36 Scott and Brehner. — On the Secondary 
apex of the root, and cortical in its older portion. These 
statements apply especially to D. reflexa 1 . 
Our own observations were made on the roots of Dracaena 
fragrans , Gawl., D. Draco , L., and D. angustifolia , Roxb. 
The last-named species was first examined, and in those 
roots which showed secondary thickening at all, we were 
surprised to find that it took place, from the first, entirely 
outside the endodermis, starting with the division of the first 
or second cortical layer. The pericycle was thick-walled, 
and neither had undergone any divisions, nor could do so at 
a later stage. So far as we saw there was no connection 
between the cylinder and the external vascular tissues. Our 
material, however, was limited, and it is probable that if we 
had been able to trace the tissues for some distance longitu- 
dinally, continuity through the endodermis would have been 
established. 
These observations, however, showed that the accounts in 
the literature are not complete. It was evident that the cor- 
tical thickening might start on its own account, without any 
previous pericyclic divisions, at any rate in the same region 
of the root. Hence we were led to investigate the relation 
between pericyclic and cortical thickening more thoroughly 
than had previously been done. 
Our best material was of D. fragrans , Gawk, of which we 
had a number of adventitious roots, up to almost an inch 
(2-3 cm.) in thickness, and showing secondary growth in all 
stages. Our most complete observations then were on this 
species. 
As a rule, the cambium only appears in large adventitious 
roots, and in them only at a late stage. In a fully formed 
adventitious root about 1 cm. thick we may expect to find 
secondary growth beginning. This, however, is very variable, 
and in one case we found a complete zone of secondary tissue 
in a root only 4 mm. thick. The primary cylinder of the 
roots varies very much in structure. In the small root just 
mentioned, for example, it showed an ordinary polyarch 
1 Strasburger, Itistologische Beitrage, III, 1891, pp. 403 and 508. 
