Burns. — Heterophylly in Proserpinaca palustris , L. 585 
leaves’ at the base* A similar example is pictured by Goebel (9) in the 
case of Ranunculus aquatilis. 
I will not give in detail a description of my experiments with such 
factors as light, temperature, nutrition, gaseous content of the water, &c., 
but only refer the reader to the work of McCallum, whose results are 
essentially the same as my own, especially agreeing with the results 
I obtained from experiments conducted in the winter. 
I repeated the experiments cited by McCallum on p. 107 of his paper. 
In the fall of 1902 I set up the following: Cuttings were made from plants 
producing the divided leaf. They were divided into four sets ; one set of 
plants was placed in a nutrient solution not quite strong enough to plasmo- 
lyze them ; the second set was placed in a weak solution of potassium 
chloride ; the third was placed in pots and cultivated as land plants ; the 
fourth was placed in battery jars under about 18 cm. of water. In every 
case the leaf-form which developed was divided, although I allowed the 
solutions to become stronger by evaporation. In the early spring I 
repeated the experiments not only on Proserpinaca , but also on other plants, 
and my results were different. This time the plants in solution of potassium 
chloride and the nutrient solution showed a tendency to produce more 
entire leaves, at least on some stems. However, about the same time my 
control plants, growing in tap water, also began to show the same tendency, 
as did also the plants growing in air. As the control plants as well as 
those in the solutions showed the same tendency, it hardly seems possible 
that these salts either directly or indirectly could be the cause of the 
production of the entire leaf. 
This phenomenon was explained by McCallum (p. 108) thus: ‘ It 
would seem here as if some of the plants after a time become accustomed 
to the stimulus and refuse to respond. Or it may be that as only the air- 
form is capable of fruiting, in the effort to produce flowers the plant has 
the ability of self-adjustment to its conditions and develops the air-form 
in spite of its environments.’ ... ‘ It is possible also that the protoplasm 
is able to adjust itself, perhaps by the expulsion of water, into that con- 
dition in which it exists when in air.’ 
Reference has been made to the biological importance of the change 
in the direction of growth of the stem which was observed in the fall. An 
effort was made to find the cause of this change. Why are all stems ortho- 
thropic in summer and plagiotropic in winter? Is it a different response 
on the part of the plant at different stages of its development ; or, is it due 
to a change in external conditions ? The facts observed in the field-work 
point to temperature as a controlling factor, but the work in the greenhouse 
excludes it as such. A solution of the question was finally reached in 
some experiments set up to study the effect of weakened illumination 
on leaf- formation. Two plants, both plagiotropic, one growing in air and 
