Roots of Cycas Seemonni , AL Braun. 67 
secondary products already considered ; these divisions first 
make their appearance in scattered cells in the layer in ques- 
tion, the exact position of which appears to be indeterminate, 
with the limitation that the process was never found to begin 
at the points opposite the primary xylem-groups. The divi- 
sions thus started eventually extend to the intervening cells, 
so as to form a single complete ring of cambium, extending 
around the vascular cylinder of the root without any inter- 
ruption, even opposite the primary groups of xylem. This 
outer cambial layer thus arises in the position usually occu- 
pied by the phellogen of the roots ; here, however, its 
character is that of a true cambium, and it produces numerous 
xylem-elements on its inner side, and hard and soft bast on 
its exterior ; thus as regards their orientation the tissues pro- 
duced of this layer are normal. As would be expected from 
the fleshy character of the root as a whole, the xylem pro- 
duced does not consist entirely of lignified elements, but 
includes a number of thin-walled parenchymatous cells 
scattered among the tracheides. 
The general structure of the root at a relatively advanced 
stage is illustrated by Fig. 3. 
It may be mentioned that in one root belonging to the type 
first described the original arrangement of the bundle was 
triarch instead of diarch ; here the phenomena of secondary 
thickening both normal and abnormal went on, mutatis 
mutandis , precisely in the same way as in the diarch 
examples, the resulting structure simply showing three inner 
rings instead of two. 
As regards the second form of thickening I have referred 
to, the primary structure of the root in question was identical 
with that of the diarch root first described, and the normal 
secondary thickening also begins in the same way; here how- 
ever the normal cambium-layer extends around the ends of 
the primary xylem-plate so as to form a single complete 
ring 1 . Before this ring is complete the first anomalous cam- 
1 As in the case described by Mettenius, 1 . c. p. 597. 
