412 Scott & Sargant. — On the Development of 
rotation does take place. It is supplemented by the dis- 
appearance of external xylem-elements, and by the formation 
of new ones within the protoxylem. In other cases no rota- 
tion whatever occurs. The xylem-groups are all approaching 
a common centre during the transition, and the larger 
elements commonly move inwards more quickly than the 
protoxylem, thus leaving it external. In every case we have 
examined, some of the xylem elements die out of each 
bundle, and a few larger vessels are formed within the stele. 
Thus in one way or another the protoxylem becomes 
external during the transition, but never by a simple process 
of rotation, and often without any true rotation at all. 
The four other seedlings from which we have cut complete 
series of sections show much variation in the structure of 
the mode. In all, the xylem-group of the midrib remains 
distinct, and is continued into one of the xylem-groups of 
the triarch root. The characteristic xylem-crescent is also 
more or less completely formed opposite the midrib in every 
case. But in two cases this crescent shows two xylem- 
groups, in one (besides the example described) it shows 
four, and in one seven. This last case is remarkable for a 
still more considerable deviation from the type. The root- 
stele when first formed is tetrarch, the fourth xylem-bundle 
corresponding in position to the stem-bundle we have called 
A. A little lower down, however, this xylem-bundle dis- 
appears from the root-stele, and the phloem-groups on 
either hand of it unite. Thus the root in the end becomes 
triarch. This variation is probably not uncommon, for in 
three series of sections through the lower part of the tuber, 
and the upper part of the root which we have cut through 
older seedlings, two show a tetrarch structure when the root 
is first formed. In one of these cases the tetrarch root 
becomes triarch later on ; in the other, the tetrarch structure 
seems to persist. 
We are inclined to consider those cases normal in which 
three distinct groups of xylem are formed in the first node and 
are continued downwards into the triarch root. These three 
