40 Scott and Brebner. — On the Secondary 
zone was attained, not at the base of the main adventitious 
root itself, but at the insertion upon it of a branch-root. 
From this point the secondary tissues thinned out both in the 
upward and downward direction, and also peripherally. Near 
the insertion of the lateral root we constantly found peri- 
cyclic thickening ; at a distance from it, in whatever direction, 
the secondary zone was invariably formed in the cortex. 
The transition was in some cases a sudden one ; more usually 
it was gradual, and in the intermediate region both pericyclic 
and cortical thickening had taken place. 
Further, we found that in these cases the thickness of the 
secondary zone bore no relation to the upper or lower side of 
the root, but was always greatest near the insertion of a 
branch root, wherever the latter might arise. 
We will describe a particular case more in detail. In 
a root of D. fragrans, 2-3 cm. in diameter, we examined radial 
sections passing through the base of a branch-root. At the 
insertion of the latter there had been abundant secondary 
development. There are numerous zones of secondary 
bundles, the inner of which slope obliquely outwards, forming 
a connection with the tissues of the branch-root. The outer 
secondary bundles have an approximately longitudinal course. 
A normal zone of thickening has in fact been built up upon 
the secondary network of bundles belonging to the base of the 
branch. The whole of this secondary mass has been formed 
by a pericyclic cambium ; the endodermis lies entirely outside 
the secondary tissues. As we recede from the insertion of 
the rootlet in either direction, the thickness of the secondary 
zone gradually diminishes. The endodermis curves rapidly 
inwards, passing obliquely through the secondary tissues, 
which in this region have been formed partly within it and 
partly to the outside. Further on still the endodermis takes 
a straight course, and borders directly on the primary cylinder. 
Here the thickening has been exclusively cortical. In the 
transitional region the endodermis is often interrupted, and 
sometimes we could trace a secondary bundle through it from 
the inner into the outer zone. As regards the actual dimen- 
