Page:Once a Week Volume V.djvu/269

262 is found both in the sea and in freshwater ditches, attains a length of more than two feet, and a diameter of two-thirds of an inch.

The cladophoræ and confervæ form tufts of light green threads, attached to stones and rocks between tide marks, and may be distinguished from each other by the threads of the latter being simple, while those of the former are much branched.

The cladophoræ are small plants, seldom exceeding a few inches in length, but the confervæ grow to a considerable size, specimens being sometimes found of one species many feet in length. These seaweeds are of very simple construction, each thread consisting of a series of cylindrical cells arranged end to end, the length and thickness of the cylinders varying much in the different species. Like the ulvæ, the cladophoræ and confervæ are reproduced by means of zoospores, and the resemblance of these moving spores to animals is in this case rendered yet more close by the presence of a minute red spot exactly like that which is found in many of the infusorial animalcules, and which, in their case, is supposed to represent an eye. The appearance of these plants under the microscope is very singular, the green colouring matter being frequently arranged in a net-like pattern upon the wall of the cell. Sometimes, too, the threads forming the net seem to be strung with small bright beads, which, by the use of proper chemical tests, may be shown to be granules of starch.

But there is yet another reason why these simple seaweeds are peculiarly interesting to the microscopist. Owing to their great transparency and the power which small fragments possess of continuing to grow when detached from the plant, if they are kept in water and exposed to the light, the whole process of their growth may be observed without any difficulty. To enable the reader to comprehend the nature of this process, it is necessary to say a few words about the vegetable cell. A full account would require not a few lines but a volume, and volumes have been written upon the subject, but enough for the present purpose may soon be told.

Every plant, the largest tree as well as the smallest alga, is built up entirely of cells, varying much in size, shape, and colour, but all constructed upon the same plan. The most simple form of these cells consists of a closed membranous bag, containing a fluid sap. The bag is formed of two layers, the outer thick and conspicuous, the inner very delicate, and not easily seen. This inner layer, which has been called by vegetable physiologists the primordial utricle, is supposed to be in a peculiar degree the seat of the life of the cell. The seaweeds which we are now considering consist, as we have said, of a single series of such cylindrical cells placed end to end, and the several threads of which the plant is composed increase in length by the division of one of these cells into two smaller cylinders, each of which then grows to the dimensions of the original cell from which they were derived.

If we place upon the stage of the microscope a growing fragment of cladophora we shall easily be able to observe the mode in which this division takes place. It is to the terminal cell of one of the branches that we must direct our attention, since it is for the most part in these only that division is going on. If we observe a cell in which division is just commencing, we shall see that the delicate membrane which lines the wall of the cell is gradually contracting at a point near the middle of the cell, so as to divide the green contents into two nearly equal halves. The contraction slowly increases until that part of the cell in which the change is going on has assumed nearly the form of an hour-glass. We may imitate the change of form closely by taking a small tube of gutta percha, or any other elastic substance, passing a string round it, and gradually drawing the ends tight. At length the two halves of the hour-glass become entirely separated, the outer wall of the cell having in the meantime undergone no sort of change. Next a new cell wall is secreted around each of the two halves so formed, so that two new cells are produced, both included within the outer wall of the original cell. The two halves then increase in length, until each has attained its full size, and a fresh division commences in the new cell, which now forms the termination of the branch. In the simple unbranched confervæ there is no other mode of division than this, but in the branched cladophoræ we find another slightly modified plan of growth. When a new branch is about to be formed, the lining membrane of one of the cells (in this case not a terminal cell), begins to project sideways, pushing the outer cell wall before it, so as to form a protuberance on the side wall of the cell. After this outgrowth has reached a certain size, a process of division takes place, exactly similar to that which we have already described, by means of which the newly-formed cell is separated from the old branch, and becomes the first cell of a new one. Cells in every stage of division may often be seen in a small fragment of cladophora, some showing the projection just commencing, others the hour-glass-like contraction of the contents of the cell, and others again the completion of the division, and the formation of the new cell wall.

Very different in structure from any of these plants is the pretty little Bryopsis plumosa, whose dark green feathered silken tufts, from one to four inches high, may often be met with between tide-marks, growing upon rocks, stones, or other seaweeds. This plant belongs to the order of green seaweeds, called siphonaceæ, or tube-like seaweeds, the stem and the principal branches being composed of a single large branched tubular cell. Zoospores are produced in this plant in all the smaller branches with which the principal branches are feathered, but the phenomena resemble too nearly those presented by the ulvæ and confervæ to need a separate description.

We have now nearly completed our sketch of the green sea-weedsseaweeds [sic]. Some we have passed over as presenting few points of interest; but we must not omit to mention one very curious tribe of plants, more common perhaps in fresh waters, but of which some species inhabit the sea,—the oscillatoriæ, or oscillating algæ. These plants are not attached to rocks, but float freely in the water, forming a dense layer of very fine green threads. There is nothing striking or attractive