green colouring matter, while those cells which form the colourless half of the leaf are quite empty. If your microscope is a good one, and you know how to manage it, you will be able to detect in the centre of each of the empty cells a very small hole or pore through which, probably, the vanished colouring matter has escaped. Scattered among the empty cells may be seen a few which are not yet quite empty, and within these one or two more zoospores may plainly be seen moving. At first you think you are mistaken; you wipe imaginary dust from your eye-piece, alter the arrangement of your light, and look again, but only to be fully convinced that the moving bodies actually are within the cell. Returning now to the green half of the frond, you will see that in the cells situated near the boundary line the internal colouring matter is beginning to form itself into small clusters, and in one or two of these cells you will observe a strange motion which has been termed ‘‘swarming,” from the resemblance which it has been supposed to bear to the motion of a swarm of bees thickly clustered together. Other cells, again, you may see in which the zoospores are perfectly formed, and seem to be trying to find a means of escape, and perhaps in one of them you may be fortunate enough to detect a zoospore in the act of escaping through the pore. May we not conclude from these facts that the pear-shaped bodies which we saw swimming about in the water are identical with those which we detected moving within the cells of the plant, and that they are, in fact, the zoospores, or moving spores, of the ulva formed from the green colouring matter with which its cells are originally filled?
Doubtless if you are inclined to be sceptical it will be possible for you to construct a theory by which most of the facts may be explained without allowing to plants the power of spontaneous motion. Suppose, it may be said, these moving pear-shaped bodies are really, as they seem to be, animals; suppose that they feed upon the green contents of the cells of the laver, and that the white portion of the frond consists of cells whose contents they have devoured. There will then be nothing strange in their occurrence within the cells into which they may easily be supposed to have penetrated in search of food, having first pierced the small hole which you have seen in the cell wall. Now is there anything absurd in such a supposition? I do not know that there is, though I much doubt whether it would meet with ready acceptation from any one who has actually witnessed the process which I have attempted to describe, nor do I think the peculiar appearance known as swarming could be thus accounted for, or the occurrence of the moving bodies in great numbers near the line which divides the green from the white half of the leaf, and their total absence from any other part of the former half.
But if we carry our observations a little further we shall find ample reason for rejecting as inadmissible any theory involving a belief in the animal nature of the zoospores. If these moving spores are really, as we have supposed, the produce of the ulva, we may expect that they will at some period of their existence grow into the likeness of their parent. If then we can keep them alive for a short time, and watch what eventually becomes of them, we shall probably find the means of settling the question. Now there is little difficulty in effecting this. Take a small glass cell containing a few drops of sea-water and some of the zoospores, and, in order to prevent the water from evaporating, place the cell upon a layer of moist sand and cover it with a bell-glass or a tumbler, and you will be able to preserve the zoospores alive for any length of time, and to trace from time to time any changes which take place in them. In a few days it will be found that they have ceased to move, and have attached themselves by their smaller end to the glass in which they are contained, generally to that part which is most freely exposed to the light. Then they will begin to grow in the manner which we shall presently describe,—a long filament being first produced which is gradually converted into a broad frond. When this process has once commenced it will soon reach a point at which even the most sceptical will be compelled to allow that the organism before him is undeniably a plant. These observations must, of course, be made under the microscope, for though the final result of the process is a plant of no small size, attaining sometimes a length of two feet and upwards, yet the zoospores themselves are so small as to be absolutely invisible to the naked eye.
There is good reason to believe that these zoospores do not truly represent the spores of the ulva, but are rather analogous to the bulbils by which some flowering plants are reproduced, and that true spores, probably motionless, exist in these as in almost all plants with which we are acquainted. These supposed spores, however, have not yet been detected by any observer.
Probably you will feel some curiosity as to the agency by which the strange animal-like movements of the zoospores are produced. Each zoospore is furnished with four, or in some cases with two, cilia or vibrating threads attached to its smaller end, and by the motion of these it is urged through the water. The cilia can only be seen, satisfactorily, when object-glasses of a high magnifying power are employed, and then only when they have been coloured and rendered opaque by the addition of a small quantity of tincture of iodine to the water, or when the zoospores have been dried at a gentle heat upon the glass slide. Of the force by which the cilia are set in motion nothing at present is known.
Very similar to the ulvæ in their habits and modes of growth, and not unlike in appearance to young filaments of the same seaweed, are the enteromorphæ or sea grasses, some species of which are common on most of our coasts. Closer examination, however, shows us that the frond of enteromorpha is not flat but tubular, the walls of the tube being composed of small cells like those which we have already seen in the frond of the ulvæ. The formation and motion of the zoospores may be watched in the enteromorphæ as well as in the ulvæ; but as there is no essential difference between the two cases, we need not again describe the process here. The commonest species of the sea grasses, the Enteromorpha intestinalis, which