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.
One of the principal charms of the marine algæ lies in the great variety of colour that they display. They all contain chlorophyll—that remarkable green colouring matter which enables a plant, under the influence of light, to feed on the carbonic acid gas existing in the atmosphere, or held in solution in water; and with its aid the sea weeds can utilise this product of decay and animal respiration that would otherwise accumulate in the water of the sea. But, in addition to this bright green chlorophyll, many of the sea weeds contain a second colouring substance, and in these the great variety of tint is dependent on the nature of the latter and on the proportion in which it is present as compared with the chlorophyll itself.
The different means by which the algæ reproduce their kind forms a most engrossing subject, and to the botanist a most important one, for it has much to do with the classification of the species. The affinities of plants may be better determined by the nature of their reproductive processes than by any other features, but unfortunately this is not so well understood with regard to the algæ generally as compared with many other divisions of the vegetable kingdom; and, as a consequence, there is still a considerable difference of opinion, not only as to the extent of the whole group, but also as to its divisions and subdivisions. The reason for this is clear; for while it is quite an easy matter to trace a flowering plant through its complete cycle from seed to seed, it requires a much more careful examination, combined with much microscopic work, to trace a lowly organised plant from spore to spore.
Some of the algæ may be reproduced without the agency of any sexual elements; that is, without the aid of parts that correspond with the ovules and the fertilising pollen of a flowering plant. Some of these are reproduced by a repeated subdivision, or by the separation of a portion of the plant that is capable of independent growth; while others produce spores that do not result from the fusion of two different cells. In most, however, sexual differences are to be observed, some cells being modified into female organs, containing one or two more minute bodies that are capable of developing into new plants after they have been fertilised, and other cells produce the male elements by means of which the fertilisation is accomplished. The fertilised cells are spores, but are named differently according to the nature of their development. They all differ from true seeds in that they never contain an embryo plant, but germinate by the elongation of some particular part, which subsequently grows by a continuous process of cell-division; or the cell-division may originate directly in the spore without any previous elongation or expansion.
The sea weeds are usually classified according to the colour of their spores; but, since this colour generally corresponds with that of the plant itself, we may almost say that they are grouped according to their general tints. There are three main divisions:—
The Chlorospermeæ, or Green-spored;
The Rhodospermeæ, or Red-spored; and
The Melanospermeæ, or Brown-spored.
The Chlorospermeæ contain no colouring matter other than the chlorophyll. They are mostly small weeds, of a delicate green colour; and, although they are not particularly conspicuous on our shores, they contribute very considerably to the beauty of the rock pools, where their delicate green fronds contrast richly with the olive Melanosperms and the pink and white corallines. The thallus or plant-body is very varied in form, sometimes consisting of a broad membrane, but more commonly of tufts of slender green filaments or of narrow, flattened fronds.
These weeds are most beautiful objects for the microscope, and they are generally so thin and transparent that no section-cutting is necessary, nothing being required except to mount very small portions in a drop of water. In this simple manner we may study the beautiful arrangement and the various forms of the cells of which they are composed. The more delicate species will be found to consist of a single layer of cells only, while in the larger forms—the Ulvaceæ, for example—the thallus may be formed of two or three distinct layers, and some of the cells may be elongated into tubes.