546 
Bancroft.—A Contribution to our 
adaxial metaxylem elements (PL XXVII, Fig. i ; Text-fig. io). 1 The 
endodermis and the remaining stelar tissues close round the xylem, and the 
leaf-base separates from the stem in the same way as an ordinary branch. 
In its outward passage the petiolar bundle may turn slightly away 
from the stem stele, this deflection being apparently more marked in a than 
in /3 types (cf. Text-fig. 9 with Text-fig. 8, and PI. XXVI, Figs. 3 and 7). 
ii. a petioles .—These are circular in outline and are smaller than the 
stems, having an average diameter of 1 *5 mm. They each possess a single 
xylem strand—0-45 mm. x 0-3 mm. in average dimension—which varies in 
shape from semilunar, at its lower levels, to elliptical or bluntly oblong as 
seen in transverse section ; there is occasionally 
a slight indentation on the abaxial margin of the 
strand (Text-fig. 11, a). The position and beha¬ 
viour of the protoxylem elements is exceed¬ 
ingly variable. The initial group always exhibits 
a tendency to divide, and typically there is a 
definite division at some point above the separa¬ 
tion of the petiolar strand from that of the stem ; 
the resulting condition of diarchy, however (Text- 
fig. 11), does not appear to set in at any fixed level. 
As the leaf-base passes away from the stem, the 
adaxial metaxylem elements, when present, tend to 
break down and disappear (PI. XXVII, Fig. 4 ; see 
also Text-fig. 11), so that the protoxylem is again 
external at a slightly higher level in the petiole. 
It may appear to occupy two shallow grooves, or 
to form two blunt points (Text-fig. 11,0 and b). 
In cases where there is no definite division of the 
protoxylem group, the more or less flattened 
adaxial margin of the xylem seems to consist almost entirely of small 
elements 2 (Text-fig. 9). 
The cortex and epidermis of a petioles present the same characters as 
those of a stems. 
iii. j 3 petioles .—These also are circular in transverse section, and are 
1 Hick (’ 96 ), p. 10 : ‘the smaller segment seems to develop little or no xylem on its inner side.’ 
In PI. I, Fig. 5, Hick shows an a stem and petiolar bundle from slide Q 101, Cash collect.; the 
protoxylem of the leaf-trace appears to be slightly immersed. In Figs. 3 and 4 (from Q 103 
and Q 102) are shown two stages in the separation of a (3 petiole, in which the protoxylem is 
external. 
2 Neither Hick, Scott, nor Seward mentions the number of protoxylem groups in the petiolar 
bundle of A. cylindrica. Hick (’ 96 ), p. 11, says the petiolar bundle is ‘semilunar in form with 
small elements on the convex side’. According to Scott (’ 08 ), p. 333, ‘ the nearly straight bundle 
has the protoxylem points all on the same side’. Seward (’ 10 ), p. 438, describes the leaf-traces as 
‘ semilunar in section, with the protoxylem on the flatter side \ These three accounts indicate the 
variability of the petiolar vascular system. 
Text-fig. io. Diagram 
of a leaf-trace just above its 
separation from the stem-stele. 
Note the undivided protoxylem 
group of the leaf-trace, and the 
occurrence of adaxial meta¬ 
xylem. x 35. (From slide 
187, Williamson collect.) 
