The plant life of the sea side may be divided into two natural groups (i) of plants living on the shore near the sea and (ii) of plants living in the sea, for part of each day at least. The former group contains many plants of exactly the same kind as occur far inland, together with a few typically sea-side plants such as Thrift or Sea Pink. They are, however, one and all land plants. In this chapter we shall confine ourselves to the real sea plants, the seaweeds.
Before we study any of these interesting plants under the microscope, it will be useful to learn a little about seaweeds in general, because they are so totally unlike land plants in every respect. They belong to the great plant division known as the Algæ, to which also belong the Diatoms, Desmids, Volvox, Spirogyra and many of the other plants we mentioned in our chapter on Pond Life. So we see that, although all seaweeds are Algæ, not all Algæ are seaweeds. They are higher in the scale of development than Fungi, to which Bacteria, Yeast, Mould, etc., belong, though they are not so highly developed as Ferns and not nearly so advanced as flowering plants. A very short acquaintance with Algæ will show us that they are either green, brown or red. The green Algæ are nearly all fresh water forms, though a few are to be found in the sea; on the other hand brown and red Algæ are common in the sea and rare in fresh water.
As we study our seaweeds on the shore, if we are really observant, we shall notice that they live in zones or belts according to their colour. There are exceptions to this rule but, in general, the green seaweeds dwell in situations where they are only covered by the sea at high tide; the red seaweeds are to be found mostly where they are always below water and, between the two, the brown seaweeds occur. In some parts, this colour scheme is very striking. Frequently red seaweeds may be found above high-water mark it is true, but in such cases they nearly always occur in rock pools and they are invariably sheltered by brown seaweeds.
In our chapter on plant life we mentioned that many coloured plants contained the green colouring matter, chlorophyll, just as do the ordinary green leaves. We showed too that by boiling some green leaves in methylated spirits we could extract the chlorophyll and that its solution had the peculiar property of appearing green when light passes through it and red when light is reflected from it. If now we take almost any brown or red seaweed, we cannot see a trace of chlorophyll anywhere. Let us leave our specimen, however, in fresh water for a few days when we shall find that the brown or red colouring matter as the case may be, is dissolved by the water and a green plant remains. By treating the seaweed, deprived of its distinctive colour, with methylated spirit as described above, we can obtain a solution of chlorophyll.
The microscopist who is anxious to make a study of seaweeds, will find little scope for his hobby on a sandy shore. Just as the most interesting marine animals are to be found where rocks abound, so must we hunt in similar situations for our Algæ. A few thread like Algæ are able to anchor themselves to the sand but most of them require a substantial support. A bare rock is a much favoured situation and before we have learned the peculiarities of these plants we may marvel how they obtain any sustenance from so barren a resting place. As a matter of fact they derive no nourishment from the rocks on which they rest. The part of the sea weed which, in our ignorance, we may have dubbed a root, is nothing of the kind. It bears no relationship to the roots of higher plants and is a mere anchor, designed to fasten its owner to a support.
None of these plants have roots, none have true stems or leaves, though the parts resembling stems and leaves are often so called; none of them flower and so fruits and seeds are unknown to them. Their food is absorbed from the sea water over the whole of their surface.
We may well begin our study of the seaweeds with an examination of the external structure of as many different kinds as we can find. Some of them are flattened and very thin forms and of them the Sea Lettuce may be taken as typical. This plant, known to scientists as Ulva Lactuca, occurs at high-water mark. In its fully developed form it is pale green and so thin as to be almost transparent; its structure may be studied under the microscope without difficulty. Then there is the very common, green, Compressed Enteromorpha which grows in great profusion on the rocks of the shore, rendering them exceedingly slippery. The closely related Intestinal Enteromorpha as it floats in the water resembles a green, membranous tube and those of us who have ever done any zoological dissection will appreciate how well named this plant is. The structure of both the Enteromorphas can easily be seen. Many of the brown and red Algæ will provide us with a good deal of occupation in making out their structure. Some of them, the brown Ectocarpus Siliculosus for example, may be found, growing in moss like tufts, which are usually attached to one of the larger Algæ living between the tide marks. It is one of the simplest of the brown sea weeds, consisting of branched threads, but one cell in thickness. The Wracks, of which the Common Bladder Wrack or Pop weed with its little air filled bladders is familiar to everyone, are more complicated in structure, in fact they appear to be possessed of stems and leaves, but we shall return to them in a moment.
