100 DIFFUSION AND OSMOTIC PRESSURE 



press the cell sap out of the numerous cells of the gland, 

 and this may accumulate on the external surface. Owing 

 to evaporation this droplet of exuded sap must immediately 

 begin to increase in concentration, and from that time on 

 Wilson's observations are sufficient to explain the maintenance 

 of liquid in the nectary. 



It has been shown, 1 however, in certain cases, that if the 

 nectar be removed, new nectar will be secreted. According to 

 the above hypothesis, this must be due to another increase 

 of turgidity and another exudation of cell sap. Thus it must 

 be supposed that, after the first discharge, the conditions 

 again come into equilibrium, and the plasmic membranes 

 again become semi-permeable, only to repeat the former 

 process of excretion if the nectar is again removed. Per- 

 haps the evaporating, and therefore concentrated, solution 

 on the surface acts as a plasmolyzing agent, keeping the 

 turgidity of the gland cells down by extraction of water. If 

 this were so, the removal of the nectar might easily cause 

 an increase in turgidity, which might, in turn, bring about 

 the response of exudation. 



Wilson found the shining droplets of water which occur 

 on certain molds to be of the same nature as the artificial 

 droplets which he was able to produce by sprinkling the 

 hyphse with pulverized sugar. When these natural drop- 

 lets are removed they again return, but this does not occur 

 if the hyphae are carefully washed with distilled water. 

 Careful examination of the places where droplets had been 

 removed without washing showed minute crystals of sugar 

 upon the surface. Thus, if the air becomes dry, evapo- 

 ration may become so great that the droplets disappear, but 

 as soon as evaporation is checked, the crystals of sugar lying 

 upon the surface go into solution in the imbibed water of 



1 H. HAUPT, " Zur Secretionsmechanik der extrafloralen Nektarien," Flora, Vol. 

 XC (1902) , pp. 1-41. On the general subject of secretion, see W. PFEFFER, Studien zur 

 Energetik der Pflanzen, Leipzig, 1892. 



