There are various genera of the pond sc.u.ms, differing in the form of the chloroplasts and also in the position of the spores. Of these may be mentioned _Zygnema_ (Fig. 19, _A_), with two star-shaped chloroplasts in each cell, and _Mesocarpus_ (Fig. 19, _B_, _D_), in which the single chloroplast has the form of a thin median plate. (B shows the appearance from in front, _C_ from the side, showing the thickness of the plate.) _Mesocarpus_ and the allied genera have the spore formed between the filaments, the contents of both the uniting cells leaving them.

[Ill.u.s.tration: FIG. 20.--Forms of Desmids. _A_, _B_, _Closterium_.

_C_, _D_, _D'_, _Cosmarium_. _D_, and _D'_ show the process of division. _E_, _F_, _Staurastrum_; _E_ seen from the side, _F_ from the end.]

Evidently related to the pond sc.u.ms, but differing in being for the most part strictly unicellular, are the desmids (Fig. 20). They are confined to fresh water, and seldom occur in ma.s.ses of sufficient size to be seen with the naked eye, usually being found a.s.sociated with pond sc.u.ms or other filamentous forms. Many of the most beautiful forms may be obtained by examining the matter adhering to the leaves and stems of many floating water plants, especially the bladder weed (_Utricularia_) and other fine-leaved aquatics.

The desmids include the most beautiful examples of unicellular plants to be met with, the cells having extremely elegant outlines.

The cell shows a division into two parts, and is often constricted in the middle, each division having a single large chloroplast of peculiar form. The central part of the cell in which the nucleus lies is colorless.

Among the commonest forms, often growing with _Spirogyra_, are various species of _Closterium_ (Fig. 20, _A_, _B_), recognizable at once by their crescent shape. The cell appears bright green, except at the ends and in the middle. The large chloroplast in each half is composed of six longitudinal plates, united at the axis of the cell.

Several large pyrenoids are always found, often forming a regular line through the central axis. At each end of the cell is a vacuole containing small granules that show an active dancing movement.

The desmids often have the power of movement, swimming or creeping slowly over the slide as we examine them, but the mechanism of these movements is still doubtful.

In their reproduction they closely resemble the pond sc.u.ms.

ORDER IV.--_Siphoneae_.

The _Siphoneae_ are algae occurring both in fresh and salt water, and are distinguished from other algae by having the form of a tube, undivided by part.i.tion walls, except when reproduction occurs. The only common representatives of the order in fresh water are those belonging to the genus _Vaucheria_, but these are to be had almost everywhere. They usually occur in shallow ditches and ponds, growing on the bottom, or not infrequently becoming free, and floating where the water is deeper. They form large, dark green, felted ma.s.ses, and are sometimes known as "green felts." Some species grow also on the wet ground about springs. An examination of one of the ma.s.ses shows it to be made up of closely matted, hair-like threads, each of which is an individual plant.

In transferring the plants to the slide for microscopic examination, they must be handled very carefully, as they are very easily injured. Each thread is a long tube, branching sometimes, but not divided into cells as in _Spirogyra_ or _Cladophora_. If we follow it to the tip, the contents here will be found to be denser, this being the growing point. By careful focusing it is easy to show that the protoplasm is confined to a thin layer lining the wall, the central cavity of the tube being filled with cell sap. In the protoplasm are numerous elongated chloroplasts (_cl._). and a larger or smaller number of small, shining, globular bodies (_ol._). These latter are drops of oil, and, when the filaments are injured, sometimes run together, and form drops of large size. No nucleus can be seen in the living plant, but by treatment with chromic acid and staining, numerous very small nuclei may be demonstrated.

[Ill.u.s.tration: FIG. 21.--_A_, _C_, successive stages in the development of the s.e.xual organs of a green felt (_Vaucheria_). _an._ antheridium. _og._ oogonium. _D_, a ripe oogonium. _E_, the same after it has opened. _o_, the egg cell. _F_, a ripe spore. _G_, a species in which the s.e.xual organs are borne separately on the main filament.

_A_, _F_, 150. _G_, 50. _cl._ chloroplasts. _ol._ oil.]

When the filaments are growing upon the ground, or at the bottom of shallow water, the lower end is colorless, and forms a more or less branching root-like structure, fastening it to the earth. These rootlets, like the rest of the filament, are undivided by walls.

One of the commonest and at the same time most characteristic species is _Vaucheria racemosa_ (Fig. 21, _A_, _F_). The plant multiplies non-s.e.xually by branches pinched off by a constriction at the point where they join the main filament, or by the filament itself becoming constricted and separating into several parts, each one const.i.tuting a new individual.

