foimded on the Structure of their Seedlings. 35 
they get near the base of the sheath. In that region they 
approach each other until the protoxylem elements unite 
to form a single group. 
The plumular traces are inserted either on the double 
trace of the cotyledon or on each other. One finally enters 
the hypocotyledonary stele. The root is triarch, and is 
derived from the two cotyledonary traces and the single 
plumular trace according to Van Tieghem’s type 1. 
Three species of Yucca have been examined. They agree 
in possessing large stout seedlings with very well-developed 
and persistent primary roots. The first leaf appears earlier 
in Y. gloriosa and Y. aloifolia than in Y. arborescens. 
Yucca arborescens. The cotyledon contains a number of 
massive bundles. These are reduced to six or seven in the 
neck of the cotyledon, just outside the seed, and they are 
continued downwards into the sheath, and form a semi-circle 
in the thickest part of it. Four smaller bundles are commonly 
found in the wings of the cotyledonary sheath. 
The plumular traces are in the end all inserted on the 
traces of the cotyledon before the transition to a root-structure 
begins. The stele of the hypocotyl contains cotyledonary 
traces only. The six or seven main traces, which are all 
alike, are continued directly into it, and a seventh or eighth 
trace is added by the fusion with each other of the four 
wing-traces. 
The root when first formed is heptarch or octarch. The 
protoxylem has become external by the branching of the 
xylem groups and the fusion of adjacent branches in pairs. 
Yucca aloifolia. Three bundles are found in the cotyledon : 
when they reach the sheath the central one opens out into 
a typical double bundle. No wing-bundles appear. 
The four cotyledonary bundles are continued downwards 
into the hypocotyl, and three traces — ultimately reduced to 
two — enter it from the plumule. The root is hexarch, 
and the transition to a root-structure takes place as in Y. 
arborescens . 
