8 MR. R. H. COMPTON: AN INVESTIGATION OF THE 
are very like those in Acacia spp. The ring of xylem enlarges in diameter 
and breaks at the collet region, at the protoxylems, so that four metaxylem 
bands are formed, each with protoxylem edges. A prominent phloem group 
lies exactly over each of these xylems, so that the structure is stem-like, 
except that the xylem is differentiated tangentially instead of centrifugally. 
This structure continues up the hypocotyl to within 3 mm. of the cotyledonary 
node. At this level a feature appears which recalls that found in some 
Ku-Mimosex. Each of the four xylem bands gives off a branch towards the 
major axis of the elliptical stele ; consequently, of the four xylem bundles 
supplying a cotyledon, the two inner ones are smaller than the two outer. 
(Cf, Mimosa pudica, Leucena glauca, &c.) Two phloem groups enter each 
cotyledon. 
The transition phenomena thus appear to combine features characteristic of 
two tribes, the Acacieze and the Eu-Mimosez. А zone of thick-walled, small- 
celled tissue occurs in the cortex of the hypocotyl beneath the second or 
third subepidermal layer (Pl. 2. fig. 24). Fibres occur in the primary 
phloem of the hypocotyl. 
PrrHECOLOBIUM SAMAN, Benth. (РІ. 9. figs. 25-28.) | 
A tall tree. The hypocotyl tapers gradually to a stout primary root. The 
cotyledons are oblong, almost sagittate at the base, erect, fleshy, caducous. 
The seedling is figured by De Candolle (1825, p. 19). 
The root (fig. 25) contains a good-sized pith throughout. A few mm. below 
the external collet the root is octarch, containing eight Y-shaped primary 
xylems surrounding a large pith and alternating with as many phloems. No 
fibres are present. Just below the collet eight protoxylems are still present, 
but only every other one is at all well marked; the root is in process of 
change to a tetrarch condition (fig. 26). Tetrarchy is attained just above the 
collet, the four alternate xylems having completely disappeared. The xylem 
is now arranged in four broad tangential bands of metaxylem with median 
externally projecting protoxylem points. The phloem remains in eight 
groups, one lying dorsally to each of the metaxylem wings; there are 
occasional signs, however, of the phloems fusing in pairs. The pith has now 
dilated very considerably, and continues to do so at the expense of the cortex 
as we ascend the hypocotyl. Fibres appear at the base of the hypocotyl, 
lying just outside the phloems. The xylem is much less bulky in the 
hypocotyl than at the base of the root, and continues to decrease all the way 
up the hypocotyl. Half-way up the hypocotyl the eight phloem groups have 
been reduced to four by fusion of adjacent pairs between the planes of the 
protoxylems. The xylems now break up—each polar xylem into a median 
protoxylem and a pair of metaxylems ; each lateral xylem into two groups of 
metaxylem with protoxylem on their adjacent edges (fig. 27). The stele 
then becomes somewhat elliptical, and the lateral groups separate further and 
