SUBMARGINAL VENATION OF FOLIAGE LEAVES 
ROBERT B, WYLIE 
An earlier paper* gave la brief account of experiments dealing 
with the conductive efficiency of certain types of vein systems in 
foliage leaves. By cutting the blade in various ways it was 
possible to show some of the advantages and also the disadvant- 
ages of different types of venation. The results indicated that 
larger veins, while highly efficient for conduction along their 
length, may constitute real barriers to movement across them, es- 
pecially when interrupted by breaks in the veins, which cuts 
doubtless lead 'to leakage. On the other hand, regions free from 
larger veins showed marked capacity for conduction in all direc- 
tions. The relatively small veins constituting the islet-borders 
were found to be capable not only of ready conduction in any 
direction in the blade but were demonstrably capable of carry- 
ing a very great overload. 
By isolating peninsulas of blade with narrow isthmus at the 
base one could readily show that the ability of the minor ven- 
ation to supply water to areas is far in excess of what would 
be ordinarily required in the normal leaf. Such results revealed 
the importance of islet-borders both for conduction and for 
mechanical support. The submarginal regions, relatively free 
from larger veins, function not only for general conduction but 
may provide for increased or even reversed flow of materials. 
This part of the leaf is thus peculiarly fitted to deal with serious 
wounds or breaks, and also to adapt the leaf to the shifting de- 
mands of transpiration as different parts of the blade may through 
external influences vary in their water loss. 
Monocotyledons with their parallel venation and marginal vein 
present relatively uniform conditions throughout the area of the 
leaf. Dicotyledons on the other hand generally show marked 
differences in various parts' of the blade due to the tree-like ar- 
rangement of the principal veins with its larger branches and 
smaller twigs. In such leaves there is reduction in size of veins 
with remoteness from midrib and the submarginal region of the 
blade becomes the portion of greater vascular uniformity. In 
* Wylie, Robert B., concerning the capacity of foliage leaves to withstand wound- 
ing. Iowa Acad. Sci., Vol. XXVIII, pp. 293-304; 1922. 
