 

 VIII (1896), pp. 1-22. 



2 J. B. DANDENO, "An Investigation into the Effects of Water, etc., on Foliage 

 Leaves," Trans. Canad. Inst., Vol. VII (1901), pp. 238-350. 



3 J. W. MOLL, " Untersuchungen flber Tropfenausscheidung u. Injection von Blat- 

 tern," Verslagen en Mededeel. d. k. Akad. v. Wetensch. te Amsterdam, Vol. XV (1880), 

 pp. 237-337. 



* W. P. WILSON, " The Cause of the Excretion of Water on the Surface of Nec- 

 taries," Unters. aits d. bot. Inst. zu Tubingen, Vol. I (1881), pp. 1-22. 



