We have now to deal more particularly with those marine _Algæ_ that are commonly known as Sea Weeds, and which add so much to the beauty of our rocky coasts. These exhibit such a variety of graceful forms, and such charming colours, that they are admired and treasured by thousands of sea-side ramblers, who are attracted by them merely on account of their pleasing general appearance; but the naturalist has all this and a great deal more to interest and instruct him, for the sea weeds possess quite a number of peculiar and characteristic features that render them well worthy of a detailed study, especially when they are compared and contrasted with the better-known flowering plants of our fields, woods, and hedgerows. It has already been observed that sea weeds differ from the majority of flowering plants in that they have no true roots or leaves, though they are often attached to rocks and other substances by a root-like disc, and sometimes have leaf-like expansions that are supported by stem-like rods. The root-like structures, however, serve simply for the attachment of the plant, and are never concerned in the absorption of nourishment like the true roots of higher plants; and the leaf-like expansions, though they are sometimes symmetrical in form, never exhibit the spiral arrangement that obtains in the leaves of higher plants, from which they also differ in function. The plant-body of a sea weed is called a _thallus_, and differs considerably in the various species. Sometimes it has no expanded portion whatever, but is more or less cylindrical in all parts, and may be either branched or simple; and in some species it forms a simple crust or a soft jelly-like covering on a rock. All portions of a sea weed are made up of cells, and these are never modified into vessels such as we see in the stems, leaves, and roots of higher forms of vegetable life; and one who is commencing the study of the algæ will find much interesting work in the examination of their microscopic structure. Thin sections of various parts of the larger weeds, cut with a sharp knife or a razor, and examined in a drop of water under a cover-glass, will show the cellular structure perfectly; while minute fragments of the small and slender species are sufficiently thin and transparent to display the form and arrangement of their cells without any previous preparation.