The s.e.xual organs are formed on special branches, and their arrangement is such as to make the species instantly recognizable.

The first sign of their development is the formation of a short branch (Fig. 21, _A_) growing out at right angles to the main filament. This branch becomes club-shaped, and the end somewhat pointed and more slender, and curves over. This slender, curved portion is almost colorless, and is soon shut off from the rest of the branch. It is called an "antheridium," and within are produced, by internal division, numerous excessively small spermatozoids.

As the branch grows, its contents become very dense, the oil drops especially increasing in number and size. About the time that the antheridium becomes shut off, a circle of buds appears about its base (Fig. 21, _B_, _og._). These are the young oogonia, which rapidly increase in size, a.s.suming an oval form, and become separated by walls from the main branch (_C_). Unlike the antheridium, the oogonia contain a great deal of chlorophyll, appearing deep green.

When ripe, the antheridium opens at the end and discharges the spermatozoids, which are, however, so very small as scarcely to be visible except with the strongest lenses. They are little oval bodies with two cilia, which may sometimes be rendered visible by staining with iodine.

[Ill.u.s.tration: FIG. 22.--_A_, non-s.e.xual reproduction in _Vaucheria sessilis_. _B_, non-s.e.xual spore of _V. geminata_, 50.]

The oogonia, which at first are uniformly colored, just before maturity show a colorless s.p.a.ce at the top, from which the chloroplasts and oil drops have disappeared (_D_), and at the same time this portion pushes out in the form of a short beak. Soon after the wall is absorbed at this point, and a portion of the contents is forced out, leaving an opening, and at the same time the remaining contents contract to form a round ma.s.s, the germ or egg cell (Fig. 21, _E_, _o_). Almost as soon as the oogonium opens, the spermatozoids collect about it and enter; but, on account of their minuteness, it is almost impossible to follow them into the egg cell, or to determine whether several or only one enter. The fertilized egg cell becomes almost at once surrounded by a wall, which rapidly thickens, and forms a resting spore. As the spore ripens, it loses its green color, becoming colorless, with a few reddish brown specks scattered through it (_F_).

In some species the s.e.xual organs are borne directly on the filament (Fig. 21, _G_).

Large zoospores are formed in some of the green felts (Fig. 22, _A_), and are produced singly in the ends of branches that become swollen, dark green, and filled with very dense protoplasm. This end becomes separated by a wall from the rest of the branch, the end opens, and the contents escape as a very large zoospore, covered with numerous short cilia (_A_ ii). After a short period of activity, this loses its cilia, develops a wall, and begins to grow (III, IV). Other species (_B_) produce similar spores, which, however, are not motile, and remain within the mother cell until they are set free by the decay of its wall.

ORDER V.--_Characeae_.

The _Characeae_, or stone-worts, as some of them are called, are so very different from the other green algae that it is highly probable that they should be separated from them.

The type of the order is the genus _Chara_ (Fig. 23), called stone-worts from the coating of carbonate of lime found in most of them, giving them a harsh, stony texture. Several species are common growing upon the bottom of ponds and slow streams, and range in size from a few centimetres to a metre or more in height.

The plant (Fig. 23, _A_) consists of a central jointed axis with circles of leaves at each joint or node. The distance between the nodes (internodes) may in the larger species reach a length of several centimetres. The leaves are slender, cylindrical structures, and like the stem divided into nodes and internodes, and have at the nodes delicate leaflets.

At each joint of the leaf, in fruiting specimens, attached to the inner side, are borne two small, roundish bodies, in the commoner species of a reddish color (Fig. 23, _A_, _r_). The lower of the two is globular, and bright scarlet in color; the other, more oval and duller.

Examined with a lens the main axis presents a striated appearance. The whole plant is harsh to the touch and brittle, owing to the limy coating. It is fastened to the ground by fine, colorless hairs, or rootlets.

[Ill.u.s.tration: FIG. 23.--_A_, plant of a stone-wort (_Chara_), one-half natural size. _r_, reproductive organs. _B_, longitudinal section through the apex. _S_, apical cell. _x_, nodes. _y_, internodes. _C_, a young leaf. _D_, cross section of an internode.

_E_, of a node of a somewhat older leaf. _F_, _G_, young s.e.xual organs seen in optical section. _o_, oogonium. _An._ antheridium. _H_, superficial view. _G_, _I_, group of filaments containing spermatozoids. _J_, a small portion of one of these more magnified, showing a spermatozoid in each cell. _K_, free spermatozoids. _L_, a piece of a leaf with ripe oogonium (_o_), and antheridium (_An._).

