[Ill.u.s.tration: FIG. 6.--Blue-green slime (_Oscillaria_). _A_, ma.s.s of filaments of the natural size. _B_, single filament, 300. _C_, a piece of a filament that has become separated. _s_, sheath, 300.]
As a representative of the group, we will select one of the commonest forms (_Oscillaria_), known sometimes as green slime, from forming a dark blue-green or blackish slimy coat over the mud at the bottom of stagnant or sluggish water, in watering troughs, on damp rocks, or even on moist earth. A search in the places mentioned can hardly fail to secure plenty of specimens for study. If a bit of the slimy ma.s.s is transferred to a china dish, or placed with considerable water on a piece of stiff paper, after a short time the edge of the ma.s.s will show numerous extremely fine filaments of a dark blue-green color, radiating in all directions from the ma.s.s (Fig. 6, _a_). The filaments are the individual plants, and possess considerable power of motion, as is shown by letting the ma.s.s remain undisturbed for a day or two, at the end of which time they will have formed a thin film over the surface of the vessel in which they are kept; and the radiating arrangement of the filaments can then be plainly seen.
If the ma.s.s is allowed to dry on the paper, it often leaves a bright blue stain, due to the blue pigment in the cells of the filament. This blue color can also be extracted by pulverizing a quant.i.ty of the dried plants, and pouring water over them, the water soon becoming tinged with a decided blue. If now the water containing the blue pigment is filtered, and the residue treated with alcohol, the latter will extract the chlorophyll, becoming colored of a yellow-green.
The microscope shows that the filaments of which the ma.s.s is composed (Fig. 6, _B_) are single rows of short cylindrical cells of uniform diameter, except at the end of the filament, where they usually become somewhat smaller, so that the tip is more or less distinctly pointed. The protoplasm of the cells has a few small granules scattered through it, and is colored uniformly of a pale blue-green. No nucleus can be seen.
If the filament is broken, there may generally be detected a delicate, colorless sheath that surrounds it, and extends beyond the end cells (Fig. 6, _c_). The filament increases in length by the individual cells undergoing division, this always taking place at right angles to the axis of the filament. New filaments are produced simply by the older ones breaking into a number of pieces, each of which rapidly grows to full size.
The name "oscillaria" arises from the peculiar oscillating or swinging movements that the plant exhibits. The most marked movement is a swaying from side to side, combined with a rotary motion of the free ends of the filaments, which are often twisted together like the strands of a rope. If the filaments are entirely free, they may often be observed to move forward with a slow, creeping movement. Just how these movements are caused is still a matter of controversy.
The lowest of the _Cyanophyceae_ are strictly single-celled, separating as soon as formed, but cohering usually in ma.s.ses or colonies by means of a thick mucilaginous substance that surrounds them (Fig. 7, _D_).
The higher ones are filaments, in which there may be considerable differentiation. These often occur in ma.s.ses of considerable size, forming jelly-like lumps, which may be soft or quite firm (Fig. 7, _A_, _B_). They are sometimes found on damp ground, but more commonly attached to plants, stones, etc., in water. The ma.s.ses vary in color from light brown to deep blackish green, and in size from that of a pin head to several centimetres in diameter.
[Ill.u.s.tration: FIG. 7.--Forms of _Cyanophyceae_. _A_, _Nostoc_. _B_, _Glotrichia_, 1. _C_, individual of _Glotrichia_. _D_, Chroococcus. _E_, _Nostoc_. _F_, Oscillaria. _G_, _H_, _Tolypothrix_.
All 300. _y_, heterocyst. _sp._ spore.]
In the higher forms special cells called heterocysts are found. They are colorless, or light yellowish, regularly disposed; but their function is not known. Besides these, certain cells become thick-walled, and form resting cells (spores) for the propagation of the plant (Fig. 7, C. _sp._). In species where the sheath of the filament is well marked (Fig. 7, _H_), groups of cells slip out of the sheath, and develop a new one, thus giving rise to a new plant.
The bacteria (_Schizomycetes_), although among the commonest of organisms, owing to their excessive minuteness, are difficult to study, especially for the beginner. They resemble, in their general structure and methods of reproduction, the blue-green slimes, but are, with very few exceptions, dest.i.tute of chlorophyll, although often possessing bright pigments,--blue, violet, red, etc. It is one of these that sometimes forms blood-red spots in flour paste or bits of bread that have been kept very moist and warm. They are universally present where decomposition is going on, and are themselves the princ.i.p.al agents of decay, which is the result of their feeding upon the substance, as, like all plants without chlorophyll, they require organic matter for food. Most of the species are very tenacious of life, and may be completely dried up for a long time without dying, and on being placed in water will quickly revive. Being so extremely small, they are readily carried about in the air in their dried-up condition, and thus fall upon exposed bodies, setting up decomposition if the conditions are favorable.
