308 Scott and Wager. — On the Floating-Roots 
The floating-tissue of the plant in question forms very con- 
spicuous spongy masses on some of the internodes of the 
procumbent stem, which is thus enabled to maintain its 
position on the surface of the water. In Rosanoffs figure t 
(Plate X, 1. c.) the tissue is shown at c and d. The drawing, 
however, gives but little idea of the very soft, loose texture of 
the swollen masses. The diameter of the whole body may be 
nearly an inch, while that of the unswollen parts of the stem 
is only about of an inch. The length of each mass may be 
as much as two inches. The external tissue is, in the mature 
condition, very loosely attached to the stem on which it 
grows. Humboldt 1 regarded it as extraneous to the plant, 
and indeed the impression it makes at first sight is quite that 
of the mycelium of some luxuriant parasitic fungus. The 
surface is very irregular, showing deep longitudinal furrows. 
Microscopic examination shows that the tissue consists of very 
loosely arranged cells, which are greatly elongated, and are 
sometimes branched. The intercellular spaces are very large, 
and it is in them only that air is contained. The cells them- 
selves always possess a delicate primordial utricle, a nucleus, 
and starch-grains ; their cavity is at all stages filled with cell- 
sap, and never contains air. On the inside the floating-tissue 
passes gradually over into a dense periderm, with its cells in 
regular radial rows. This is continuous internally with the 
phellogen. Neither the cell-walls of the floating-tissue itself, 
nor those of the dense periderm, give the reactions of cork ; 
they consist of unchanged cellulose. In moderately young 
stems it is easy to trace the remains of the epidermis and of 
the outermost layers of the primary cortex outside the floating- 
tissue. Sections through a young internode, when the forma- 
tion of this tissue is beginning, show that the phellogen 
arises by division of the third or fourth layer below the epi- 
dermis. At first, ordinary periderm is produced all round the 
stem. The modification of the periderm into floating-tissue 
begins irregularly at various points of the circumference. At 
1 Cited by Rosanoff, 1. c. 
