of Sesbania acute at a, Pers . 313 
2. This tissue, though thus falling under the definition of 
periderm, differs from cork in its permanently living cells, its 
non-suberised cell-walls, and its large intercellular spaces, in 
which alone air is contained. In all these respects it agrees 
with the floating-tissue of the stem of Neptunia . 
3. The phellogen originates immediately outside the endo- 
dermis, thus differing from the phellogen of most roots with 
typical periderm. 
It may be mentioned that the development of phelloderm, 
on the inner side of the phellogen, is quite insignificant in 
amount. 
In the light of recent investigations on roots as organs of 
respiration 1 it seems not improbable that one function of the 
floating-tissue may be to facilitate the supply of oxygen to 
the organs on which it occurs. This tissue would then present 
an analogy with lenticels — a point not without interest, con- 
sidering the similarity, above noticed, in the development of 
the two organs. 
It has long been known that some species of the genus 
Jussiaea also possess modified roots, which serve as floats, 
and develop a special floating-tissue. The existing investiga- 
tions tend to show that this tissue here forms part of the 
primary cortex, but a renewed investigation of these plants 
seems desirable 2 . 
I have to thank Mr. Thiselton Dyer, Director of the Royal 
Gardens, Kew, both for first calling my attention to the plants 
investigated, and also for the supply of material. 
1 Goebel, Uber die Luftwurzeln von Sonneratia, in Berichte d. Deutschen Bot. 
Gesellschaft, Bd. IV, Heft 6, July, 1886. Ludwig Jost, Ein Beitrag z. Kenntniss 
der Atbmungsorgane der Pflanzen, in Bot. Zeitung, 1887, p. 601. 
2 Martins, Sur les racines aeriferes des especes aquatiques de Jussiaea, in Mem. 
Acad, de Montpellier, Tom. VI (1866). 