Most of the common red Algæ are so delicate in structure that they require a fairly high magnification for their examination. The thin, membranous fronds of the beautiful crimson Delessera Sanguinea, may be sought below low tide limit or may be found washed up upon the shore. Superficially the plant resembles a red hart’s-tongue fern, with much more delicate fronds than ever fern of that species possessed. We may well compare its structure with that of the sea lettuce, for it is equally transparent.
In the rock pools of many parts of the coast we may happen upon a most curious almost white sea weed, known as Coralline or Corallina Officinalis. Its branched, feathery stems are hard and stony and the whole plant bears a superficial resemblance to a coral, hence its name. The plant absorbs a substance known as calcium carbonate from the sea water and deposits it in the form of a hard, stony covering over its surface. Calcium carbonate does not occur in sea water everywhere, at least not in sufficient quantity to be of use to the Coralline, that is the reason the sea weed is not quite so common as some of the others we have mentioned. The curious armour-plating of this sea dweller, should be studied under the microscope.
The chief scientific interest of the sea weeds, however, lies in their mode of increase, it is so totally different from that of any of the higher plants. The most simple method of increase is known as vegetative reproduction, it does not occur in every kind of seaweed and is nothing more or less than the growth of a broken piece of plant into a new individual. This form of increase is not unknown higher in the plant world; begonia leaves may be induced to send forth roots and grow into new plants, many garden favourites are propagated by means of cuttings and both these methods are similar to the breaking away and growth of portions of a seaweed; the garden plants, however, are assisted by man, the seaweed does its own work.
The simplest forms of increase occur amongst those giants of the sea, the Laminarias or Tangles as they are often called. These brown seaweeds often attain enormous sizes, they all grow below the limits of low tide and appear to thrive best where the water is frequently lashed by storms. To see these plants at their best we must look down upon them in their watery home. There are spots on the North-Eastern coast of Ireland, where one may look from the cliffs upon a veritable forest of Tangles. There thrives the “Devil’s Apron,” short of stem but with a flat ribbon of a frond, which may attain a length of a dozen feet and a width of as many inches. There too we can behold the Fingered Tangle, with stem, maybe, six feet in length and a crown of large finger-like fronds, “Sea Laces” or “Dead Men’s Ropes,” with fronds resembling slender ropes, in length, at times as much as forty feet, ride gracefully on the ever changing currents. Safely hidden in this marine forest lurk queer fishes and crabs and shell fish. About the broad fronds of the “Devil’s Apron” sea mice and sea cucumbers disport themselves; the Tangle home is a paradise for marine life. Yet with all their vigorous growth they increase simply by liberating spores which give rise to new plants.
In our chapter on Plant Life we described spores very briefly; we said that from a strictly scientific point of view they were not comparable to seeds but that for our purpose they might be looked upon as seeds for the reason that, by their germination, new plants were formed. All the spores of land plants are minute, they are carried from the mother plant to suitable spots for germination by wind. The spores of seaweeds also are small, but they are very different to the little wind-blown, land-dwelling spores. They possess a pair of the curious little whip-like structures we have observed in so many water plants and animals. By the lashing of these little whips they are able to swim about in the water till they find a suitable spot to settle down and grow into plants similar to those whence they came. On account of their animal-like movements they are called zoospores.
