584 Worsdell .— The Origin and Meaning of Medullary 
inasmuch as the medullary bundles are shown as arising from the internal 
phloem-strands, it follows that these latter must be regarded as forming part 
and parcel of the same system of medullary bundles. 
Acanthosicyos horridus . 1 
The material of this plant was collected by the writer on a sand- 
mound at a Hottentot village about two miles inland from Walfisch Bay in 
South-west Africa. 
Aerial Stem . 
The rigid branched stems are devoid of leaves, the place of each leaf 
being occupied by a pair of woody cylindric stipules in the form of spines. 
The cortex is very narrow. The sclerotic zone limiting the central 
cylinder externally has a very sinuous outline, forming a number of deep 
intrusions on the flanks of and around the points of which the periderm 
occurs. These sclerotic intrusions correspond in number to, and are 
opposite, the main bundles of the cylinder. In the bays between the 
sclerotic intrusions occur the very small bundles, one, as a rule, in each bay, 
of the outer ring or series of the cylinder. Each has a small internal 
phloem-strand which is very loosely, i. e. far from intimately, connected 
with the xylem. In one bay there were three small bundles, only one 
of which, viz. the middle one, possessed internal phloem. In the outer 
(cortical) bay formed by each sclerotic intrusion occurs the green assimilat¬ 
ing tissue of the stem. 
The inner (main) series of bundles of the cylinder, about sixteen or 
seventeen in number, are very well-developed, and each has a large, very 
rounded internal-phloem strand. Of these latter one to three possess xylem, 
consisting of vessels and fibres and a large quantity of parenchyma, on 
the outer side between the phloem and the protoxylem of the bundle. 
There is a fairly wide pith. 
Subterranean Stem. 
This is very thick and woody. 
The same structure is found as in the aerial stem, but there is 
relatively less xylem attached to the large internal phloem-strands, a fact 
which is probably correlated with the very large amount of wood developed 
in the bundles of the ring. 
Peduncle of Fruit. 
The fruits had at this time (April, 1910) attained a fair size (about that 
of a croquet-ball), but were not yet ripe. 
The structure of the peduncle is, in essentials, the same as that of the 
aerial stem. All the internal-phloem strands have, on their outer side, 
1 Marloth has written an interesting account of the habit and structure of this plant. 
