. The families of flowering plants. Plants; Phanerogams. 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 Potamoyefon, is the difference in shape often exhibited between the immersed leaves and those that float on the sur- face of the water. Thus P. KpiriUH» has its iloating leaves oval or elliptical, while its submersed leaves are narrowly linear; this peculiar- ity is the result of the plant's adaptation to the conditions under which it grows, the
. The families of flowering plants. Plants; Phanerogams. 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 Potamoyefon, is the difference in shape often exhibited between the immersed leaves and those that float on the sur- face of the water. Thus P. KpiriUH» has its iloating leaves oval or elliptical, while its submersed leaves are narrowly linear; this peculiar- 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 Ajxmogetori, composed of about 15 Old A\'orld species, contains some beautiful ornamental aijuatics, known as lattice-plants on account of the open lace-like tissues of their leaves, which ai)pear 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 })erfect flowers born in spikes or racemes. The perianth is in two series, thus showing traces of diffprentiation into a calyx and corolla; the ovary is composed of several carpels, slightly united. Iriglochin, 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 AlismaceaeT—Water-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 or dioecious; but they are always borne in
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