484 Pearson. — Anatomy of the Seedling of 
preceded by a definitely triarch stage as described and 
figured in this case. When the triarch arrangement does 
not occur, the xylem-masses pass to the centre, as in 
the case of the triarch cylinder, and the phloem forms 
two masses, one on each side of the slightly-elongated band 
of xylem. 
The concentration of the vascular mass in the centre of 
the hypocotyl proceeds, and the cambium becomes extended, 
and finally more or less completely surrounds a central 
mass of xylem. At this stage the centre of the stele is 
occupied by a mass composed of three groups of protoxylem 
(Fig. 14,/. xyl)i no pith being present. The cambium gives 
rise to secondary xylem and phloem and wide medullary 
rays (Fig. 14): but the cambium and the tissues formed from 
it never completely surround the central mass of protoxylem. 
The diameter of the whole cylinder is, however, very small 
compared with that of the hypocotyl. The cylinder has 
this structure at the point of origin of the cotyledons, and 
for a very short distance (about J mm.) below it. 
The Stem. 
The plumule undergoes very slight development, and the 
stem is, in consequence, very short. The growing-point 
becomes displaced laterally by the leaves, and is enclosed 
in the sheathing base of the youngest of them. 
At the insertion of the cotyledons the central vascular mass 
(Fig. 14) breaks up into two equal portions, from which are 
derived the cotyledonary leaf-traces. The remainder*of each 
portion divides into several smaller bundles, which in the 
lower part of the stem are arranged in a more or less regular 
ring around the enveloped pith. Above this the stem- bundles 
pursue a very irregular and generally oblique course, mean- 
while giving off traces which pass to the foliage-leaves. 
The xylem is entirely centrifugal in origin (Fig. 15). As 
has been shown for other Cycads *, the secondary wood is 
1 Worsdell, loc. cit., Ann. Bot., vol. x, 1896, &c. 
