Ipomoea versicolor , Meissn. 175 
cotyledonary bundles as soon as the latter enter the stem, 
so that the hypocotyl is traversed by the cotyledonary traces 
only. 
Each pair of bundles in the hypocotyl has a common strand 
of internal phloem lying just within the two converging pro- 
toxylem-groups. As we trace the bundles down towards the 
root we find that in each pair the protoxylem-groups slowly 
approach each other, and as this goes on the internal phloem- 
strand passes out between them. Three stages of their course 
are shown in Figs. 3, 4, and 5, taken respectively at the levels 
a , b , and c shown in the sketch of the seedling (Fig. 1). As 
the root is approached the two xylem-groups of each pair 
meet and fuse, turning their protoxylem outwards. At this 
point the internal phloem is seen forming two strands, one on 
each side of the xylem plate (Fig. 4. ph 2 ). Fower down still 
the internal phloem passes completely to the outside of the 
xylem (Fig. 5) and ultimately joins the external phloem. 
The later stages of the transition are passed through within a 
very short space, as is shown by the relative positions of the 
sections as marked in Fig. 1. The main root is tetrarch, 
and its four xylem plates, corresponding to the four pairs of 
cotyledonary traces, eventually meet in the centre, the pith 
dying out. It will be seen that the course of the bundles 
agrees with the well-known type of Phaseolus. Its regu- 
larity is very favourable to tracing the passage of the internal 
phloem. 
The transitional region between stem and root shows remark- 
able peculiarities when secondary growth begins, and these 
are now to be described. 
The upper and younger part of the stem is cylindrical and 
typical in structure, as already stated. The lower part of 
the stem, however, has a flattened form ; its greater diameter 
may be as much as double the lesser, the latter being equal 
to the diameter of the cylindrical portion. The region show- 
ing this flattened form may be many feet in length in well- 
grown specimens, but the peculiarity is most marked in the 
neighbourhood of the cotyledons. The flattened appearance 
N 2 
