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AFFINITIES OF DAVIDIA INVOLUCRATA. 305 
l. VASCULAR STRUCTURE OF THE SEEDLING AND GENERAL ANATOMY. 
The vascular structure of the seedling of Davidia is of interest on account of its degree 
of variability. The vascular skeleton before secondary growth occurs consists of two 
opposite cotyledonary traces, each composed of two or three bundles. These traces 
alternate with plumular vascular tissue (Pl. 32. figs. 1, 4). Each cotyledon trace passes 
intact throughout the hypocotyl into the root. In the root, however, adjacent phloem 
horns of the opposed arcs ultimately unite (Pl. 32. figs. 3,8). Whether the cotyle- 
donary trace consists of two or three bundles, the xylem of each bundle bifurcates, the 
parts diverge towards mid-hypocotyl—change from endarch to exarch structure taking 
place by rotation of protoxylem pairs towards one another—and then converge towards 
the root (Pl. 32. figs. 1-8). Each xylem group of the trace thus preserves its identity. 
Each phloem group bifurcates, the parts diverge downward towards the root, and 
portions of adjacent phloem groups unite. When the cotyledonary trace is two- 
bundled the root is tetrarch, when three-bundled, at first hexarch. The behaviour of 
the xylem in converging towards both ends is similar to that in the well-known case of 
Fagus sylvatica and appears to differ only in degree from the cases mentioned by 
Sargent, Hill, and others, where the xylem of a trace-bundle becomes exarch without an 
appreciable divergence of parts. Early developmental stages of this system are well seen 
in the very young germinating seedling before the endosperm has entirely disappeared. 
The cotyledonary traces are distinct and consist each of a single arc of conducting tissue. 
The bundles of the trace become distinguishable during the growth of the petiole. In 
the hypocotyl, when the arrangement is symmetrical, there are eight or twelve protoxylem 
groups converging in pairs downward towards the as yet undifferentiated root and 
upward into the cotyledonary arc. In the case of tetrarch seedlings the groups are not 
always of equivalent value and their elements may be somewhat scattered. In 
subsequent growth these irregularities become emphasized and one or more of the 
protoxylem groups become subordinate or apparently suppressed. The four protoxylem 
groups at the horns of the ares do not become suppressed, but the bundles associated 
with them show an increase in size correlated with subordination or suppression of the 
median groups. "There is, therefore, almost a complete series of stages towards the 
development of four xylem strands passing from cotyledon to root without bifurcation, 
each individual strand rotating, so that the xylem strands of each trace rotate toward 
one another into a tetrarch root. 
Secondary growth sets in at the time when the first foliage leaves are unfolding, and 
the plumular vascular tissue increasing in importance in the hypocotyl supplants 
the cotyledonary system. At this time the fibres become prominent and increase 
in number in transverse section owing to sliding growth, but the characteristic 
arc-shaped appearance in tangential section (see p. 306) is not yet well-marked. Their 
distribution in the hypocotyl is indicated by the diagram (Pl. 32. fig. 9) and the 
following table :— 
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