MULSOW: DIFFERENTIATION OF OAKS. 275 



divides longitudinally, and the inner ends bend up to meet the 

 ends of the upper layer which are bent down. This middle 

 layer of phloem also extends out into a few of the larger side 

 veins, but only to a distance of about 4 mm. This method and 

 place of ending was the same in the three trees of Q. rubra 

 studied. (Figs. 17-19.) 



In Q. schneckii this middle layer of phloem extends about 

 two-thirds the distance from the base to the apex of the leaf, 

 which is usually the point where the vein of the upper largest 

 lobe separates from the midrib. The middle of the phloem 

 ends by forming the partition between the midrib and the 

 side vein branching from it. This usually unites with the 

 upper layer just before the partition is formed between the 

 vein and the midrib. The upper layer of phloem bends down 

 in the middle to unite with this middle layer of phloem, which 

 is almost divided longitudinally to unite with the two portions 

 of the lower phloem layer which form the lower layers of the 

 midrib and vein. This middle layer of phloem also extends 

 out into the side veins about one-half the distance to the apex 

 of the lobe. These characteristics were the same in the three 

 trees of Q. schneckii studied. (See figs. 23-25.) 



In Q. coccinea this middle layer does not end with the larger 

 side veins as was the case with the other leaves studied. It 

 extends about three-fourths the distance to the apex of the 

 leaf. It first divides longitudinally, then each part unites with 

 the outer ends of the upper layer. This middle layer extends 

 up the side veins about two-thirds of their length. (See figs. 

 20, 21, 22.) 



In Q. macrocarpa this layer of phloem extends nearly three- 

 fourths the distance to the apex of the leaf, where small veins 

 branch off. It forms the partition between the veins and the 

 midrib, and there are usually two veins branching off at the 

 same place or nearly so. (See figs. 26 to 30.) This phloem 

 extends into only the two large side veins, and extends up them 

 about two-thirds the distance to the apex of the lobes. 



STEM. 



In cross sections of the current year's growth of the stem 

 the division into vascular bundles is more noticeable in Q. ruba 

 and Q. coccinea than in Q. schneckii and Q. macrocarpa. In 

 Q. macrocarpa the cork-cambium usually produces enough cork 



3 — Univ. Sci. Bull., Vol. IX, No. 20. 



