294 Harris M. Benedict 



dry situation, has a combined length of veins to the square millimeter of 

 leaf blade of 1065 millimeters; while Trifolium medium, a moisture- 

 loving plant, has a length of veins of but 460 milHmeters. This, however, 

 does not eliminate the effects of heredity. Schuster's investigation, on 

 the other hand, was a detailed study of the effect of definite envu'on- 

 mental factors on the same species. His results will be referred to 

 frequently in the course of this paper. 



The study of the arrangement of the finest veins of the leaf has the 

 advantage that these veins are directly in contact with the active cells 

 of the leaf, and must therefore be influenced by the state of the activities 

 of these cells. The direct stimuH causing the development of branches 

 from the sides of the existing veins, which thus subdivide the leaf blade, 

 are not yet definitely known, but they must be affected to some degree 

 by the conditions of the cells in the area into which the new branches 

 penetrate. This matter will be taken up later. It is referred to here 

 in order to emphasize the fact that the arrangement of the nerves in the 

 leaf is related to the activities of the other cells of the leaf. 



Uniformity in size of vein islets on different parts of a leaf 



Schuster (1908) states that there is a definite uniformity as regards 



venation in all parts of the leaf, except close to the midrib and at the 



apices. Many tests were made of different parts of the same leaf of 



Vitis vulpina and a variation of less than three per cent was found. One 



test on a leaf of vine 2 gave the following figures: 



Square 

 millimeters 



Average of areas of five pieces from the right side .... . 4560 



Average of areas of five pieces from the left side .4567 



Largest average of one piece from either side 0.4613 



Smallest average of one piece from either side 0.4511 



In other words, the largest average found was one per cent above the 

 average for the whole leaf, while the smallest was one and one-tenth per 

 cent below, and this may serve as a typical example. 



Influence of size of leaf blade on size of vein islets 

 Schuster's results indicate that the relation between the size of the leaf 

 and the combined length of veins m a given area is not of marked impor- 