A simple experiment to show this may be performed by taking two test tubes and partly filling them with an infusion of almost any organic substance (dried leaves or hay, or a bit of meat will answer). The fluid should now be boiled so as to kill any germs that may be in it; and while hot, one of the vessels should be securely stopped up with a plug of cotton wool, and the other left open. The cotton prevents access of all solid particles, but allows the air to enter. If proper care has been taken, the infusion in the closed vessel will remain unchanged indefinitely; but the other will soon become turbid, and a disagreeable odor will be given off. Microscopic examination shows the first to be free from germs of any kind, while the second is swarming with various forms of bacteria.
[Ill.u.s.tration: FIG. 8.--Bacteria.]
These little organisms have of late years attracted the attention of very many scientists, from the fact that to them is due many, if not all, contagious diseases. The germs of many such diseases have been isolated, and experiments prove beyond doubt that these are alone the causes of the diseases in question.
If a drop of water containing bacteria is examined, we find them to be excessively small, many of them barely visible with the strongest lenses. The larger ones (Fig. 8) recall quite strongly the smaller species of oscillaria, and exhibit similar movements. Others are so small as to appear as mere lines and dots, even with the strongest lenses. Among the common forms are small, nearly globular cells; oblong, rod-shaped or thread-shaped filaments, either straight or curved, or even spirally twisted. Frequently they show a quick movement which is probably in all cases due to cilia, which are, however, too small to be seen in most cases.
[Ill.u.s.tration: FIG. 9.--_Euglena_. _A_, individual in the active condition. _E_, the red "eye-spot." _c_, flagellum. _n_, nucleus. _B_, resting stage. _C_, individual dividing, 300.]
Reproduction is for the most part by simple transverse division, as in oscillaria; but occasionally spores are produced also.
CLa.s.s III.--GREEN MONADS (_Volvocineae_).
This group of the protophytes is unquestionably closely related to certain low animals (_Monads_ or _Flagellata_), with which they are sometimes united. They are characterized by being actively motile, and are either strictly unicellular, or the cells are united by a gelatinous envelope into a colony of definite form.
Of the first group, _Euglena_ (Fig. 9), may be selected as a type.
This organism is found frequently among other algae, and occasionally forms a green film on stagnant water. It is sometimes regarded as a plant, sometimes as an animal, and is an elongated, somewhat worm-like cell without a definite cell wall, so that it can change its form to some extent. The protoplasm contains oval ma.s.ses, which are bright green in color; but the forward pointed end of the cell is colorless, and has a little depression. At this end there is a long vibratile protoplasmic filament (_c_), by means of which the cell moves. There is also to be seen near this end a red speck (_e_) which is probably sensitive to light. A nucleus can usually be seen if the cell is first killed with an iodine solution, which often will render the flagellum (_c_) more evident, this being invisible while the cell is in motion. The cells multiply by division.
Previous to this the flagellum is withdrawn, and a firm cell wall is formed about the cell (Fig. 9, _B_). The contents then divide into two or more parts, which afterwards escape as new individuals.
Of the forms that are united in colonies[2] one of the best known is _Volvox_ (Fig. 10). This plant is sometimes found in quiet water, where it floats on or near the surface as a dark green ball, just large enough to be seen with the naked eye. They may be kept for some time in aquaria, and will sometimes multiply rapidly, but are very susceptible to extremes of temperature, especially of heat.
[2] The term "colony" is, perhaps, inappropriate, as the whole ma.s.s of cells arises from a single one, and may properly be looked upon as an individual plant.
[Ill.u.s.tration: FIG. 10.--_Volvox._ _A_, mature colony, containing several smaller ones (_x_), 50. _B_, Two cells showing the cilia, 300.]
The colony (Fig. 10, _A_) is a hollow sphere, the numerous green cells of which it is composed forming a single layer on the outside.
By killing with iodine, and using a strong lens, each cell is seen to be somewhat pear-shaped (Fig. _B_), with the pointed end out.
