52 Thompson — The Structure and Developmetit of 



tion stages, for two bundles have acquired internal phloem, 

 while two bundles are as yet devoid of it. The region of the 

 first leaf node shows the acquired condition of the presence 

 of internal phloem in all four bundles. 



The physiological significance of this acquisition, and the 

 causes that led to it, are not clear. It is a noteworthy fact 

 that internal phloem appears only in parts of this plant where 

 pith is present. Although present in the stem, internal phloem 

 is absent throughout the greater length of the petiole. It is 

 present in the upper portion of the hypocotyl, but is absent in 

 the lower part where the pith area is becoming constricted by 

 inward growth of the xylem. Both internal phloem and pith 

 are absent in the root. In plants like Strychnos, whose roots 

 possess medullary phloem, pith is always present. 



The view may be advanced, that to utilize the pith area, 

 either for more perfect protection of the phloem, in these twisted 

 and at times contorted stems, or to increase the total amount 

 of it, a portion of the external phloem, during the evolution of 

 the plant, dipped in from the bases of the petioles, through the 

 fissures formed by the leaf traces in the vascular cylinder, and 

 became internal in position. The climbing habit of this plant 

 may be one of the factors in its evolution. 



Summary of Results. 



1. The internal phloem arises primarily as four longitudinal 

 strands, which are an integral part of the leaf trace bundles. 



2. The origin of the internal phloem is simultaneous with, 

 or slightly later than, the protoxylem and external phloem, so 

 that the leaf trace bundles are bicollateral from the first. 



3. The internal phloem patches are bounded internally by 

 a two-celled phloem sheath. 



4. The internal phloem patches grow centrifugally by 

 means of a medullary cambium, the inner and older layers 

 in time becoming crushed and obliterated. 



5. Death of the pith occurs early in the first year. 



