June, 1923] SINNOTT AND DURHAM — ANISOPHYLLY IN ACER 
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not. uniform but varies somewhat from twig to twig. Furthermore, an 
examination of twigs growing in all orientations shows that anisophylly 
appears very slightly in those twigs which are but slightly inclined from the 
vertical and becomes more and more marked as we approach the horizontal. 
The upper leaf is therefore relatively smaller in proportion to the lower in 
some of our twigs than in others; and since we have shown that the amount 
of leaf material at a given node tends to be independent of the way in which 
this material is apportioned between the two leaves, it follows that these 
differences in relative size between upper and lower leaves will be much 
greater in proportion to the size of the leaf in the upper leaves than they are 
in the lower, and that the variability in size of the upper leaves will therefore 
be greater than that of the lower, the result which we actually find to exist. 
The horizontal leaves, being relatively unaffected as to their volume, area, 
petiole length, and midrib length by these slight differences, will tend to 
be the least variable of all. In these leaves, however, the difference in 
length between the upper and lower main lateral veins also varies with 
the degree of anisophylly and thus with the orientation of the twig, in¬ 
creasing as we approach the horizontal. In a group of twigs varying 
slightly in the degree of anisophylly which they show (as do ours), these 
lateral veins will therefore tend to be more variable in length than the mid¬ 
rib, as we found actually to be the case. These differences in degree of 
variability to which we have called attention are probably only the natural 
result of slight differences in the intensity of the anisophylly displayed, 
which in turn seem to be due to, or at least to parallel, differences in the 
orientation of the twigs to the horizontal. Although this does not prove that 
gravity is the major factor here operating, it certainly favors such a 
conclusion. 
As to exotrophy and other internal factors which involve a relation 
between the twig and its mother-shoot and are believed by Wiesner and 
others to stimulate growth on the outer (thus in horizontal twigs the 
lower) side of the stem, our evidence is not of decisive value. Such a 
factor may be operative but it is evidently not constant in its effect, since 
we find that twigs vary in the degree of their anisophylly. It may be 
argued that the size of the angle between twig and mother-shoot may 
determine the intensity of anisophyllous development, but a study of 
individual branches does not confirm such a conclusion but rather suggests 
that absolute orientation to the horizontal is the chief factor. 
Evidence from this biometrical study of anisophylly therefore suggests 
that gravity is the major factor in producing the phenomenon in Acer. 
Other factors, both external and internal, may perhaps play a part, and the 
only conclusive evidence as to their relative importance must be derived 
from carefully controlled experimental work. 
