Lichens are extremely simple in their construction. They are composed of two parts, the nutritive and the reproductive system. The nutritive portion is called the thallus, which, in the typical plant, spreads equally on all sides from the original point of development, in the from of an increasing circle; the circumference of which is often healthy, while the central parts are decayed or completely wanting. Nature has bestowed upon the lichens a peculiar mode of reproduction which appears quite different from that of the higher orders of the vegetable kingdom; and yet they are propagated with as unerring certainty and as great rapidity as the most prolific family of flowers. Every one who has an attentive eye must have often noticed the curious round disks or shields, usually of a different color from the rest of the plant, with which their surface is often studded. These are called apothecia, and correspond with the flowers of the higher plants; for in them are lodged the seeds or germs by which the lichens are perpetuated. When examined under the microscope they are found to consist of a number of delicate flask-shaped cells, called thecæ, containing 4, 8, 12, or 16 sporidia, that is, cells of an oval form, with spores or seeds in their interior. The mode in which these spores are ejected affords as wonderful a proof of design as in the case of ferns and mosses. [Illustration: Typical Nuts and Tree-Products 1, Cinnamon; 2, Camphire (Camphor); 3, Pomegranate; 4, Sycamore Figs; 5, Olive Twig and Fruit; 6, Theobroma Cacao (Chocolate)] Lichens are very slow-growing plants. They spring up somewhat rapidly during the first year or two, as is evinced by the luxurious growth which they form over young fruit-trees and espaliers in gardens; but after a circular frond is formed, they subside into a dormant state, in which they remain unaltered for many years. The foliaceous and scrubby species are the most fugacious, though even these have great powers of longevity. We have no data from which to ascertain the age of tartareous species, which adhere almost inseparably to stones. Some of them are probably as old as any living organisms that exist on the earth. In the Arctic regions--those outer boundaries of the earth where eternal winter presides--these humble plants constitute by far the largest proportion of the flora, and by their prodigious development, and their wide social distribution, give as marked and peculiar a character to the scenery as the palms and tree-ferns impart to the landscapes of the tropics. In the Southern Hemisphere also lichens extend almost to the pole. They mark the extreme limit at which land vegetation has been found; one scrubby species, with large, deep, chestnut-colored fructification, called Usnea fasciata, having been observed by Lieutenant Kendal on Deception Island, the Ultima Thule of the Antarctic regions.