44 Scott and Brehner . — On the Secondary 
a number of distinct points. One of these starting-points 
may be the base of the root itself. The chief formation of 
secondary tissues, however, begins at the bases of rootlets, 
and thence extends both up and down the root, and also 
peripherally. 
(2) At the base of the rootlet the thickening takes place 
entirely by means of a pericyclic cambium. At a distance 
from it there is usually only cortical cambium, and conse- 
quently the whole of the secondary tissues are here external 
to the endodermis. In the transitional region there may be 
first a pericyclic, then a cortical cambium, and the second- 
ary tissues are here formed on both sides of the endodermis. 
(3) The connection between the vascular tissues inside and 
outside the endodermis is not only maintained through the 
transitional region, but also by means of special bundles which 
traverse the endodermis at various points. 
(4) The important part played by the cortex in the forma- 
tion of secondary vascular tissues in these roots, shows that 
the morphological distinction between central cylinder and 
cortex is not necessarily correlated with a permanent dif- 
ference of function. 
III. The Secondary Growth in Thickness of 
Aristea Corymbosa, Benth. (N.O. Irideae). 
Within the natural order Irideae, which now includes 
between 900 and 1000 known species, there is a little group 
of shrubby forms. Only four such species are at present 
known to science ; all belong to the tribe Sisyrinchieae, sub- 
tribe Aristeae, and all are natives of the south-western 
provinces of the Cape Colony. The plants in question are 
Aristea fruticosa , Pers., A. corymbosa , Benth., Witsenia maura , 
Thunb., and Klattia partita , Baker. The two first-named 
species now form the subgenus Nivenia of the genus Aristea, 
of which twenty-seven species are known in all. Witsenia , 
Thunb., as at present limited, and Klattia , Baker, are both 
monotypic genera 1 . 
1 See Baker, Handbook of the Irideae, 1892, pp. 145 and 146, where the 
