and Histogeny of Strychnos. 299 
the cambium, and sometimes in the pith. In the Cruciferae 
a group of phloem is first formed, then a ring of cambium 
surrounding it gives rise to additional phloem-elements on the 
inside, and to a little xylem on the outside 1 . According to 
Weiss, these concentric bundles are continuous with leaf-trace 
bundles of the stem 2 . 
In Bryonia , on the other hand, where the new formation 
takes place around a normal group of xylem, the phloem lies 
externally, and the xylem internally, with reference to the 
special cambium 3 . These interxylary bundles are only indi- 
rectly connected with our immediate subject. They bear the 
same relation to true phloem-islands as complete medullary 
bundles bear to the internal phloem-groups of a normal ring of 
bicollateral bundles. 
Summing up the results of our survey, we see that the few 
satisfactorily investigated examples of phloem-islands are about 
equally divided between the centripetal and centrifugal modes 
of development. Thus HeraiFs ‘ general law ’ above cited does 
not hold good universally, any more than does the opposite 
generalization of Solereder. Every case must be investigated 
on its own merits. 
It is worth mentioning that interxylary phloem-islands 
in the stem generally occur in connection with bicollateral 
bundles. According to Solereder, the exceptions to this rule 
are limited to the five genera Dicella , Sarcostigma, Salvador a , 
Dobera , and Hexacentris 4 . The same author finds that the 
occurrence of phloem-islands is not so constant a character 
for large groups as is the presence of bicollateral bundles. 
This is quite what would have been expected a priori. 
Phloem-islands are a secondary formation arising late in the 
development, while bicollateral bundles are part of the primary 
structure formed on the first differentiation of the tissues. 
1 See Figs, i and 2 of Weiss’s paper in Flora. 
2 Bot. Centralblatt, xv, p. 407. 
3 Flora 1 . c. Figs. 5 and 6. By an oversight this is wrongly described in the 
text of the paper, p. no. 
4 1. c. p. 33. 
