of Sesbania aculeate,, Pers. 3 1 1 
are very large lacunae filled with air, the structure thus being 
of the usual aquatic type. It may be added that the larger 
roots have a persistent parenchymatous pith, while in the 
smaller lateral roots the groups of primary xylem meet in 
the middle of the vascular cylinder. 
The mature floating-tissue is in most respects similar to 
that described in Neptunia. The cells are much elongated 
in the radial direction, but as a rule they remain unbranched 
(see Fig. 4). They are so arranged as to leave large inter- 
cellular spaces, containing air, between them. Each cell re- 
tains its protoplasm, nucleus, and cell-sap, throughout its 
existence, and air is never found inside the cells. The walls 
of the great majority of the cells give the reactions of un- 
altered cellulose. The tannin-sacs, which occur here, as well 
as in the primary cortex, form the only exception to this 
rule ; their walls appear to be slightly cuticularised. These 
tannin-sacs also differ from the other cells of the floating- 
tissue in their form, which is almost spherical. Their number 
is insignificant compared to that of the elongated cells, and 
they may be left out of account in considering the general 
character of the tissue. 
On a superficial examination, the idea suggests itself that 
the floating-tissue might be formed simply by the elongation 
of the cells of the primary cortex. Investigation of the 
development shows, however, that this is not the case. If 
transverse sections be made of a moderately young root, it 
is easy to find cases in which some layers of the long-celled 
floating-tissue are already well developed, while outside them 
the round-celled lacunar tissue of the primary cortex is 
still present, though somewhat torn by the extension of the 
internal structures (see Fig. 3). 
The floating-tissue passes over on the inside into a zone 
of closely-packed cells arranged in regular radial rows, and 
these again fit accurately into a layer of thin-walled cells, 
showing recent tangential divisions, and having all the charac- 
ters of an internal phellogen. Great as is the difference in 
form between the elongated cells of the mature floating-tissue 
