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IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
many leaves of this group there is a more or less pronounced ten- 
dency to combine branches in the peripheral region into a sub- 
marginal path for conduction. The outer portion of the leaf has, 
therefore, a greater freedom of conduction, especially as regards 
the direction of flow, and probably acts as a general equalizing 
system between the various parts of the lamina. 
With the great diversity in form and structure of Dicotyledon 
leaf it is not surprising to find that even those possessing similar 
outline have marked differences in their venation and consequent 
variety in the submarginal zone. While no attempt is made at 
this time to present a detailed discussion of these differences, a 
few examples may suggest the range of differentiation. Towards 
one extreme of the series is the situation represented by Ulmus 
fulva, or Betula nigra in which the major laterals run almost to 
the edge of the leaf and so break up the submarginal strip. It 
should be noted, however, that even in such leaves the outer por- 
tion of the blade is the part most nearly free from obstructions. 
Less strict are the conditions in such leaf as that of Catalpa 
bignonioides where the major laterals are widely separated and 
stop short of the margin leaving a peripheral plexus consisting 
chiefly of islet borders with occasional larger connectives. 
Throughout a large part of this leaf there are seen only the ver- 
satile islet-borders of proven efficiency for varied conduction. 
In Lilac one finds the slanting major laterals connecting nearer 
the margin with a peripheral meshwork of veins that are inter- 
mediate in size. These anastomotic veins stretch around the outer 
part of the leaf and penetrate with larger meshes back between 
the chief lateral veins, Filling the entire area of the blade are, 
of course, the islet-borders. Lilac thus presents a triple vascular 
system but avoids large cross veins and never approaches the 
rectangular type seen in Tilia (1. c., p. 303). The graduated sub- 
marginal plexus favors distribution around the circumference and 
also radially between the larger veins. 
Asclepias syriaca possesses a sharply defined submarginal strand 
located about one-eighth of the distance from the edge of the 
leaf to midrib. The chief laterals run out nearly at right angles 
to the central vein. Without giving off important branches these 
laterals are joined in the submarginal region by heavy outcurving 
connectives thus building a prominent sinuous vein parallel to 
the edge of the leaf. Still nearer the margin the minor veins 
also unite to form a smaller strand roughly parallel to the leaf 
margin, but much less conspicuous than the principal submarginal 
vein just mentioned. 
