Internal PhloLiii m Gfi-SDiuum scmpervirens, Ait. 5 1 



Origin of the Internal or Medullary Phloem. 

 In the growing apex of the stem the first cells to differen- 

 tiate from the primary meristem are the spiral trachejE of the 

 protoxylem, which are arranged in radial rows. On their outer 

 border appear groups of very small, thin walled cells, whose 

 division walls lie in all planes. Soon thereafter similar groups 

 of small cells are differentiated on the border of the pith area. 

 These represent the internal phloem patches. The course of 

 the internal phloem has been traced in older stems into the 

 petioles, so it may be regarded as an integral part of the 

 leaf trace bundle. It owes its origin to the same primary 

 meristem that gives rise to the external phloem, and the pro- 

 toxylem. Certain primary meristematic cells on the inner face 

 of the protoxylem represent the medullary cambium. To 

 the later activity of these cells the secondary growth of the 

 medullary phloem is due. A radial arrangement of the later- 

 formed medullary phloem cells is to be obser\'ed, and is an indi- 

 cation of their cambial origin. The medullary phloem appears 

 in the hypocotyl some time after the differentiation of protoxy- 

 lem and external phloem. Its origin, however, is from embryo- 

 nic cells that are a part of the original primary meristem of the 

 bundle. The appearance of these embryonic cells, on the 

 inner side of two bundles in the hypocotyl, at a definite 

 point below the cotj'ledonary node, and of similar cells in the 

 two opposite bundles in the epicotyl, just below the first leaf 

 node, may be explained as follows on phylogenetic grounds. 

 Internal phloem is a secondary character acquired during the 

 evolution of the plant. Since the hypocotyl and cotyledons 

 are embryonic structures representing the primitive stages of 

 growth of the plant, characters that have been acquired by, 

 and are adapted to, the adult stem, may reasonably be found 

 absent throughout the whole, or a part, of the hypocotyl. 

 In this plant the lower portion of the hypocotyl exhibits the 

 ancestral condition in the absence of internal phloem. The 

 upper portion of the hypocotyl and of the epicotyl are transi- 



