376 BOTANICAL GAZETTE [November 



the expanse is increased." Matte (6) assigns almost exactly the 

 same cause. 



A definite cause for the girdling cannot be given at present, for it 

 seems to be deeper seated than at first suspected. The even distri- 

 bution of the four strands of each leaf in the vascular cylinder appears 

 to be the dominating factor. When the primordium appears on the 

 stem tip, its distance from the potential vascular cylinder is very short, 

 and the same conditions that determine cell division in the develop- 

 ment of the leaf, cause the differentiation of cells along certain paths 

 that run from definite points in the procambium to definite places into 

 the developing leaf. After the traces have been started, they con- 

 tinue to develop with the further growth of all the tissues; new organs 

 appear and intercalary growth continues; thus the strands are 

 lengthened more and more and their curves are widened to keep pace 

 with the ever increasing growth of the plant. 



Summary 



i. The vascular cylinder of the embryo is a protostele, but becomes 

 a siphonostele in the seedling. It is very short and squarish in cross- 

 section, one of the diagonals of the section being at right angles to the 

 inner faces of the cotyledons, and the other parallel with them. Near 

 each of the four corners is a group of protoxylem cells, the long 

 diameter of whose section extends along the diagonal. 



2. The four protoxylem groups extend downward to form the 

 protoxylem of the root. 



3. From each protoxylem group a foliar strand runs outward for 

 a short distance and then branches, the branches separating at a 

 wide angle and continuing outward until under the cotyledon, when 

 they ascend it; the result is four strands for each cotyledon. The 

 two protoxylem groups on the diagonal perpendicular to the inner 

 faces of the cotyledons give rise to the two inner strands of each coty- 

 ledon; while the two protoxylem groups on the diagonal parallel with 

 the inner faces of the cotyledons give rise to the two outer strands 

 of the cotyledons, that is, one branch runs into the outer edge of the 

 one and the other branch (from the same group) runs into the edge 

 of the other cotyledon (opposite the first). 



4. For each leaf or scale four strands leave the vascular cylinder 



