310 Scott and Wager . — On the Floating-Roots 
a merely superficial likeness ; but further comparative investi- 
gation will be necessary before this point can be decided. 
Sesbama aculeata , Pers., is a very different plant from the 
Mimosa- like Neptunia . Sesbania belongs to the papilio- 
naceous tribe Galegeae, and therefore has no near relation- 
ship with the plant already dealt with. In spite of this we 
shall find many points of agreement between them as regards 
the tissues under consideration. 
Sesbania has a tall upright stem, rising high above the 
water ; in this case the floating-tissue is developed in the 
aquatic roots. In external appearance the tissue much re- 
sembles that on the stem of Neptunia , forming soft spongy 
masses as described above. The roots are much branched, 
and it is only on the relatively main roots that the floating- 
tissue is formed, the finer lateral branches retaining their 
normal structure. 
The general anatomy of the root is of the usual type ; 
the vascular cylinder is tetrarch, pentarch or hexarch, each 
phloem-group having a strand of bast-fibres on the outside, 
as is so common in the Leguminosae. The pericambium 
is at first one layer of cells only in thickness, but its cells 
undergo tangential divisions, beginning opposite the xylem- 
groups. In the oldest roots examined the pericambium 
was always three layers at least in thickness. This 
multiplication of the pericambial layers is a very common 
phenomenon \ quite apart from any formation of internal 
phellogen, with which, as we shall see, the pericambium 
here has nothing to do. Opposite the xylem-groups 
the inmost layer of the pericambium is of course used up 
to complete the cambial ring. Secondary thickening takes 
place in these roots in the normal manner, as in Phaseolus 
or Vicia. The endodermis is distinctly marked, and shows the 
characteristic structure of its radial walls. The cell-walls of 
the endodermis ultimately become cuticularised throughout. 
The primary cortex consists of rounded cells, among which 
1 Cf. Olivier, Appareil tegumentaire des Racines, in Ann. des Sci. Nat. ser. 
VI, tom. XI. 
