Igor} PHYSIOLOGY OF POLYMORPHISM IN ALGAE 299 
rapid and marked in these cases. If one has filaments and 
wishes the palmelloid form, he has but to leave the dish uncov- 
ered for a few days to attain this end. Evaporation affects the 
plant in the same way whether it be in the case of sugar solution, 
mineral solution, or porous plate culture. It was impracticable 
to get results by evaporation from gelatin on account of bacteria, 
by the growth of which the culture was destroyed before a 
response could be expected. 
SUMMARY. 
1. Non-electrolyte solutions have the same effect as electro- 
lyte solutions ; the osmotic pressure is the controlling factor in 
determining the form of the plant. This is effective through 
changes in the water content of the cells. 
2. The threshold of stimulation for inhibition of zoospore 
production is at a higher concentration in a solution of non- 
electrolytes than in one of electrolytes. Also, the plant with- 
stands the killing effect of the solution at a higher concentration 
for non-electrolytes than for electrolytes. 
3. This threshold of stimulation for the palmella form is at 
a higher concentration for cane sugar than for lactose. 
4. Cells supported upon gelatin or porcelain plates moistened 
by a solution respond in the same way as though they were 
immersed in the solution. 
5. Prolonged darkness has no effect upon the form of the 
plant; the response of polymorphism does not depend upon the 
photosynthetic process. 
6. Increase in concentration caused by evaporation from any 
solution or from porcelain plates brings about the normal response 
for a concentrated solution. 
Il. PHYSICO-CHEMICAL TESTS. 
In order to determine the extent of the error introduced into 
these experiments by the author’s assumption of complete ioni- 
zation in the mineral solutions, a series of tests of the actual 
3 Loc, cit., p. 297. 
