128 de Frame. — The Seedling Structure of certain Cactaceae. 
a strand of epicotylar phloem bridges the space between the cotyledonary 
phloem groups. Soon after their separation the protoxylem elements 
arranged in the tangential position begin to move inwards, leaving the 
metaxylem and the radially placed protoxylem in an isolated position 
(Diagram i, Fig. 3). Before reaching the centre of the axis these lateral 
branches pass outwards again, and the adjacent ones fuse ; thus the two 
original protoxylem groups have, by branching into three and the subsequent 
fusion of adjacent lateral arms, given rise to four protoxylem strands in 
a manner which strongly recalls Miss Sargant’s Anemarrhena type. The 
four groups of protoxylem formed in the manner just described very soon 
become connected, by the differentiation of tracheides, in such a way that 
a cross-shaped xylem plate is formed (Diagram 1, Fig. 4). This arrange¬ 
ment, however, does not long persist; the first indication of a change is 
given in the gradual breaking down of the phloem arcs opposite to the 
last-formed xylem groups; all the epicotylar ploem from this stage slowly 
dies out. Concurrently with these changes the intercotyledonary xylem 
arms gradually disappear, until finally only the two cotyledonary ‘ double ’ 
bundles with the protoxylem in the exarch position remain (Diagram 1, 
Fig. 5). All the details of the transition above described take place very 
rapidly and are restricted to the upper part of the long hypocotyl; no 
other changes occur until the root region is almost reached. At the base 
of the hypocotyl an isolated xylem element then arises in a position similar 
to that marked A (Diagram 1, Fig. 5); a little later this is followed by 
a second at B ; new elements are then rapidly formed in the centripetal 
position until finally a solid core of xylem occupies the centre of the stele 
(Diagram 1, Fig. 6). The root is thus seen to be typically tetrarch, but 
two of its protoxylem groups are disconnected from the protoxylem of the 
higher regions of the hypocotyledonary axis. 
The upper part of the root is, for a short distance, very thickly clothed 
with long root-hairs; below this region a cork cambium appears in the 
outer layers of the cortex, but at the age at which the seedlings were 
examined it had not been very active. 
Pereskia Pititache , Karw. In general external characteristics the 
seedlings of this species are extremely like those of Pereskia n . sp., 
except that the difference in size between the two seed-leaves is much less 
marked in them than in the latter. The structure of the cotyledons is also 
similar to that of Pereskia n. sp., and the transition follows the course 
indicated in Diagram 1, Figs. 1-5. There is, however, no further change 
after the gradual disappearance of the intercotyledonary xylem arms, and 
the root structure is of the type which will be described later as character¬ 
istic of the genus Cerens, for it possesses two xylem-strands and four well- 
marked phloem-bundles. 
In the seedlings of P. Pititache there is considerably less metaxylem 
