[Illustration: Lichen.] 5. FILICES (Ferns). The Ferns are a very numerous order of acrogenous plants, growing in the temperate regions from a rhizome or underground stalk (commonly called its root), which throws up "fronds" or leaves with a strong midrib; this midrib is commonly called its stalk, but in tropical countries the fern stalk rises above ground to the height of 30 or 40 feet, and then it is seen that the ordinary stalks are but the midribs of the fronds. There are between two and three thousand species of fern. The fronds open in a peculiar manner, unwinding as it were from a round ball. The "sorri" or seed-cases are situated at the back of the fronds in little brown spots, each of which is found to consist of a heap or collection of round capsules, and if these be placed under the microscope they have the appearance of little membraneous cases covered with net-like markings, and at the upper part a striated band of a brown colour, which after a time stretches out into a nearly straight line, tearing open the bag or capsule, and a number of seeds escape. Ferns grow best in damp and shady situations, and will thrive well in damp mould under a glass case. [Illustration: Fern.] 6. MARCHANTIACEÆ (Liver-worts). [Illustration: Liver-worts.] These Liver-worts are much like Lichens, growing between and upon stones near springs and moist places, and forming a broad thallus or root from which grow cup-shaped sporangia or seed-cases. The _Marchanta polymorpha_ is one of its chief members. 7. JUNGERMANNIACEÆ (Scale-mosses). [Illustration: Scale-moss.] The Scale-mosses grow in moist places and under the stems of trees and plants. Their appearance is scaly, and they have a sort of stalk. 8. BRYACEÆ (Urn-mosses). [Illustration: Urn-moss.] Urn-mosses comprise the most ordinary of the mosses, the "Brium," which grows on the ground everywhere, forming tufts; its fructification resembles an urn, and hence its name. 9. LYCOPODIACEÆ (Club-mosses). [Illustration: Club-moss.] These are very beautiful mosses, of a bright green colour, growing in moist places. There are about 200 species of club-mosses known. The common Lycopodium is its most characteristic member. It grows well with ferns under a glass case. 10. EQUISETACEÆ (Horsetails). The Equisetaceæ are the highest of the Acrogens, and nearly resemble the Endogens; they grow in ditches and pools, have a hollow stem with joints at regular intervals from each of which a whorl of green modified leaves arises; they are very full of earthy matter (silica), and one kind (_Equisetum Hyemale_), called Dutch Rush, is so rough with it that it is used for polishing and scraping many articles. [Illustration: Horsetail.]