FAMILIES OF FLOWERING PLANTS. 19 
parted perianth. The ovary is composed of several distinct carpels 
which become small stone fruits or nutlets. A curious feature of the 
pondweeds, particularly in 2’otamogeton, is the difference in shape often 
exhibited between the immersed leaves and those that float on the sur- 
face of the water. Thus P. Sp/r///us has its floating leaves oval or 
elliptical, while its submersed leaves are narrowly linear; this pecular- 
ity is the result of the plant’s adaptation to the conditions under which 
it grows, the exposure to air and sunlight tending to the production of 
expanded blades. In species which grow wholly submerged there is 
no difference in the shape of the leaves. (See Fig. 14.) The ‘genus 
Aponogeton, composed of about 15 Old World species, contains some 
beautiful ornamental] aquatics, known as lattice-plants on account of 
the open lace-like tissues of their leaves, which appear as if pierced 
by many holes. 
Family Scheuchzeriaceae.—(Juncaginaceae). Arrow-grass Family. 
Four genera and only about 10 species, of wide distribution. All are 
marsh plants, with rush-like or sedge-like leaves and small perfect 
flowers born in spikes or racemes. The perianth is in two series, thus 
showing traces of differentiation into a calyx and corolla; the ovary is 
composed of several carpels, slightly 
united. Zr/glochin, the best known 
genus, with three North American 
species, is found in salt marshes along 
the coast and in alkaline situations in 
the interior of the continent, its small 
greenish spike and linear or filiform 
leaves being quite inconspicuous. 
Family Alismaceae?—W ater-Plan- 
tain Family. Thirteen genera and 
about 65 species, widely distributed in 
fresh water swamps. ‘The plants are 
herbs with long petioled leaves and 
scapose flowers borne in broad panicles 
or racemes. The flowers vary greatly ; ; 
in structure, being perfect, monoecious jitaigs Gea le olaek 
or dioecious; but they are always borne ee vit euleteed fel ren ence 
in whorls on the peduncle, and possess — Nortn. v. s.) 
a perianth, usually in two distinct 
series. The ovaries are often numerous, each carpel becoming a small 
