in the Root and Stem of Dicotyledons. 283 
the inner side of the cambium. The tertiary strands are 
differentiated from the older secondary woody parenchyma, 
when already remote from the cambium, and are especially 
characteristic of fleshy roots. Intraxylary (medullary) phloem 
has so far only been found in the roots of Strychnos and 
Chironia . 
II. Special modifications in the structure of 
the Stem. 
We are not aware that any cases have hitherto been 
recorded of the formation of medullary wood by an 
internal cambium in plants with undoubtedly bicollateral 
structure of the bundles. As was pointed out in the intro- 
duction, Weiss, in agreement with Vesque, has made the 
constant absence of medullary wood a criterion by which the 
internal phloem of bicollateral bundles can be distinguished 
from that which belongs to an independent bundle-system in 
the pith (e. g. Tecoma). On this ground, among others, Weiss 
has removed the internal phloem of certain Cichoriaceae, 
Campanulaceae, and Acanthaceae, from the former to the latter 
category 1 . We have found, however, that in some plants 
with typical bicollateral structure a true internal cambium is 
active, producing new wood as well as new phloem, the 
orientation of the two being inverted as compared with the 
normal parts of the bundle. 
1. Apocynum cannabinum , L. The Apocynaceae are enu- 
merated by Vesque, De Bary, Weiss, Solereder, &c. among 
plants characterized by bicollateral structure of the bundles, 
and the character is here of ordinal value. The above species 
(the so-called ‘Indian Hemp’ ) is thoroughly typical in this 
respect. The internal phloem is developed from the procam- 
bium simultaneously with the xylem and external phloem, 
and from the first exceeds the latter in extent. It lies close 
to the protoxylem-groups of the bundles, only one or two 
1 Loc. cit, Bot. Centralblatt, XV. pp. 321, 396, &c. 
