TURNING, now, to living things which are unquestionably ‘plants ‘in the strict sense of the word, we find first two largeof relatively simple kinds called ‘Algae ‘and ‘Fungi.’ The former group includes all the marine seaweeds, a host of their freshwater relatives, and very numerous single-celled plants. Some of the latter, called Diatoms, are enclosed in minute flinty shells, exquisitely sculptured, and consisting of two parts, or valves. But a greater number lack these protective coverings, and dwell together in colonies of innumerable individuals spread out in close contact as greenish or yellowish films on the surfaces of rocks, tree-trunks or undisturbed soil.
The large, many-celled Alga; appear to owe their origin to an extension of this communal mode of life. For we may suppose that the individual cells, instead of separating completely after fission, tended more and more to retain physical contact with one another, thus forming threads or thin plates of cells whose needs and normal habitat were identical. By an extension of this process such aggregates of cells would be likely to increase in size as time went on, and to arrive at some simple specialisation, or division of labour, among the various groups of cells forming the mass. Thus, although seaweeds cannot be said to possess truly functional, or , many of them seem to have taken the first step or two along the road which leads to the development of such organs.
All Alga? are self-supporting. That is to say, their substance contains chlorophyll, which enables them—like most of the higher plants—to build up food for their own use from materials present in their surroundings.1 Algaj are grouped by botanists into four classes, more or less according to their typical colouring, that is, (I) blue-green, (2) green, (3) brown, and (4) red.
In members of the first group, many of which are inconspicuous, green chlorophyll is replaced by blue-green. The individual cells often occur in colonies, or loosely united as filaments, which in their turn may be matted or interwoven. One simple kind forms patches of scum on the moist window-panes of hot-houses; while another, somewhat more complex,
is apt to appear suddenly as gelatinous masses on footpaths after rain. Before the rain it lay dormant, and quite invisible to the naked eye, among the particles of the surface soil.
The majority of the green Alga? are also unicellular or small, but include a few large kinds, of which the Sea Lettuces are probably the most familiar. These share with some of the red seaweeds the name of ‘Laver.’ In some districts, Laver is esteemed a table delicacy, more especially as a sauce to be eaten with roast mutton. But it is a fact that most strangers who have been induced to sample this alleged dainty regard the taste for it as an acquired one!
HOW SEAWEEDS PROTECT THEMSELVES FROM THE LIGHT MOST of the large seaweeds which we see growing on rocks or washed up on the shore after a storm belong to the brown and red classes. Their visible colouring seems to serve as a filter which stops out certain light-rays likely to injure the green colouring matter—in much the same way as a photographer protects his sensitised plates, or a druggist his delicate chemicals, by means of deeply tinted glass. This is a legitimate inference because, if we soak these seaweeds in warm fresh water, their red or brown pigment is found to ‘run,’ and eventually dissolves out completely, leaving the plant-body ‘green ‘in colour.
Alga; most commonlythemselves either by simple fission, or by the production of spores—from which, as we have seen, the new generation arises by direct growth when the spore-walls split open. But in many instances we find that a system has been developed which may quite properly be called ‘sexual,’ even when—as often happens—the two kinds of organs concerned occur on one and the same individual plant. Briefly, the products of these special organs, which we may call ‘male ‘and ‘female ‘respectively, are minute unicellular bodies, differing in constitution and function. ‘Fertilisation ‘is said to take place when a pair of these —one of each kind—comes into contact and unites to form what is known as an ovum, or fertilised egg-cell, from which in due course a new individual of the particular species takes its origin. This type of is in vogue among the brown ‘wrack ‘weeds which often thickly drape the tidal rocks of our coasts. It is not difficult to locate the reproductive organs, which should be looked for at the tips of the branching fronds during the summer and autumn months; but the
actual process can only be watched through a high-power microscope.
