Though some of the Ferns are so common that almost every one must have seen them, very few persons are aware how very curiously they are constructed. In the first place, they may be said to have neither stems nor leaves, and neither flowers nor seeds. The different parts of the plant spring from a rhizoma, and the leaves, which are called fronds, have their veins neither branched nor in parallel lines, but forked. On the back of the leaves are some curious brown spots of various shapes called sori; and these, which generally form under the outer skin or cuticle of the leaf, and which always spring from one of the veins, contain a number of small grains, called the thecæ, which are in reality cases containing the sporules or seeds. When the sorus forms under the cuticle of the leaf, the membranous part raised, which resembles a blister, is called the indusium; but sometimes the sori are naked, that is, they are formed on the outside of the cuticle; and sometimes they are found on the margin of the leaf, which folds over them, and supplies the place of the indusium. The order is generally divided into two sections, called Polypodiaceæ and Osmundaceæ. The first contains those plants which unroll their leaves, when they rise from the stem, and which have their sori either on the back or on the margin of the frond. The thecæ are on stalks, and they are furnished with a ribbed, elastic, articulated but incomplete ring, which seems to serve as a sort of hinge when they burst. This elastic ring is a continuation of the stalk of the theca, which always bursts on the opposite side. The following are the principal genera in this division: Polypody (_Polypodium_), sori without any indusium; Shield Fern (_Aspidium_), Bladder Fern (_Cistopteris_), and Spleenwort (_Asplenium_), all of which have their fronds pinnate or pinnatifid; Maiden Hair (_Adiantum_), Hart’s-tongue (_Scolopendrium_), the frond of which is simple and shaped like a tongue, and the sori oblong; and Brake (_Pteris_), the leaves of which are pinnatifid, with the sori placed round the margin so as to form a continuous line, and the edge of the leaf turned over them. The rhizoma of the Brake is eaten in many countries, and the fronds, when burnt, yield alkali, which is used in making both soap and glass.