AQUATIC PLANTS. 71 
this form of tissae.* Scshcmia acicleata, Poir., the " ])aiichi " of India, 
a tropical leguminous plant, is described f as a marsh plant giving off 
floating roots from the base of the stem, covered with a spongy 
aerenchyma, whilst in Neptunia oleracea, Lour., which belongs to an 
altogether different subdivision of the same great order, the Mimoseoiy 
it is a floating stem which produces this spongy tissue. A terrestrial 
species, N. j)lena, has no such structure. These aerenchymatous struc- 
tures are now generally believed to serve to conduct down to the, more 
completely submerged structures the oxygen required for respiration. I 
think, therefore, that Kerner's description % of another tropical aquatic 
mimosad, Desmanthiis natans, is physiologically inadequate. 
In Desmanthus natans [he says] an actual swimming apparatus is developerl, 
not in the leaf-stalks, but in the stem itself. It takes the form of a large-celled, 
spongy, air-containing mantle, arising here below the epidermis of the internodes, 
which renders sinking impossible. The Mimosa-like foliage-leaves rise up from the 
nodes of these floating stems like masts with flags. When the leaves turn yellow the 
stems rid themselves of their swimming organs, which are no longer needed, and 
indeed it appears to be an advantage to the leafless stems to be able to sink down and 
to obtain a period of rest at the bottom. 
Without for one moment questioning the fact of the plant floating, I 
would suggest that it does so, not merely, as in Trapa and Eichornia, to 
facilitate assimilation, but also for respiratory oxygen. The disappear- 
ance of the aerenchyma in the resting stage is an interesting case of 
rapid adaptational change in structure. These instances from the 
Leguminosce lend additional interest to the variety offered by two species 
of the tropical genus of Onagracem Jussieua, figured and described by 
Goebel.§ One, /. suffrzUicosa, L., has an erect stem which only develops 
aerenchyma over the lower part of its surface when the plant is growing 
in water, whilst the other, /. repens, L., when in water, produces some 
ascending aerenchymatous roots which reach the surface, as in the Man- 
groves. Among British plants the Great Water Dock {Buincx Hydro- 
lapathuin, L.) sometimes produces roots of this character, whilst the 
Gipsy-wort {Lycopus europceus, L.) and the Purple Loosestrife {Ly thrum 
Salicaria, L.) produce aerenchyma on submerged parts of their stems. 
Many are the structures in plants which are susceptible of a purely 
mechanical explanation. The weight of branches and leaves, for instance, 
necessitates, in an aerial stem that does not climb, the presence of more 
or less rigid woody " mechanical tissue," as it is called. The buoyancy 
of water, however, renders this unnecessary. 
Firm woody cells and strands of elastic bast-fibres [says Kerner |] ] which play such 
an important part in the aerial portions of plants, are wanting here. Woody plants 
neither grow in the sea, nor in fresh water. Aquatic plants, indeed, quickly collapse, 
in consequence of the absence of wood and bast, when brought from the depths into 
the air : the leaves collapse of their own weight, and sink flaccidly on to the substra- 
tum. They are able to retain an erect position in the water, because a portion of their 
tissue is penetrated by comparatively large air-spaces, by which means their specitio 
gravity, compared with that of the water, becomes much diminished. 
Goebel, Berichte d. Deutsch. Botan. Gesellschaft, iv. (1880), Berlin, p. 249, and 
PJianzenbiologische Schildcrungen, i. 139. 
f J. C. Willis, op. cit. ii. 35i. 
X Op. cit. i. 669. 
§ Pflanzenbiologische Scliilderungcn, ii. 256. 
II Op. cit. i. 424. 
