Once a Week (magazine)/Series 1/Volume 5/Green seaweeds
GREEN SEAWEEDS.
Most people who have visited the sea-side know some little about seaweeds. They are familiar with the characteristics of the three great orders into which this class of plants has been divided, have learned to distinguish some of the commoner and more elegant species, and have perhaps made a small collection of specimens more or less skilfully dried upon paper. They know, too, something of the purposes to which some of the seaweeds are applied, have probably tasted laver and Irish moss, and have heard that the Scotch eat dulse, and that the ashes of the fuci are made into kelp. And with this amount of knowledge very many rest content. Yet there are few objects which offer more points of interest than the seaweeds to any one who is willing to look a little way below the surface, and to spend a few hours in learning something of their modes of growth and propagation.
The name seaweed can in its strict sense be applied only to plants growing in the sea. If, however, we use it as a translation of the botanical term algæ, it becomes applicable not only to these, but to a considerable number also which are never found in salt water. Every river and brook, every pond, ditch, and roadside puddle has its algæ—even on damp earth and walls they are seldom wanting. On the other hand, at least one plant growing in the sea, the Zostera marina, sometimes called seagrass, is a true flowering plant, and cannot therefore properly be called a seaweed. It has been found convenient to divide the algæ into three great divisions: the confervoideæ, otherwise called chlorospores, or green-spored algæ; the florideæ rhodospores, or red-spored algæ; and the fucoideæ melanospores, or dark-spored algæ. Colour, which is generally a mark of very small value in classification, is here a tolerably safe guide, so that we shall not go far wrong in calling all the brown seaweeds melanospores, all the red seaweeds rhodospores, and all the green seaweeds chlorospores. It would be impossible in the space of a single article to give even the slightest sketch of the history of these three orders, and we shall therefore for the present confine our attention to the last only, the chlorosperms or green-spored algæ, and to those only of this order which are found in salt water.
The green seaweeds which we find growing on our seaside rocks belong most commonly to one of the four genera: conferva, cladophora, enteromorpha, and ulva. The broad bright-green fronds of the Ulva lactuca, the lettuce ulva, or laver, are too well known to need any description, but there are some curious episodes in its history which are far less generally known.
Among the strange facts which modern improvements in the microscope have brought to light, there are few more unexpected and more startling to our preconceived ideas than the possession by certain plants of a faculty of motion so like in every respect to that of some of the lower animals, that the first observers had no suspicion of their vegetable nature. You may perhaps feel inclined to doubt the possibility of the existence of these moving plants; you may think that the story is a mere scientific figment contrived for the purpose of puzzling the unlearned, and you ask why we cannot be contented with the good old definition of our younger days, “Plants live and grow; animals live, grow, and move.” If you happen to be by the sea-side, you may soon satisfy your doubts, and in no way more easily than by bestowing a little attention upon the common green laver. Go down to the beach at almost any state of the tide, and in the first rock pool you see you will probably find some of the conspicuous bright green fronds of this seaweed. Probably, too, if you look closely you will see that some of the fronds seem to have lost their brilliant hue and are limp, transparent, and colourless, not unlike wet tissue paper. They are faded, you say, and dying. By no means. Never were they more full of life than they are now. Gather one of these colourless fronds, or rather one which is half green, half colourless, and take it home with you for examination. Cut off carefully a small piece from that part of the frond through which the line passes, dividing the green from the uncoloured half, and having placed it in a little sea water on the stage of the microscope, examine it with an object-glass of a quarter of an inch focus. You will, in the first place, see that the frond of laver is composed of a network of small, many-sided, irregularly-shaped cells densely packed together with their sides in contact, and by slightly altering the focus you will find that another and precisely similar layer constitutes the under surface of the frond. Next, if you turn your eye from the seaweed to the water in which it is floating you will see, nimbly swimming about, a considerable number of small green bodies in shape something like a pear. These, if you now see them for the first time, you will, beyond all doubt, pronounce to be animals, probably belonging to the class of the infusorial animalcules. But you will be very far from the truth in your guess, for these pear-shaped bodies are the zoospores, or moving spores of the green laver, and have probably just emerged from one of the cells of the fragment at which you are looking. Now look again at the seaweed, and observe particularly the cells of which the coloured portion is made up. These you will find to be filled with a 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 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-weeds. 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 in their appearance, and few people probably ever notice them, or could be easily induced to believe that there is anything in them worthy of notice. But place a small portion under the microscope, and you will not long be in doubt as to where the interest of the oscillatoriæ lies. You will see an evident and undeniable plant actively moving. In the case of the moving zoospores, which we just now described, it is only by seeing them within the cells of the parent plant, or by patiently watching their growth, that you can convince yourself of their vegetable nature; but that the oscillatoriæ are plants, notwithstanding their strange motions, your eyes will at once convince you. And strange indeed their movements are. Here you will see a thread moving from side to side like the pendulum of a clock, one end vibrating, while the other remains fixed; here a second twists itself about like a worm or a caterpillar, while a third combines this with an onward progressive motion. “If a piece of the stratum of an oscillatoria,” says Dr. Harvey, “be placed in a vessel of water, and allowed to remain there for some hours, its edge will first become fringed with filaments, radiating from a central point, with their tips outwards. These filaments, by their constant oscillatory motion, are continually loosened from their hold on the stratum cast into the water, and at the same time propelled forwards; and as the oscillation continues after the filament has left its nest, the little swimmer moves along, till it not only reaches the edge of the vessel, but often, as if in the attempt to escape confinement, continues its voyage up the sides till it is stopped by dryness. Thus, in a very short time, a small piece of oscillatoria will spread itself over a large vessel of water.” The cause of these singular movements is a mystery to which at present we have no clue. No cilia can be detected, nor any other organs of motion; and when we have said that the movements of these plants are rendered more active by heat and light, and checked by any strong chemical agent, we have exhausted the whole stock of our present knowledge.
Strange as are some of these facts when told, they seem far more strange when seen. The history of the seaweeds is one which words can but very imperfectly relate. Probably no one fully realises the idea of moving plants, until he has seen for himself the swarming of the zoospores within their parent cells, or the oscillatoriæ performing their singular rhythmical movements. Here, too, as in all other branches of natural history, he who wishes truly to know, must not rest contented with the descriptions of others, but must take the earliest opportunity of verifying or correcting them by his own observation.