HOW FUNGI FORAGE FOR THEIR RATIONS
SEXUAL methods of propagation are also found among the fungi, including some of the simple types called ‘moulds; ‘but practically all the larger kinds—known popularly as Mushrooms, Toadstools and Puffballs—rely wholly upon the production of spores, which they often scatter abroad in prodigious numbers. Indeed, a mushroom or a toadstool is the special spore-producing outgrowth of a vegetative plant-body which consists merely of branching threads, or ‘hyphie,’ and whose life is passed burrowing in earth, decaying wood, or some other substance from which it can obtain food.
Fungi differ chiefly from their relatives the alga; in being destitute of chlorophyll. Hence, they cannot make food-materials for themselves,1 but must needs forage for their rations. Many species subsist on decaying organic matter of all sorts, including the humus of the soil; but not a few have become actively parasitic—that is, they attack the living tissues of other plants, and even of animals.
Among the parasitic fungi is the curious ‘Caterpillar Fungus,’ whose red, club-shaped ‘toadstool ‘not infrequently springs up in garden beds during the autumn. Careful examination of the soil beneath invariably brings to light the mortal remains of some grub or pupa which the fungus has done to death !
From the economic standpoint this particular species may be counted as beneficial, since it destroys numerous insects which we call ‘pests.’ But many fungi work incalculable injury to man’s interests by destroying forest trees, crops of many kinds, and even (in the case of ‘dry rot ‘) the timber used in the construction of houses.
TWO PLANTS IN PARTNERSHIP: THE LICHEN’S ODD LIFE-STORY BEFORE leaving this Thallophyte division of plants, some mention must be made of the group called ‘Lichens.’ These are usually very moss-like in appearance, but in point of fact are composite organisms, made up of the matted strands of a fungus among which colonies of single-celled alga; are imprisoned. Exactly how this strange combination originated is one of Nature’s secrets. It is fairly certain,
however, that both parties derive benefits from the union, for scientists have discovered that the alga? share with the fungus the food products which—thanks to their chlorophyll —they are able.to fabricate from the raw chemical constituents of their surroundings, while the fungus, for its part, collects and conserves the moisture (containing dissolved mineral salts) without which the alga? could not carry on their work. By these means lichens are able to flourish in situations— for instance, the sun-baked surfaces of granite rocks—where neither of the partners could possibly thrive independently.
In the matter of propagation, the fungus element seems always to play the leading part. The methods vary; but in many instances minute spore-like bodies are produced which may be seen under the microscope to consist of several alga? wrapped round with a strand of detached fungus thread. When these are carried by the wind to suitable situations, growth commences, and in process of time new lichen plants are formed.
THE CASE OF THE CORD MOSS
BRYOPHYTA—otherwise the Liverworts and Mosses—are simple in structure, though less ‘all-of-a-piece ‘than the Algre and Fungi; for we can often distinguish parts which resemble superficially the, roots and branches of the higher plants. Their most important characteristic, however, is that their life-history always proceeds through two distinct, alternating stages, each culminating in the formation of reproductive cells, either spores or ova, as the case may be.
Suppose, for example, we start with the minute spores of the Common Cord Moss which often forms large patches on walls and rocks, as well as on open ground, especially where forest or heath fires have occurred. In propitious circumstances these spores germinate, and develop eventually into little green plants bearing numerous tiny leaflets arranged spirally round a shortfixed to the soil at its base by rootlike filaments. After a period of vegetative growth, sexual organs appear at the tip of the shoot, and may be either ‘male ‘or ‘female,’ since the two kinds are produced by separate plants.
Of the egg-cells born by a ‘female ‘plant, one at least is likely to be fertilised by a ‘male ‘or sperm-cell, which— when a film of moisture covers the moss colony and the surrounding soil—swims through this to its objective from
a neighbouring ‘male ‘plant. Now this fertilised egg-cell does not develop directly, as we might expect, into a new moss-plant, but growing upward becomes a spore-capsule of complex structure attached by a long stajk to its parent, from which it draws its nourishment. It is from the spores so produced—not from the fertilised egg-cells as such—that new moss-plants subsequently grow.
