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MR. К, Н, COMPTON: AN INVESTIGATION OF THE 
the higher internodes of Pisum sativum, where the stem has increased in 
diameter and the pith is permanent. 
The structure of tetrarch seedlings of Vicia Faba has been shortly described 
by Chauveaud (1911, p. 350), who mentions the two cotyledonary and two 
plumular xylems, and also the medullation of the first epicotyledonary 
internode. 
The structure of the hypocotyledonary axis varies in different specimens. 
The number of xylem poles in the root may be four, five, or six. Five seems 
to be the commonest number. A seedling with the tetrarch root will be first 
described. 
(1) There are in the root four xylem plates each consisting of 2-3 rows 
of vessels, arranged radially around a fair-sized pith which extends to the 
root apex. Alternating with these are four groups of phloem, which consist, 
in the upper part of the root, of a central mass of stout fibres, encircled 
by a ring of clear-celled parenchyma, this again being surrounded by patches 
of sieve-tubes and companion-cells. 
As we ascend the axis, through its region of gradual dilation below the 
cotyledons, the xylem plates broaden internally and the pith increases in size. 
The stele becomes somewhat elliptical, and the xylems on the minor axis 
spread out almost tangentially, and each tends to break into three groups— 
a median group containing the protoxylem and some metaxylem, and two 
lateral metaxylem groups. The median groups suffer a re-arrangement of 
their elements and gradually become endarch, the change being much the 
same as that which occurs in a cotyledon trace in the region of transition ; 
in the endarch position these median bundles of the triads enter the epicotyl. 
The lateral metaxylems also enter the epicotyl without undergoing much 
change. The median bundles retain signs of a double structure for some 
time, and become the midrib bundles of the first two leaves respectively, 
which lie in the intereotyledonary plane. The lateral metaxylems become 
the lateral “ faisceaux réparateurs ” of the first epicotyledonary internode. 
To return to the xylem bundles which lie on the major axis of the elliptical 
stele below the cotyledons. They quickly assume a mesarchiarrangement, 
owing to the development of the metaxylem on the flanks of the protoxylem 
instead of internally to it; at the same time the metaxylem becomes mixed 
with parenchyma and comes to consist of spiral vessels опу. In the mesarch 
condition the double bundles pass out to the cotyledons. 
The phloem bundles with their contained fibres give off branches below 
the cotyledons, and each cotyledonary double xylem bundle takes with it 
portions of the two adjoining phloems. The portions that remain behind 
give off branches in the opposite directions, which fuse in the intercotyle- 
donary plane dorsally to the median leaf trace xylem bundles ; the residual 
bundles become the lateral pairs of phloems of the epicotyledonary central 
stele, one of them lying dorsally to each of the “ faisceaux réparateurs.” 
