286 
AMERICAN JOURNAL OF BOTANY 
{Vol. io. 
Summary 
1. In horizontal twigs of certain species there are marked differences 
between the leaves borne on the upper and those on the lower sides of the 
twig. Difference of opinion exists as to the causes of this anisophylly. 
2. In horizontal anisophyllous twigs of a single tree of Acer saccharum, 
measurements were made of the various linear dimensions and of blade area 
and volume in iooo leaves, these being divided into four groups according 
to their position on the twig—the upper and lower members of the ver¬ 
tically oriented pairs and the right and left members of the horizontally 
oriented pairs. 
3. The vertical upper leaves are the smallest, the vertical lower the 
largest, and the horizontal leaves are intermediate between these. At a 
given node the combined size of the two members of a leaf pair is essentially 
the same regardless of whether the pair is” a vertically or a horizontally 
oriented one. 
4. In thickness of leaf-blade, the four groups are almost identical. 
5. In proportion to their midrib length, the vertical upper leaves are 
the broadest and have the shortest petioles; the vertical lower leaves are 
the narrowest and have the longest petioles, and the two horizontal leaves 
are alike and intermediate between the two former. In each horizontal 
leaf the lower lateral vein is markedly longer than the upper. 
6. The vertical upper leaves tend to be the most variable in their 
dimensions, the vertical lower next, and the horizontal leaves the least. 
In the horizontal leaves the two main lateral veins are more variable than 
the midrib. These differences in variability are evidently due to the fact 
that the twigs studied differ somewhat in the degree of anisophylly which 
they display. This is probably due, in turn, to slight differences in the 
orientation of the twigs with reference to the horizontal, since a study of 
the degree of anisophylly in this species indicates that its intensity is pro¬ 
portional to the degree to which the twig diverges from the vertical. 
7. Evidence here presented indicates that gravity is more important 
than light or other environmental or internal factors in producing aniso¬ 
phylly in Acer. 
Connecticut Agricultural College, 
Storrs, Connecticut 
BIBLIOGRAPHY 
Boshart, K. 1911. Beitrage zur Kenntnis der Blattasymmetrie und Exotrophie. Flora 
103: 91-124. 
Doposcheg-Uhlar, J. 1913. Die Anisophyllie bei Sempervivum. Flora 105: 162-183. 
Figdor, W. 1897. Uber die Ursachen der Anisophyllie. Ber. Deutsch. Bot. Ges. 15: 
( 7 o)-( 79 ) : 
--. 1904. fiber den Einfluss ausserer Faktoren auf die Anisophyllie. Ber. Deutsch. 
Bot. Ges. 22: 286-295. 
-. 1909. Die Erscheinung der Anisophyllie. Leipzig and Wien. 
