and Histogeny of Strychnos . 283 
with medullary phloem the periderm is internal, so that the 
normal phloem comes to lie dangerously near the surface. 
This is not the case in Strychnos. In the species examined 
the periderm was always external (hypodermal in some, 
epidermal in others, e. g. S. Grayir Griseb.), and though the 
primary cortex is often thin the external phloem is very 
well protected by sclerotic tissue, and is exposed to no 
greater risks than in most normal plants. 
It may be noticed that in the older specimens referred to 
S. spinosa , groups of very beautiful stone-sclerenchyma occur 
in the pith. 
More advanced stages of development were chiefly observed 
in 5. nnx vomica , L. The anatomy of the stem differs in some 
points from that of >S. spinosa. The primary cortex is thicker, 
consisting of about seven layers of cells. The pericycle is 
limited externally by a single layer of very long fibres with 
thick cellulose walls. These elements are multi nucleate when 
young. Within the fibres there are one or two layers of 
parenchyma, and then comes a sclerotic ring, usually two 
layers thick, of very thick-walled lignified cells, which are 
parenchymatous in form, but of very variable length 1 . The 
minute groups of external phloem either abut directly on 
the sclerotic ring, or in some cases are separated from it by a 
single parenchymatous cell. These phloem -groups are even 
more rudimentary than in S . spinosa . The centripetal activity 
of the cambium is for a time very slight. When the xylem is 
from ten to fourteen cells in thickness, its outer margin is 
often separated from the sclerotic ring by three cells only, 
including the cambial layer. The wood itself shows no special 
peculiarities. Besides the spiral vessels of the primary xylem, 
it consists of vessels with bordered pits, very abundant and 
thick-walled libriform fibres, woody parenchyma, and medullary 
rays one to three cells in thickness. The groups of medullary 
phloem are of large size. At the stage we are describing the 
larger groups are about fourteen cells and the smallest about 
1 The pericycle of this species was described by Morot, Recherches sur le 
pericycle, in Ann. des Sci. Nat. ser. 6, T. xx, 1885, p. 264. 
