278 Scott and Brebmr.—On Internal Phloem 
the corresponding medullary phloem-groups arise. Each 
group is formed by the subdivision of an elongated cell of the 
young pith, or more than one such cell may take part in the 
process. Chironia then is a plant to which Heraii’s objection 
to the term bicollateral would be especially applicable. The 
internal phloem arises late compared with the rest of the 
bundle and not, strictly speaking, from the same procambial 
strand 1 . But to these differences we are not disposed to 
attach great importance. The medullary phloem accompanies 
the bundles on their exit to the leaf and only disappears in the 
finer branches 2 . 
The interxylary phloem-strands begin to be formed when 
the secondary wood is about fifteen elements thick. They 
are similar to those in the pith, only more regular owing to 
their cambial origin. We have satisfied ourselves that 
Vesque is right in stating that they are developed on the 
inner side of the cambium, so here we have a case of centri- 
fugal interxylary phloem as contrasted with the centripetal 
development in Strychnos. These phloem-islands are formed 
in great numbers. In a thickened stem the normal phloem is 
quite insignificant in amount compared to the medullary and 
interxylary systems jointly. 
Before we leave the stem it may be pointed out that here 
also there is a typical endodermis and uniseriate pericycle, 
both of which are easily recognised even in the thick stems. 
So far as we know no observations have previously been 
made on the root of Chironia. A thickened root has the 
structure represented in Fig. 9. The cortex, which is not 
thrown off by periderm, is very lacunar ; it shows a distinct 
exodermis. Its innermost layer, the endodermis, undergoes 
radial cell-division, enabling it to keep pace with the growth 
of the vascular cylinder within. The pericycle is one or two 
layers in thickness. The external phloem is only moderately 
developed. The xylem-cylinder is dotted over in all parts 
1 Our observations on the development of Strychnos may be contrasted with this. 
See Annals of Botany, vol. III. p. 281, PI. XVIII. Fig. 1. 
2 Cf. Weiss, loc. cit., Bot. Centralblatt, XV. p. 401 
