328 
Hill and de Fraine.—On the 
to be in a state of transition, their protoxylems being directed towards each 
other and outwards. The completion of this rearrangement forms one pole 
of the primary root. The bundles on the other side of the axis continue 
their course straight downwards; the protoxylem of the two strands 
situated in the cotyledonary plane, at the upper level of, or immediately 
above, the sucker, turn towards each other and outwards, and thus form 
the second pole of the root-structure, which is organized before that on the 
foot-side of the axis. 
In seedlings of the stage illustrated in PL XXIII, Fig. 14, isolated proto¬ 
xylem elements may be found at some distance above the foot, at which 
higher levels they are few in number and separated one from the other ; 
but lower down they increase in number, and ultimately become attached 
to that protoxylem pole of the root situated on the side of the axis opposite 
the foot. These isolated elements have not been observed in the corre¬ 
sponding position on the sucker side of the axis. This peculiarity, in all 
probability, is due to the elongation of the axis after the rearrangements of 
the vascular tissues have been accomplished. 
The remaining changes take place on normal lines; the phloem- 
masses of the strands directly concerned in the transition pass to one side 
and join with the bast of the bundles situated in the intercotyledonary 
plane, which close up and so produce a proper root-structure which, in the 
species examined, G. scandens , G. Gnemon, and G. moluccense , is diarch. 
Attention may now be drawn to a few minor points in the structure of 
the hypocotyl. In seedlings of about the stage indicated in PI. XXIII, Fig. 14, 
a lignification of the elements of the central ground-tissue occurs in the 
lower regions of the axis ; this alteration takes place centripetally, but 
immediately above the foot the thickening is not nearly so extensive as at 
higher levels. In the same parts of the axis, and at about the same time, 
an interfascicular cambium arises and also a phellogen which is epidermal 
in origin. The periderm formation below the sucker, i. e. in the root 
region, is internal. 
It is clear that the transition-phenomena in the seedlings of Ephedra , 
Welwiischia , and Gnetum are essentially the same ; each pole of the diarch 
root-structure being formed by the rearrangement of the vascular tissue 
of two seed-leaf-traces. In the case of Gnetum this similarity is obscured 
by the larger number of cotyledonary bundles and by the numerical 
increase of these in the hypocotyledonary axis. In all cases the transition 
to root-structure occurs at the lower end of the hypocotyl. On the other 
hand, there are some differences, the chief of which are in the number 
of vascular bundles in the cotyledons. In Ephedra each seed-leaf has two 
strands which retain their identity until the transition is accomplished; 
in Welwitschia the number of traces in the blade of each cotyledon is 
eight or more, but just before entry into the axis, these fuse together; thus 
