916] COSENS & SINCLAIR—AERIFEROUS TISSUE 21 
submerged portions are divided, while those on the aerial portions 
are entire. Explanations, on experimental grounds, have been 
offered by various workers, notably McCattum, Burns, and 
GOEBEL, to account for this phenomenon of heterophylly. 
McCativum (4) conducted a series of experiments with Proser- 
pinaca palustris L., in which both submerged and aerial plants were 
grown under various degrees of illumination, and found that the 
type of leaf developed was entirely independent of the light rela- 
tions. Further experiments were peered in which pats were 
keptina perfectly saturated at taining normal amounts 
of carbon dioxide and oxygen, and it was observed that the water 
form of leaf persisted. It was then assumed that the essential 
factor common to moist air and water was inhibition of transpira- 
tion, and confirmatory experiments from this standpoint were 
carried out, in which transpiration was induced in submerged plants 
by high osmotic pressure. The conclusion arrived at was that the 
aerial type of leaf is developed, even under water, if sufficient 
moisture is withdrawn from the protoplasm. 
From the preceding experiments the conclusion deduced was 
that the only factor, constant in all cases where the water form of 
leaf developed; was the checking of transpiration and constant 
increase in the amount of water in the protoplasm, while those 
chemical and physical conditions, resulting from partial withdrawal 
of water, are associated with the aerial type. The main point is 
that the modification in the form of the leaf was construed as 
environmental, that the form of the leaf is especially plastic under 
the influence of the transpiration factor. 
Burns (1) studied the same plant and arrived at conclusions 
entirely different from those formed by McCatium. He experi- 
mented with cuttings and seedlings, and found that in the case 
of the former the disturbance of the vegetative activity caused a 
change that might be accompanied by a reversion, but concluded 
that environmental factors did not determine, in McCaLium’s 
sense, the type of leaf produced. A verification of this was 
obtained from the production of seedlings under varied condi- 
tions of light, temperature, moisture, etc. He also found in the 
case of plants grown from seed that the first leaves to appear were 
