312 Scott and Wager. — On the Floating-Roots 
and the flat cells of the phellogen, it is easy to find every 
intermediate stage. In the younger stages the radial rows can 
be traced outwards from the phellogen far into the floating- 
tissue ; in the mature state the regularity of the radial arrange- 
ment is of course less distinct, owing to the displacements 
which must necessarily be associated with the formation of 
the intercellular spaces (Fig. 4). 
In the oldest roots examined the floating-tissue had become 
detached, leaving behind the densely-packed cells immediately 
outside the phellogen. The walls of these cells then become 
suberised, so that the old root has a corky periderm of the 
usual character. 
The most interesting point which remains to be noticed 
is the place of origin of the phellogen. In a large proportion 
of roots with secondary thickening the pericambium, as is well 
known, is the layer from which the peridermal structures take 
their rise. To this rule, however, there are many exceptions, 
as has been especially shown by Olivier ( 1 . c.), who has observed 
that in a large number of dicotyledonous roots the periderm 
is external, arising in the outermost living layer of the cortex. 
In Sesbania we have a case different from either of those 
I have referred to. Sections from the younger roots show 
clearly that the divisions to which the phellogen owes its 
origin begin immediately outside the endodermis, in the first 
or second layer of the primary cortex (Figs. 1 and 2). In 
some cases both these layers begin to divide (Fig. 1) ; from 
comparison with later stages it is probable that in this case 
the innermost layer alone continues its divisions for any length 
of time. Careful investigation of roots in every stage of 
development leaves no doubt that all the peridermal structures 
arise exclusively from this extra-endodermal phellogen ; the 
pericambium undergoes no further development, beyond the 
two or three tangential divisions above mentioned, and the 
endodermis is persistent throughout. 
The results attained may be summed up as follows : — 
1. The floating-tissue of the roots of Sesbania is a secondary 
cortical structure, arising from a phellogen. 