Attached to this end are two vibratile filaments (cilia or _flagella_), and the united movements of these cause the rolling motion of the whole colony. Usually a number of young colonies (Fig. _x_) are found within the mother colony. These arise by the repeated bipart.i.tion of a single cell, and escape finally, forming independent colonies.
Another (s.e.xual) form of reproduction occurs, similar to that found in many higher plants; but as it only occurs at certain seasons, it is not likely to be met with by the student.
Other forms related to _Volvox_, and sometimes met with, are _Gonium_, in which there are sixteen cells, forming a flat square; _Pandorina_ and _Eudorina_, with sixteen cells, forming an oval or globular colony like _Volvox_, but much smaller. In all of these the structure of the cells is essentially as in _Volvox_.
CHAPTER IV.
SUB-KINGDOM II.
ALGae.[3]
[3] Algae (sing. _alga_).
In the second sub-kingdom of plants is embraced an enormous a.s.semblage of plants, differing widely in size and complexity, and yet showing a sufficiently complete gradation from the lowest to the highest as to make it impracticable to make more than one sub-kingdom to include them. They are nearly all aquatic forms, although many of them will survive long periods of drying, such forms occurring on moist earth, rocks, or the trunks of trees, but only growing when there is a plentiful supply of water.
All of them possess chlorophyll, which, however, in many forms, is hidden by the presence of a brown or red pigment. They are ordinarily divided into three cla.s.ses--I. The Green Algae (_Chlorophyceae_); II. Brown Algae (_Phaeophyceae_); III. Red Algae (_Rhodophyceae_).
CLa.s.s I.--GREEN ALGae.
The green algae are to be found almost everywhere where there is moisture, but are especially abundant in sluggish or stagnant fresh water, being much less common in salt water. They are for the most part plants of simple structure, many being unicellular, and very few of them plants of large size.
We may recognize five well-marked orders of the green algae--I. Green slimes (_Protococcaceae_); II. _Confervaceae_; III. Pond sc.u.ms (_Conjugatae_); IV. _Siphoneae_; V. Stone-worts (_Characeae_).
ORDER I.--_Protococcaceae_.
The members of this order are minute unicellular plants, growing either in water or on the damp surfaces of stones, tree trunks, etc.
The plants sometimes grow isolated, but usually the cells are united more or less regularly into colonies.
A common representative of the order is the common green slime, _Protococcus_ (Fig. 11, _A_, _C_), which forms a dark green slimy coating over stones, tree trunks, flower pots, etc. Owing to their minute size the structure can only be made out with the microscope.
[Ill.u.s.tration: FIG. 11.--_Protococcaceae._ _A_, _C_, Protococcus. _A_, single cells. _B_, cells dividing by fission. _C_, successive steps in the process of internal cell division. In _C_ iv, the young cells have mostly become free. _D_, a full-grown colony of _Pediastrum_. _E_, a young colony still surrounded by the membrane of the mother cell. _F_, _Scenedesmus_. All, 300. _G_, small portion of a young colony of the water net (_Hydrodictyon_), 150.]
Sc.r.a.ping off a little of the material mentioned into a drop of water upon a slide, and carefully separating it with needles, a cover gla.s.s may be placed over the preparation, and it is ready for examination. When magnified, the green film is found to be composed of minute globular cells of varying size, which may in places be found to be united into groups. With a higher power, each cell (Fig. 11, _A_) is seen to have a distinct cell wall, within which is colorless protoplasm. Careful examination shows that the chlorophyll is confined to several roundish bodies that are not usually in immediate contact with the wall of the cell. These green ma.s.ses are called chlorophyll bodies (chloroplasts). Toward the centre of the cell, especially if it has first been treated with iodine, the nucleus may be found. The size of the cells, as well as the number of chloroplasts, varies a good deal.
With a little hunting, specimens in various stages of division may be found. The division takes place in two ways. In the first (Fig. 11, _B_), known as fission, a wall is formed across the cell, dividing it into two cells, which may separate immediately or may remain united until they have undergone further division. In this case the original cell wall remains as part of the wall of the daughter cells. Fission is the commonest form of cell multiplication throughout the vegetable kingdom.
The second form of cell division or internal cell division is shown at _C_. Here the protoplasm and nucleus repeatedly divide until a number of small cells are formed within the old one. These develop cell walls, and escape by the breaking of the old cell wall, which is left behind, and takes no part in the process. The cells thus formed are sometimes provided with two cilia, and are capable of active movement.
Internal cell division, as we shall see, is found in most plants, but only at special times.