118 
Mo7iocotyledons with Petaloid Flowers . 
Natural Order 
HYDROCHARIDACEAL Tab. 92. 
Diagnosis. —Aquatic herbs, with radical or cauline leaves. Flowers 
usually dioecious, with a 3-6-leaved perianth. Ovary inferior. Seeds 
indefinite, exalbuminous. 
Distribution. —A small Natural Order, generally distributed over the Globe. 
Number of British Genera, 3; Species, 3 (one introduced). 
Stem submerged, slender, branched and leafy throughout in Water Thyme ( Elodea canadensis) ; a floating 
rhizome with tufted orbicular leaves in Frog-bit [Hydro char is); rhizomatous at the bottom of the water, with long 
narrow leaves, in Water Soldier ( Stratiotes ) and Vallisneria. 
Flowers inconspicuous in Water Thyme, the pistillate only recognised as yet in Britain, sessile in the upper 
axils ; in Frog-bit conspicuous with petaloid inner perianth-leaves and pedunculate ; in Vallisneria the small pistillate 
flowers solitary on long slender peduncles reaching to the surface of the water; staminate flowers in subsessile 
submerged heads, which become detached and rise to the surface to discharge their pollen. After fertilisation the 
spiral peduncles of the pistillate flowers are withdrawn under the surface and mature their seeds at the bottom. 
USES, &c.—But few species possess economic importance. Species of Hydrilla and some of the Naiadeae are 
used in India in “ claying” sugar. Vallisneria, introduced from Southern Europe, is everywhere cultivated in 
aquaria, for which its bright green colour, easy propagation and persistent vitality well fit it. Thin longitudinal 
slices of the leaves are suitable for exhibiting under the microscope the motion of the cell-contents, as are the 
membranous scales at the base of the petioles of Frog-bit. Water Thyme, a North American aquatic, was first 
noticed in Britain about the year 1847, since which time it has spread rapidly, in some localities to such an excess as 
to obstruct the navigation of slow streams and canals. 
