38 Scott and Brebner . — On the Secondary 
transition happens to be a sharp one. In the mixed case (3) 
it is remarkable how thoroughly homogeneous the secondary 
tissues are, whether they are of pericyclic or cortical origin. 
The same bundle may pass out from the one zone into the 
other, one part being formed by the cortical, and the other by 
the pericyclic cambium. Some of the endodermal cells are still 
thin- walled when the secondary growth begins, and conse- 
quently are not easily recognisable when the displacements 
due to thickening have taken place. Hence the endodermis 
may appear to be ruptured at more points than is really the 
case. It is very probable that the thin-walled endodermal 
cells may themselves take part in the cambial divisions, as 
was noticed by Morot 1 . We did not find a clear case of this 
however. At the base of the adventitious root, near its 
insertion in the stem, it appears that the whole endodermis 
is thin-walled, and in advanced stages it is here impossible to 
make out the limit between pericyclic and cortical formations. 
This peculiar mode of growth is really only a special case 
of the type of secondary thickening prevailing in Monocoty- 
ledons. There is not as a rule a single initial layer here, as 
there is in typical Dicotyledons and Gymnosperms 2 . The 
same cambial cell only continues active for a limited time, 
and then the divisions are taken up by an adjacent cell 
towards the exterior 3 . An extreme illustration of this 
process is afforded by Fig. 8, which shows an early stage of 
purely cortical growth in thickness in the root of D. Draco. 
Here three or even four distinct rows of cortical cells have 
already taken up the cell-division. It is essentially the same 
phenomenon when pericyclic is succeeded by cortical divisions, 
only here there is usually a thick-walled endodermis to be 
overleapt. If this physiological barrier were really continuous 
it would probably be an effectual obstacle to any such mode 
of growth. We know, however, that it is not absolutely 
1 1. c. p. 248. 
2 Some doubt has been recently cast on the constancy of the initial layer even in 
these classes; see Raatz, Stabbildungen im secundaren Holzkorper und die 
Initialentheorie, Pringsheim’s Jahrbiich., XXIII. 1892. 
3 Cf. Strasburger, Hist. Beitrage, III, p. 396. Roseler, 1 . c. p. 309. 