The sexual and spore-bearing organs vary a good deal in appearance among the different species of Liverworts and Mosses, but the same cycle of alternating generations can always be traced. We may express it in the following short formula :
Spores > ‘male ‘and ‘female ‘green plants ? fertilised egg-cells > non-green spore-developing bodies which are short-lived, being normally parasitic upon their parents ? spores; and so on through successive cycles.
Certain Liverworts, including a species common in gardens and greenhouses, supplement this method by separating off from their substance minute bodies called ‘gemma;.’ These are matured in tiny bird’s-nest-like cups with frilled edges sunk in the frond or ‘thallus ‘of the parent plant. When ripe, they are washed out by rain, and soon grow into new individuals.
THE LIFE-STORY OF A FERN
IF, as seems probable, Liverworts and Mosses were evolved from Algae which in the remote past took to living in fairly dry situations, this is almost certainly true of the Ferns and their near relations, the Horse-tails and Club-mosses. These three types, while differing considerably among themselves in appearance and habits, agree sufficiently in essentials to be classed together as a third main division of plant life—the Pteridophytes. In particular, the egg-cell, after fertilisation, develops into something quite different from the spore-bearing generation of a Moss; for it gives rise to a conspicuous plant-body, having leafy fronds or shoots which, as they contain chlorophyll, are capable of self-maintenance. This point will be made clear if we consider for a moment the life-story of a common Fern, such as the Hart’s-tongue or the Bracken.
The word ‘ferns,’ as popularly used, refers simply to the familiar spore-bearing plants, which have fronds, roots and stems. Being entirely self-supporting, they may continue to flourish for many years; whereas the corresponding stage in
a Moss’s life-cycle is nourished by its parent, and quickly withers away after its crop of spores has been matured and scattered. The Fern plant, on the contrary, continues to produce spores each year, as long as it lives, in little cases which appear as brown groups or clusters on the reverse sides of the fronds. When the spores fall on damp spots and germinate, they grow into small, green, plate-like bodies, usually heart-shaped in outline, known as ‘prothalli,’ on the underside of which the sexual organs develop.
The process of fertilisation, similar to that already described in the case of the Moss, appears to be somewhat uncertain in operation; for although numerous egg-cells are produced by a prothallus, the latter rarely gives rise to more than one young Fern. This is at first a delicate little thing with a tiny leaf and an embryo; but it gradually increases in size
and strength, developing the specific features of its parent stock, and eventually producingcrops of spores.
Thus, for comparison with our Liverwort and Moss formula set out above, we get the following : Spores »¦ insignificant bi-sexual green plants of very simple structure ? fertilised egg-cells *¦ spore-bearing green plants of large size and complex structure continuing to grow for many years >- spores.
The life-story of a Horsetail resembles that of a Fern, save that the two kinds of sex-organs are produced by separate prothalli (of which the ‘males ‘are the smaller), while the spore-cases are carried on upstanding club-shaped shoots, instead of on the backs of the fronds. Among the Club-mosses, however, we find that a kind of telescoping of the process has been effected, together with a pushing back (so to say) of the sex principle to the spores, which are of two kinds, matured in separate chambers and distinguishable as ‘small ‘and ‘large.’ Also, the prothallus is rudimentary, and is developed by spore-germination inside the chamber; so that it seems to be omitted from the life-cycle altogether.
If these details had not been established by careful research, we might have jumped to the conclusion that the Club-moss propagates itself by means of, like the higher plants. In point of fact, it comes very near to doing this. But technically speaking a ‘ ‘is a complex body, not set free from the parent organism until the fertilised egg-cell from which it takes its origin has undergone considerable development, resulting in the formation of an embryo, or baby plant. In the case of the Club-moss this development has not yet taken place when the apparent ‘seed ‘is cast adrift.