292 Scott and Brebner . — On the Anatomy 
Small groups of medullary phloem lie just within some, but 
not all, of the primary xylem-groups. The cells between the 
medullary phloem and the xylem form a distinct cambium 
(Fig. 12). The development of these phloem-groups is very 
unequal. In the same transverse section some are found with 
fully formed sieve-tubes, while others are only indicated 
by a few cambial divisions in the outer cells of the pith. It 
appears then that the medullary phloem in the root is of rela- 
tively late development. Similar structures are found in all 
the roots which have any pith. In the smallest lateral 
branches the xylem reaches to the centre, and here, of course, 
there is no medullary phloem. 
In some roots of the specimen undoubtedly belonging to 
vS. spinosa the outer zone of the pith is sclerotic, so that the 
central parenchymatous portion is of relatively small extent. 
In such roots there is usually a single excentrically situated 
medullary phloem-group. This may be separated by as many 
as three parenchymatous layers from the nearest sclerotic 
tissue. Here also a layer of cambium is present on the side 
towards the xylem. Longitudinal sections show that the 
medullary phloem-groups of the root contain typical sieve- 
tubes and companion cells, the older sieve-tubes having their 
plates obliterated by callus. 
Summary of Results. 
Before going on to compare the phenomena of development 
in Strychnos with those in some other Dicotyledons, we will 
shortly recapitulate the results of our observations : — 
1. The external phloem , though but little developed, con- 
tains sieve-tubes and companion cells of normal structure, 
with the exception that nuclei are found in the mature 
sieve-tubes. The latter fact is perhaps an indication of 
their rudimentary character. 
2 . (a) The medullary phloem-groups , as shown by their 
development and by their course, form an integral part of 
