42 MR. R. H. COMPTON: AN INVESTIGATION OF THE 
primary root. Cotyledons shortly petiolate, oblong-ovate, thick. Lubbock 
(1892, p. 427) in describing the embryo states that the position of the radicle 
varies much in different seeds, being “accumbent, or obliquely or wholly 
incumbent," and being * compressed or subterete according to position." 
(РІ. 1. fig. 5.) 
The root is tetrarch or triarch, or may show a structure intermediate 
between typical triarchy and tetrarchy. 
(i.) When the root is tetrarch it contains four narrow plates of xylem 
meeting in the centre, and four equal groups of phloem with fibres. Pith 
begins to appear 10 mm. below the collet and rapidly increases in bulk, so 
that the early stages of the transition are passed through before the hypocotyl 
is reached. At the collet the xylem is arranged in four groups of three, a 
large median protoxylem and a pair of smaller lateral metaxylems composing 
each triad. The phloem also divides, and half-bundles go to lie over each 
metaxylem. As we pass up the hypocotyl the metaxylems gradually increase 
in bulk and become mixed with parenchyma, while the protoxylems become 
erushed ; so that half-way up the hypocotyl there are eight main bands of 
xylem, each with its own group of phloem, thus forming a system of stem- 
like collateral bundles. The lateral xylems separate in the usual way, and 
the halves with their phloems join the polar bundles. 
The cotyledonary trace consists of a pair of collateral bundles and a few 
erushed protoxylem elements between. 
(11.) In the case of the seedlings with triarch root the structure and 
changes of the individual bundles are identical with those which are found in 
the tetrareh until about half-way up the hypocotyl (Pl. 6. fig. 90). Then 
a new small group of vessels appears in the intercotyledonary plano; this is 
the representative of the fourth root xylem (Pl. 7. fig. 91). It is quite 
unconnected with the other xylem elements, and can hardly function for 
water-conduction from the root until it is put in communication therewith by 
secondary developments. It never reaches the size of the other lateral xylem 
group, but behaves in an exactly similar fashion (fig. 92). The remainder 
of the transition phenomena are precisely as in the tetrarch type, except that 
the double bundles of the cotyledon traces are somewhat smaller on the side 
of the late-appearing fourth xylem than on the other. 
(ii. In the intermediate type (seen in one seedling) the root shows, 
apparently throughout its length, four groups of xylem of very unequal size 
and unequally spaced round the stele. One of the xylems is relatively small 
and consists only of protoxylem, while the others possess metaxylem also ; at 
the collet, the three others have the usual detached metaxylem wings, while 
he small lateral xylem is without them. This condition gradually changes 
to that of the typical tetrarch type as we ascend the hypocoty], and the main 
features of the transition are identical with those in case i. 
It was found that in triarch seedlings the diameter of the hypocotyl is 
