2'8 o Scott and Brebner. — On the Anatomy 
situated on their inner side, towards the pith (compare Figs. 5 
and il). Conversely, the living, functional tissue always oc- 
cupies the inner part of the phloem-islands, and the outer 
part of the medullary groups. This fact of the obliteration of 
effete portions of the phloem in both regions does not 
appear to have been noticed by any previous observers, pro- 
bably because their material was either too young, or if older 
was herbarium material, in which the soft tissue is all so dis- 
organized that it is impossible to distinguish between its active 
and its obliterated portions. The facts just mentioned afford 
a most valuable clue to the direction of development in both 
cases. Another point which is easily observed is, that in the 
phloem-islands the radial rows of cells can always be readily 
traced inwards into the wood, while no such continuity exists 
in the opposite direction. It was the observation of these 
two classes of facts which first led us to question the accuracy 
of de Bary’s account of the development, and to infer that 
the medullary phloem-groups are developed centrifugally, but 
those in the xylem centripetally 1 . 
Development of the Stem. 
The earliest stage of development observed was in S'. 
spinosa , Harv., in a stem about o-6 mm. in thickness. Within 
the epidermis is a cortex consisting of about three layers of 
short parenchymatous cells. Next to this comes the pericycle, 
which is three to four layers thick ; the outer layers are formed 
of rather elongated parenchyma (not readily distinguishable 
from the external cortex in transverse section), and the 
innermost layer consists of multinucleate prosenchymatous 
elements, often radially elongated as seen in transverse 
section. In the ring of leaf-trace bundles the protoxylem 
elements already have spirally thickened walls. The xylem 
1 As confusion has occasionally arisen from the use of these words, it may be 
well to state expressly that development is said to be centrifugal when each new 
element cut off from the cambium lies further from the centre of the axis than the 
next older element, while the converse order of development is termed centripetal. 
