VENATION OF FOLIAGE LEAVES 
235 
In the earlier paper (1. c., p. 301) the writer noted the strik- 
ing inability of Aslcepias syriaca to meet demands resulting from 
wounds. Excepting in the marginal areas the blades quite uni- 
formly failed to meet critical tests. Slashed longitudinally the 
strips usually died almost to the base with the exception, of 
course, of the zone along the midrib and the lateral zones watered 
by the submarginal strands. In these earlier experiments the out- 
side edge of the leaves was not disturbed and since this limited 
water loss in the outer zones to one edge, the observed results 
might have been due in part, at least, to the diminished loss 
through evaporation from but one margin in these outer strips. 
During the summer of 1921 wound experiments were carried 
on at the Iowa Lakeside Laboratory on various leaves including 
those of Asclepias syriaca and Lilac. Conditions in the marginal 
strips were made then equally difficult with those of the interior 
zones by removing first from the outer edge of the leaves a nar- 
row strip of tissue parallel to the margin. The leaves were then 
slashed longitudinally into roughly parallel zones ('fig. 1). Under 
the circumstances of the experiment each strip should suffer ap- 
proximately equal traumatic water loss. 
Two leaves of Asclepias syriaca, exhibiting typical results, are 
shown in the accompanying plate (fig. 1, d.e.). It will be noted 
that the middle, and marginal strips suffered no death of tissue 
while all other zones died throughout most of their length. The 
central zone was, of course, taken care of by the midvein, while 
the outer strips were obviously well supplied by the submarginal 
veins. 
The marked inefficiency of the intermediate strips deserves 
attention since they show so little adaptability to the altered de- 
mands forced by the conditions of the experiment. Though sup- 
plied with water from both the inner and outer ends, and appar- 
ently with equal liberty from both ends, these strips seemed unable 
to conduct water through any considerable distance. Reasons 
for this failure must be attributed primarily to the rectangular 
system of major veins in this leaf. The major laterals are nearly 
parallel while right and left the midvein and submarginal veins 
fence off the leaf blade into a series of closed rectangles bordered 
by large strands. However efficient this plan may be in the un- 
injured leaf, wounds of the type described, forcing conduction 
across larger veins show that they always, under such circum- 
stances, act as barriers. Asclepias syriaca seems then to possess 
right-angle efficiency but is relatively inefficient in other direc- 
tions. 
