Marsileae
A small order of aquatic plants, of variable habit. There are no true leaves. Fronds, as in Filices, proceeding from the rhizome, and rolled inwards (circinate) at the top when young; barren ones either reduced to a linear stipes or bearing a leaflike lamina divided into four digitate leaflets; fertile ones on a shorter stipes or nearly sessile, the lamina recurved with the margins united, forming an ovoid or globular utricle, usually called an involucre. Spore cases of two kinds, as in some Lycopodiaceae, but arranged, as in Filices, in sori inside the involucre (i.e., on the under surface of the recurved frond); each sorus inclosed in a membranous indusium, apparently dividing the involucre into as many cells. The order is limited to the two Australian genera, Marsilea and Pilularia, both of which have a wide range in the New, as well as in the Old, World. According to Bentham, they might well be regarded as forming a tribe of Filices, to which they are much more closely allied than to Lycopodiaceae, with some genera of which they have been frequently associated. The sporangia of Marsilea, after the pools in which the plants grow are dried up, are found in great abundance on the surface of the soil, and form a miserable article of food, which has, however, saved the lives of some of the exploring parties who have traversed Australia.