692 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
groups of protoxylem; triarch and diarch specimens also occur. Thus our fig. 7 
shows a triarch rootlet, and one of those figured in a previous memoir was probably 
also triarch.* We have figured a very characteristic diarch rootlet in fig. 5. 
Another rootlet (see fig. 8), itself probably tetrarch, bears a diarch branch. Minute 
diarch rootlets are excessively common in the preparations, but, us a rule, their 
structure is not sufficiently definite for us to be certain to what they belong. We 
have, however, tigured one of these doubtful rootlets in fig. 9. It occurs in the same 
preparation as that shown in fig. 8, so it is very probable that it may be of the same 
nature as the diarch branch shown in the latter figure, borne upon a rootlet of the 
usual structure. Fig. 9 is in itself of considerable anatomical interest. The rootlet, 
which is very young, is seen in oblique section. The diarch xylem-plate is not yet 
completely lignified in the middle; the minute trachez of the protoxylem are clearly 
shown at either end of the plate. A branch, which is obviously of endogenous 
origin, is seen in median section. Its xylem is directly continuous with one of the 
protoxylem-strands of the parent rootlet. A more typical example of root-structure 
would be difficult to find; we have therefore thought the specimen worth figuring, 
though we cannot be quite certain that it is of the same kind as the other rootlets. 
Returning to the typical specimens, we find that the rootlets, in some cases, 
possess the primary tissues only, while others have undergone a greater or less 
degree of secondary growth in thickness. Such differences are no doubt dependent 
simply upon age. Thus the rootlet shown in fig. 7 is destitute of any secondary 
tissues, while fig. 4 shows the first commencement of their formation. The diarch 
rootlet, represented in fig. 5, already has about five layers of secondary wood. A 
tetrarch rootlet, previously figured,t has a broad secondary zone, the wood of which 
is about ten layers thick. Other rootlets, with much thicker secondary wood, in one 
case amounting to sixty layers of trachez (C.N. 1309), are included in the collection. 
The structure of the wood of these rootlets is in all respects identical with that of 
the larger roots, except, of course, that in the former, owing to the absence of 
primary rays, the wood is not broken up into wedges. 
Fascicular rays, corresponding to the protoxylem-groups, can often be distinguished 
in the rootlets, as well as in the larger roots. 
Remains of the phloém are frequent in the better-preserved rootlets (see figs. 4 
aud 5). The secondary phloém does not seem to have attained any great thickness. 
The tetrarch rootlet, shown in transverse section in fig. 4, is very instructive. The 
position of the four protoxylem-groups ( pa.) is quite clear; at these four points no 
secondary growth had as yet taken place. In two cases the cell lying immediately 
outside the protoxylem has undergone a single tangential division ; at the other two 
points there seems to have been no division, though it is possible that the delicate 
septa may have perished. Opposite the protoxylem groups there is evidently an 
* Wittiamson, “ Organization,” Part XIL., Plate 27, fig. 1. 
+ ‘ Organization,’ Part XIL., Plate 30, fig. 5. 
