A remarkable feature of the green-spored weeds is the large size of the chlorophyll granules as compared with those of the other groups, and also the great variety of forms which these granules assume. They may be easily seen under a low power, and the examination of the weeds will show that the thalli are not uniformly green, but that the colour of the plants is due entirely to the chlorophyll granules, the remainder of the plant substance being quite colourless.
If a green sea weed be placed in alcohol for a short time, it will be found that the liquid assumes a green colour, while the plant itself becomes colourless. The explanation is, of course, that chlorophyll is soluble in alcohol. The presence of starch also in the weed may be proved in a very simple manner, as follows:—Mount a small piece in water, and then put a drop of iodine solution by the edge of the cover-glass. The solution will gradually diffuse itself around the object, turning the starch-grains to a deep blue colour, and so rendering them very conspicuous under a moderately high power.
The manner in which the green weeds are reproduced is very interesting also. In some cases the fragments of a thallus that have been detached by storms or other mechanical means possess the power of independent growth, and develop into plants; and this mode of reproduction may often be watched in the indoor aquarium. Another method is by the agency of little spores (zoospores) that are produced at the edges or extremities of the thallus. Certain of the cells become modified into what are called zoosporangia, and the minute zoospores are formed within them. The walls of the cells either gradually degenerate, or are fractured, and the zoospores are thus set free. The latter are provided with little vibratile cilia, by which they move about freely in the water. Some eventually settle down and germinate without any further aid, but others are unable to develop until they have been fertilised by fusion with another cell. The former is therefore an asexual development, while the latter is sexual.
Some of the delicate, filamentous green algæ are reproduced by another process termed conjugation. In this case two adjacent threads that lie close together become lightly united by a covering of gelatinous substance, and a cell of each throws out a process. The processes are directed towards each other, and unite to form a tube in which the contents of the two cells become fused together, with the result that zoospores are produced.
Among the lowest of the green sea weeds are the plants known collectively as the Confervaceæ, which consist of delicate green filaments, usually attached to rocks in dense masses, but often found floating freely in the rock pools. The filaments are composed of cells joined together at their ends, and are always unbranched.
Confervæ are found principally in the tide pools, especially near high-water mark, and often abound in hollows in the rock even above high-water mark, where the spray of the waves is mingled with rain-water or the drainage from the land. They exist in both fresh and salt water, and some species seem capable of thriving in brackish water of any degree of salinity.
Allied to the confervæ is a group of marine algæ called Cladophora, very similar to the former in general appearance, and found in similar situations, but readily distinguished by the branching of their jointed filaments. The various species of this group are very beautiful weeds, their delicate filaments looking very pretty as they float and sway in the water of the pools. They are also exquisite objects for the microscope; but, unfortunately, often lose their natural colour when preserved dry. They vary in colour, some few being of a dull green tint, while others are bright green, sometimes with a beautiful silky gloss.
One species (C. pellucida) is more rigid than most of the others; its fronds stand out erect and firm, and are repeatedly forked near the tips. It is a moderately common weed, found in the lower rock pools, and may be readily recognised by the long one-celled joints, from the tops of which the branches proceed. Another species (C. diffusa) is also very firm in structure, so much so that its bristly tufts retain their form when removed from the water, instead of becoming matted together in a shapeless mass. Its branches are rather long, and bear a few simple branchlets towards their extremities. It is found in rock pools between the tide-marks. C. lanosa is a very pretty little weed, growing in dense globular woolly tufts, an inch or more in diameter, on the olive tangles between the tide-marks. It is of a pale yellowish-green colour, which becomes much paler, or is even altogether lost, when the plant is preserved in a dry state, and, at the same time its fine glossy appearance is lost. Its fronds have straight branches, all making very acute angles, and they have also small root-like filaments. It much resembles another species (C. arcta), which grows in dense tufts on rocks, but the latter is larger, not so slender, and more freely branched. The cells, too, of C. arcta are longer, being about ten times the length of the diameter, and the fronds are silvery at the tips.
Nearly thirty species of Cladophora have been described, but it is impossible to give here a detailed description of all. We add, however, a brief summary of the distinguishing features of a few other species that are common on our coasts.
C. rupestris is common everywhere, and easily recognised by its rigid, branching, tufted fronds, of a dark greyish-green colour; its branches, which are opposite, bear awl-shaped branchlets. It is found in rock pools from half-tide downwards, and in deep water beyond the tide-marks, the plants growing in the latter situations being generally of a fine dark-green colour.