The formation of zoospores may be easily observed in the brown sea weed Ectocarpus Siliculosus we have already mentioned. This plant, as we have already remarked, consists of thin, thread-like rows of single cells and from time to time it is branched. At certain periods of the year, moderately large, pear shaped swellings occur on the threads of the sea-weed. If we are either fortunate or exceptionally patient we may chance to be examining one of these swellings under the microscope at the moment when it bursts and sets free its contents. Should we have this good fortune we must hasten to magnify more highly the zoospores which have escaped from the pear shaped spore case. Here we may add the caution that we shall only witness the bursting of the spore case if we examine our specimen in sea water; we should require more than the patience of Job to watch for its bursting in the dry state, for it will never come to pass.
A careful study of a zoospore will show that it swims in a peculiar manner. One of the little whips is directed forwards, the other trails behind. After a short period of activity the zoospore comes to rest, loses all means of propulsion, germinates and grows into a new Ectocarpus plant.
Sometimes this Algæ reaches a low ebb of vitality, it requires a new lease of life as it were, when this state is reached another form of increase takes place. The events in this case may also be witnessed under the microscope. From very similar spore cases a number of zoospores are liberated and for a time they swim about freely. If now we watch carefully we shall notice that one of the active little bodies comes to rest and that the others lose no time in swarming round it. One of these swarming zoospores fuses with the individual which first ceased swimming about, with the result that a much larger, non-swimming individual is formed which, after a short resting period, germinates and grows into a new sea weed. The remainder of the zoospores will come to rest later and germinate to form new plants just as though no fusion had taken place with two of their number. Here we see the beginnings of male and female increase amongst sea weeds; the individual which first comes to rest is looked upon as the female and the one with which it fuses as the male.
By the courtesy of Messrs. F. Davidson & Co.
The Feeler of a Cockchafer
The end of the feeler consists of a number of plates, which can be spread fanwise. The pores visible on the plates are the insect’s organs of smell.
Amongst the Wracks, of which there are a number of kinds, the methods of increase reach a higher stage. First of all let us describe the plants, so that we may know what to look for. They all belong to the group of brown Algæ. The “Channelled Wrack” is, when fully grown, about six inches long. It is much branched, often almost yellowish in colour and grows just below high-water mark. Along one side of the plant there is a moderately deep groove. Here we may note that the Wracks grow in zones from just below high water mark to low water mark. A little nearer the sea than the haunts of the Channelled Wrack, we shall find the Flat and Bladder Wracks. The former is but six inches or so in length, with flat, forked fronds, along the centres of which runs a single rib. The Bladder Wrack varies considerably in size. It may be smaller than either of the Wracks we have already mentioned or it may be two or three feet in length. It is the one seaweed familiar to everybody. Nearer to low tide mark we shall encounter the Knobbed Wrack, greenish brown in colour and often as much as six feet in length. It is so named because from the sides of its flat, leathery, strap-like fronds, there arise little stalked bladders. Right at, and often beyond, low tide mark there dwells the Notched Wrack; very similar to, though larger than, the Flat Wrack, from which it may easily be distinguished by the fact that the edges of its fronds are toothed, after the manner of a saw.
It is obvious that the structure of any one of these Wracks is much more complicated than is the case with Ectocarpus. The latter Alga was composed of a number of cells, similar to one another in every respect except size. If we tease a stem or a frond of one of the Wracks upon a slide and examine the result of our efforts under the microscope we shall see that the cells which compose the Wrack are not by any means similar to one another. Those of us who have mastered the, by no means difficult, art of section cutting, should cut sections of stem and frond and compare them with sections of leaf and stem of some higher plant. The comparison will show us that, although the seaweed does not exhibit the complicated structure of a flowering plant, it has at least three kinds of different cells—an outer layer, a central structure and an intermediate layer.