_B_, _H_, 150. _J_, _K_, 300. _I_, 50. _L_, 25.]

By making a series of longitudinal sections with a sharp razor through the top of the plant, and magnifying sufficiently, it is found to end in a single, nearly hemispherical cell (Fig. 23, _B_, _S_). This from its position is called the "apical cell," and from it are derived all the tissues of the plant. Segments are cut off from its base, and these divide again into two by a wall parallel to the first. Of the two cells thus formed one undergoes no further division and forms the central cell of an internode (_y_); the other divides repeatedly, forming a node or joint (_x_).

As the arrangement of these cells is essentially the same in the leaves and stem, we will examine it in the former, as by cutting several cross-sections of the whole bunch of young leaves near the top of the plant, we shall pretty certainly get some sections through a joint. The arrangement is shown in Figure 23, _E_.

As the stem grows, a covering is formed over the large internodal cell (_y_) by the growth of cells from the nodes. These grow both from above and below, meeting in the middle of the internode and completely hiding the long axial cell. A section across the internode shows the large axial cell (_y_) surrounded by the regularly arranged cells of the covering or cortex (Fig. 23, _D_).

All the cells contain a layer of protoplasm next the wall with numerous oval chloroplasts. If the cells are uninjured, they often show a very marked movement of the protoplasm. These movements are best seen, however, in forms like _Nitella_, where the long internodal cells are not covered with a cortex. In _Chara_ they are most evident in the root hairs that fasten the plant to the ground.

The growth of the leaves is almost identical with that of the stem, but the apical growth is limited, and the apical cell becomes finally very long and pointed (Fig. 23, _C_). In some species the chloroplasts are reddish in the young cells, a.s.suming their green color as the cells approach maturity.

The plant multiplies non-s.e.xually by means of special branches that may become detached, but there are no non-s.e.xual spores formed.

The s.e.xual organs have already been noticed arising in pairs at the joints of the leaves. The oogonium is formed above, the antheridium below.

The young oogonium (_F_, _O_) consists of a central cell, below which is a smaller one surrounded by a circle of five others, which do not at first project above the central cell, but later completely envelop it (_G_). Each of these five cells early becomes divided into an upper and a lower one, the latter becoming twisted as it elongates, and the central cell later has a small cell cut off from its base by an oblique wall. The central cell forms the egg cell, which in the ripe oogonium (_L_, _O_) is surrounded by five, spirally twisted cells, and crowned by a circle of five smaller ones, which become of a yellowish color when full grown. They separate at the time of fertilization to allow the spermatozoids to enter the oogonium.

The antheridium consists at first of a basal cell and a terminal one. The latter, which is nearly globular, divides into eight nearly similar cells by walls pa.s.sing through the centre. In each of these eight cells two walls are next formed parallel to the outer surface, so that the antheridium (apart from the basal cell) contains twenty-four cells arranged in three concentric series (_G_, _an._).

These cells, especially the outer ones, develop a great amount of a red pigment, giving the antheridium its characteristic color.

The diameter of the antheridium now increases rapidly, and the central cells separate, leaving a large s.p.a.ce within. Of the inner cells, the second series, while not increasing in diameter, elongate, a.s.suming an oblong form, and from the innermost are developed long filaments (_I_, _J_) composed of a single row of cells, in each of which is formed a spermatozoid.

The eight outer cells are nearly triangular in outline, fitting together by deeply indented margins, and having the oblong cells with the attached filaments upon their inner faces.

If a ripe antheridium is crushed in a drop of water, after lying a few minutes the spermatozoids will escape through small openings in the side of the cells. They are much larger than any we have met with. Each is a colorless, spiral thread with about three coils, one end being somewhat dilated with a few granules; the other more pointed, and bearing two extremely long and delicate cilia (_K_). To see the cilia it is necessary to kill the spermatozoids with iodine or some other reagent.

After fertilization the outer cells of the oogonium become very hard, and the whole falls off, germinating after a sufficient period of rest.

According to the accounts of Pringsheim and others, the young plant consists at first of a row of elongated cells, upon which a bud is formed that develops into the perfect plant.

There are two families of the _Characeae_, the _Chareae_, of which _Chara_ is the type, and the _Nitelleae_, represented by various species of _Nitella_ and _Tolypella_. The second family have the internodes without any cortex--that is, consisting of a single long cell; and the crown at the top of the oogonium is composed of ten cells instead of five. They are also dest.i.tute of the limy coating of the _Chareae_.

Both as regards the structure of the plant itself, as well as the reproductive organs, especially the very complex antheridium, the _Characeae_ are very widely separated from any other group of plants, either above or below them.