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.