C. Salviniales.—Next to the ferns are the members of this order, in which spores of two sorts are produced in special conceptacles re sembling small pods or spheres. There are two types of habit represented, one series (Sevin incece) floating on the surface of water, and the other (11farsiliacect) rooting in mud. Sal vinia, with oval leaves, is occasionally seen in cultivation but is rare in a wild state. is found in great abundance in Florida, some times covering the surface of ponds with its delicate branching small-leaved stems. A sec ond species occurs in California. Marsilia is the type of the second family, and has a four parted leaf resembling that of Oxalis. It grows on the wet borders of ponds, or if the water becomes high, it elongates its leaf-stalks and the leaves float on the surface like four leaf clovers. Its conceptacles are oval and are borne on short stalks. One species grows abundantly at Bantam Lake, Connecticut, and others in Florida and in the Far Northwest where they are more common. Pilularia is a second genus of the family with thread-like leaves, and spherical pill-like conceptacles. Our single species occurs in California, Oregon and Arkansas.
II. Equisetina.—About 25 species forming a single genus and family make up this order. There arc two types of growth, both common species of Equisteum, which have received dif ferent popular names. One is the field horsetail, which appears in low sandy ground early in spring, sending up a flesh-colored stalk termi nating in a cone, followed by the "horsetails," which are the vegetative parts of the plant, con sisting of a central hollow stalk with angular whorled branches. The leaves are reduced to sheaths encircling the stem. Under the surface shields that cover the cone, the spores, which in mass resemble a fluffy mold, are borne in spo rangia. The peculiar appearance of the spores is due to the fact that each is surrounded by an outer covering which splits spirally into four branches arranged cross-like at one end of the spore. These are exceedingly hygroscopic, and they absorb water and coil up at the slightest breath; then uncoiling as soon as the moisture dries, they jostle each other about. The pur pose of this mechanism is to scatter the spores gradually from the sporangia at maturity. The second type of Equisetum grows on banks with stiff, jointed, rush-like, often clustered stems surrounded at the joints by a light-colored sheath. These also bear cones in late spring similar to those of the field-horsetail. These plants are sometimes called Iscouring-rushes,' from the fact that their roughness caused by silica in the epidermis led to their use for scour ing floors and woodenware.
III. Lycopodina. A. lsoetacece.— This order likewise contains a single genus (Isoetes) and family. The plants are aquatic, either grow ing submerged or on the shores of ponds and rivers. They consist of a short fleshy base, bear ing a cluster of leaves which are broadened at the base and contain an axillary sporangium, and end in awl-like joints. The position of the order is problematical, as obscure plants seem to show unexpected affinities with several other groups. They are eusporangiate and pro duce spores of two sorts.
B. Lyropodiacete.— This order contains the ground-pines or club-mosses and the selaginellas or little club-mosses, which are alike in produc ing their spores in the axils of leaves, but differ in the spore-characters. The club-mosses (Ly eo podium) have yellow dust-like spores, which are used commercially under the name of lyco podium powder for the relief of chafing and for producing flash-lights, since they contain a large amount of oil and are highly inflam mable. The lycopods are familiar as paristmas greens, but are rarely seen in cultivation. Over 100 species are known of which at least 12 are found within the limits of the United States.
C. Selagittellacete.—This order, whose sole extant representatives belong to the genus Sela ginella, is distinguished from all extant Lycopo dacew by the possession of two kinds of spores — microspores, from which develop the male prothalli, and macrospores, from which develop the female prothalli. Seloginello is frequent in cultivation, some of the species being exceed ingly delicate and graceful; but most of our native species are inconspicuous and little known. The uresurrection-plant" has already been men tioned. Some of the tropical species attain enormous size as climbing-plants. Over 600 species have been described, of which at least 12 are found in the United States.
IV. This group of Pterido phytes, which is represented by Psilotum, a leafy epiphyte in our Southern States, used formerly to be assigned to the Lycopedinx, but it is now recognized that its closest allies are to be found in the Spenaphyllinz, an extinct class of Pterido phytes intermediate betwen the Lycopodinx and the Equisetinn.
Fossil Pteridophytes are abundant in the Car boniferous period, when indeed they dominated the entire terrestrial flora, assuming arboreal proportions. Equisetinm. Lycopodinm and Fili cinx all have their representatives. Many groups possessed two types of spores whose present relatives have but one. See PAL,E0