152 de Frame .— The Seedling Structure of certain Cactaceae. 
well developed epicotylar bundles are present in the upper part of the 
hypocotyl; these move towards the adjacent halves of the cotyledonary 
‘ double ’ bundles and fuse with them. Towards the base of the hypocotyl 
the four phloem groups have lost their accompanying metaxylem elements, 
and the root-structure is thus of the type characteristic of species of 
Cereus. 
Echinocactus denudatus , Link and Otto. Each of the cotyledons of 
this seedling has a median bundle and two small lateral ones, which at the 
base of the seed-leaf fuse with the central strand. Bifurcation and rotation 
of the phloem take place as in E. hexaedrophorus y but, at a later stage, 
two of the phloem groups fuse (ph^ + ph lt Fig. 16, A). 
t>K 
Fig. 16. Echinocactus denudatus. Formation of the tetrarch root, x 220. 
At a still lower level new xylem elements are differentiated on either 
side of px x , producing a slightly elongated strand of tracheides. Opposite 
to the middle of this xylem arc a phloem group arises (ph 5 , Fig. 16, b), 
and between ph 2 and ph 3 (Fig. 16, b) the appearance of a new xylem element 
shows the origin of a third xylem-bundle. The fourth bundle results from 
the breaking of the protoxylem group px lf one branch passing between the 
phloem groups ph 4 + ph Y and pk 5i , the other remaining in situ ; by means 
of these changes a tetrarch root is produced. 
Echinocactus bicolor , G. The top of the hypocotyl is occupied by two 
endarch cotyledonary bundles and six epicotyledonary strands ; the latter 
arrange themselves in two groups of three, alternating with the seed-leaf- 
traces, and, at a slightly lower level, two of each group fuse. The phloem 
