Pachy podium namaquanum , Welw. 937 
plates on their terminal walls ; between the two is a distinct layer of 
cambium (c, Fig. 5). On the adaxial side of the protoxylem, embedded in 
the parenchymatous ground tissue, are several small islands of tissue 
(B 1 , Fig. 5, and Text-Fig. 3) resembling phloem in appearance, but in which 
the presence of sieve-tubes has not been ascertained. In addition to these 
phloem groups there are also, on the adaxial side of the protoxylem, a few 
scattered xylem-elements, some of which are in contact with some of the 
phloem groups. Some of the latter, however, are entirely without xylem- 
elements. As the petiole bundle characteristic of the order is bicollateral, 1 
it was at first thought that the bundle in P achypodium was derived from 
a bicollateral bundle by the degeneration of the adaxial band of phloem to 
these small isolated groups. This, however, gives us no explanation of the 
scattered xylem-elements on the adaxial side of the protoxylem. After 
carefully following the course of the leaf-trace through the protuberance it 
was found that these isolated bundles in the petiole were the continua- 
tions of the medullary bundles which accompany the leaf-trace, and in 
which, as has been stated before, the xylem had gradually decreased. 
1 Solereder ( 5 ), p. 529. 
