320 M. R. ENSIGN 
more, it appears that with increasing maturity there is a corresponding in- 
crease in the size of the vein-islets until maturity is reached. 
Twenty immature leaves of varying size showed a very marked varia- 
bility in the size of the islets, while in the mature leaves they tended to be 
Fig. 3. Photomicrograph showing the venation in an immature leaf of Citrus grandis. 
X 30. 
constant. The immature leaves are distinguishable from the mature also 
by their lighter green color, and by a more resinous feel. 
One may conclude, therefore, that the main skeleton work of the vascular 
system in the leaf is laid down quite early; that little additional differentia- 
tion takes place after this as the leaf continues to expand, and that the 
spaces between the bundles consequently increase in size until the leaf is 
mature. The bundles themselves become much larger in cross section as 
the leaf advances toward maturity. In very young leaves the stained 
bundles appear as fine blue threads in striking contrast to the large, rigid 
veins found in mature leaves. One thing which seems inexplicable is the 
fact that the plant seems to predetermine how much differentiation shall 
be allowed this or that leaf, so that at maturity the number of intersecting 
veins in a unit area of leaf surface shall be more or less nearly constant, 
independent of the size of the leaf. 