If we secure a specimen of the Channelled Wrack, at the end of the summer, we shall notice that the tips of certain of the fronds are swollen. Examination of these swellings under a low magnification will reveal a number of wart-like structures, and at the end of each wart there is a little pore. If we open up one of these little warts, very carefully with our mounted needles, we shall find that each little pore opens into a cavity, within which we can find two kinds of structures, hidden amongst a number of hair like growths. We shall see a number of dark, oval bodies, at the base of the hairs, these are the egg cells; more careful search will show us a number of much branched structures also partly concealed by the hairs, these are the male organs of the plants. The purpose of the hairs, by the way, is to keep the little chamber moist, when the plant is left high and dry. If we watch the pores of the other warts carefully we may be fortunate enough to see the process of increase taking place, for it occurs outside the plant and not within the chamber. The egg cell divides into two and its contents pass out of the chamber by way of the pore; each cell of the male organs gives rise to sixty-four oval little structures, each provided with a pair of minute whip-like threads by means of which it swims from the chamber and goes in search of the egg cells. Many of these little navigators are lost by the way but one of them will reach and fuse with each egg cell. After fusion the new-formed cell germinates at once into a new Channelled Wrack.
That this is an advance is shown by the fact that the little swimming bodies which fail to fuse with the egg cell, do not develop into new plants as in Ectocarpus nor does the egg cell, which has failed to fuse with a swimming body, germinate.
In the Bladder Wrack, a very similar process takes place. There are, however, certain important differences, differences which show that the plant is still more highly developed. If we examine the cavities, in the little warts of the Bladder Wrack we shall find that some of them contain egg cells, some contain male organs but none contain both. We noticed that the egg cell of the Channelled Wrack produced two eggs, that of the Bladder Wrack produces eight. In other respects the two plants behave similarly.
The methods of increase amongst the red seaweeds are rather more complicated and as our object is to interest and not to puzzle our readers we will content ourselves with a few general remarks. Microscopists who are anxious to probe more deeply into the subject will soon devise ways and means for themselves. The little swimming bodies which lend an added attraction to the study of the brown seaweeds are replaced, amongst the red Algæ, by organisms with but one whip-like structure apiece and that without the power of propelling its owner through the water. As with Ectocarpus, increase may take place in two ways. On these red plants we may find the now familiar swellings, which we have learned to know are spore cases, but instead of the multitudes of free swimming organisms which are set free on the bursting of the brown sea weed spore case, we now witness the expulsion of but four inert spores, which settle down in the water and immediately grow into new plants.
In the second method of increase, where male and female organs are concerned, we find that both these structures grow on the outside of the plant and not in cavities. Let us take the common, pink, much branched seaweed, known by the fearsome name of Callithamnion Corymbosum as our example. The male organs grow in little fungus like tufts about the branches of the plant and they give off enormous numbers of little organisms which have no power of swimming to the female organs. Either on the same or on another plant we shall find the female organs; we need not describe them in detail but there is one point of very great interest. From each of the female organs there grows a long jelly-like hair. As we have remarked, the organisms set free by the male organs cannot swim about but float aimlessly in the water. Obviously the majority of them simply perish, one perchance may touch a sticky hair to which it adheres, with which it fuses and passes down to the female cell, resulting in the production of a new seaweed.
It may be remembered that in writing about the pollen grains of flowering plants, we mentioned that those plants dependent upon wind for the distribution of their pollen, have stigmas ingeniously contrived for catching and retaining the grains. It is curious that the red seaweeds should have very similar contrivances for capturing and retaining the male cells.
The sea will also provide us with a rich harvest of those beautiful microscopic objects, the Diatoms. They may be sought on seaweeds, their yellowish brown colour often betrays their whereabouts, on rocks and sand and in mud. The salt water forms are as varied and as beautiful as their fresh water cousins.
To the microscopist who merely uses his microscope for the pleasure he can derive from it, rather than for serious study, it may appear that the plant life of the sea falls short of the animal life as far as interest is concerned. He may disabuse himself at once of this idea. There is no class of plants more interesting than the seaweeds and in few branches of plant life is there greater scope for new discoveries. The garden of the sea is largely an unexplored territory and there is no coast-line in the world of equal extent which provides so many different sea dwelling plants as our own.