P achy podium riamaquanum , Welw. 939 
Perhaps the most probable view of the morphology is that we have 
here two serial buds placed one above the other in the axil of an adnate 
leaf. The upper of these is an inflorescence whose vascular supply is 
derived from the vascular ring of the stem. The lower is the apex of the 
protuberance which receives its bundles from the medullary region only. 
This view leaves the morphology of the spines themselves an open question. 
They arise from the lower of the two buds, i. e. the apex of the protuber- 
ance, and not being merely emergences they are either stem-spines or 
modified leaves. That the middle one is a leaf and the two lateral stipules 
is improbable in view of the facts that stipules are uncommon in the order 
and when present are intrapetiolar. The three spines are therefore all stem 
structures, or all modified leaves, or the two lateral are leaves, the terminal 
and shorter being a stem. Against the first of these is the absence of any 
sign of reduced leaves subtending the stems ; against the second is the 
apparently terminal position of the median spine ; the third then seems the 
most probable, and is supported by the fact that the median spine has 
sometimes been seen branching into two. 
If the above interpretation of the morphology of the spines and pro- 
tuberances is correct, we have here two serial axillary buds differing entirely 
in the origin of their vascular supply. It will be seen that the knowledge 
of the origin of the medullary bundles, which has been impossible to deter- 
mine, is necessary before the discussion can be carried any further. The 
facts that the mature spine loses its water-storing capacity, and that by the 
time it has matured the inflorescence has fallen off, suggest that the spines 
store water which is used up in the formation of the inflorescence belonging 
to the same protuberance. When the flower has fallen off the spines may 
perhaps be of biological importance in protecting the superficial tissues 
of the stem from the intense sunlight. 
5. Anatomical Relationships. 
The most striking anatomical feature which distinguishes Pachypodium 
from all other members of the order is the presence of an anastomosing net- 
work of medullary bundles and the absence of intraxylary phloem bundles. 
With regard to this one of two opinions may be held. 
I. That intraxylary phloem bundles, wherever found, are due to the 
degeneration of medullary bundles. In all other members of the 
order this degeneration has taken place. This is the opinion 
of Mr. Worsdell, and is supported by the appearance in the petiole 
of medullary bundles degenerated into internal phloem groups. 
II. That in Pachypodium the intraxylary phloem bundles are present 
and have acquired xylem as an adaptation due to the immense 
water-storing pith. 
