568 Hatfield . — Anatomy of the Seedling and 
to four by fusion above the cotyledonary node. The strands are prevail- 
ingly mesarch throughout the greater portion of their length, becoming 
endarch or nearly so at the node. The protoxylem is in seriation with the 
centripetal metaxylem, and usually there is some parenchyma separating 
centripetal and centrifugal woods. There seems to be a tendency for the first 
elements of the centripetal xylem to develop on the flanks — i.e. right and 
left — of the protoxylem, and then, later, behind it. In the younger seed- 
lings, where germination has only just started, no cambium is visible in the 
cotyledonary traces, and the structure is that described above. In older 
plants there is an obvious cambium, which is active, adding secondary 
xylem in the usual way. It would thus seem certain that a few elements of 
the centrifugal xylem of the cotyledons are primary in origin — a conclusion 
which agrees with that arrived at by Marsh 1 from an examination of the 
foliar traces of Stangeria paradoxa : ‘ At the base we have a centrifugal 
xylem not arranged in rows ; . . . this portion is probably primary, and thus 
connects up the Cycadean foliar trace with the truly mesarch bundles of the 
Cycadofilicalesd 
(c) Plumule . The plumule is enclosed in the cotyledonary tube during 
the earlier stages of its development. The axis is extremely short, and 
bears its earliest leaves almost, but not quite, opposite each other (Text- 
fig. 3, A). Each successive leaf, as it arises, misses by a very little being 
exactly opposite the previously formed one, and so, gradually, the typical 
spiral leaf-arrangement which characterizes the older plant is evolved. 
When very young, the leaves are hooded, fleshy structures, consisting of 
parenchymatous cells ; as they develop, the lower portion becomes hollow, 
enclosing the next formed leaf by its two flanks. Apart from the leaf- 
traces, there is no development of xylem in the epicotyl of the seedling ; it 
consists solely of parenchymatous tissue, which, shortly below the growing- 
point, is separated into pith and cortex by a dome-shaped mass of extremely 
meristematic tissue — the procambium. 
(d) Hypocotyl . In the hypocotyl, a very short* distance below the 
cotyledonary node, the vascular cylinder is a protostele (Text-fig. 1). This 
is circular in transverse section in the younger seedlings, and consists 
of a central portion made u£) of primary xylem elements and parenchyma- 
tous tissue surrounded by protophloem, the whole enclosed by a pericycle 
of fibrous elements with lignified walls (Text-fig. 1, Pc.). This vascular 
cylinder is exceedingly short. In the direction of the root the xylem 
elements of the central portion of the stele are seen to separate and group 
themselves round two centres, at opposite ends of a diagonal, to form 
a diarch root with its two poles situated in the cotyledonary plane ; at the 
same time the phloem becomes concentrated into two masses in the 
intercotyledonary plane. 
1 Marsh : loc. cit. 
