Elements of Structural and Systematic Botany Part 3

Closely resembling _Protococcus_, and answering quite as well for study, are numerous aquatic forms, such as _Chlorococc.u.m_ (Fig. 12).

These are for the most part dest.i.tute of a firm cell wall, but are imbedded in ma.s.ses of gelatinous substance like many _Cyanophyceae_.

The chloroplasts are smaller and less distinct than in _Protococcus_. The cells are here oval rather than round, and often show a clear s.p.a.ce at one end.

[Ill.u.s.tration: FIG. 12.--_Chlorococc.u.m_, a plant related to _Protococcus_, but the naked cells are surrounded by a colorless gelatinous envelope. _A_, motionless cells. _B_, a cell that has escaped from its envelope and is ciliated, 300.]

Owing to the absence of a definite membrane, a distinction between fission and internal cell division can scarcely be made here. Often the cells escape from the gelatinous envelope, and swim actively by means of two cilia at the colorless end (Fig. 12, _B_). In this stage they closely resemble the individuals of a _Volvox_ colony, or other green _Flagellata_, to which there is little doubt that they are related.

There are a number of curious forms common in fresh water that are probably related to _Protococcus_, but differ in having the cells united in colonies of definite form. Among the most striking are the different species of _Pediastrum_ (Fig. 11, _D_, _E_), often met with in company with other algae, and growing readily in aquaria when once established. They are of very elegant shapes, and the number of cells some multiple of four, usually sixteen.

The cells form a flat disc, the outer ones being generally provided with a pair of spines.

New individuals arise by internal division of the cells, the contents of each forming as many parts as there are cells in the whole colony. The young cells now escape through a cleft in the wall of the mother cell, but are still surrounded by a delicate membrane (Fig. 11, _E_). Within this membrane the young cells arrange themselves in the form of the original colony, and grow together, forming a new colony.

A much larger but rarer form is the water net (Fig. 11, _G_), in which the colony has the form of a hollow net, the s.p.a.ces being surrounded by long cylindrical cells placed end to end. Other common forms belong to the genus _Scenedesmus_ (Fig. 11, _F_), of which there are many species.

ORDER II.--_Confervaceae_.

Under this head are included a number of forms of which the simplest ones approach closely, especially in their younger stages, the _Protococcaceae_. Indeed, some of the so-called _Protococcaceae_ are known to be only the early stages of these plants.

A common member of this order is _Cladophora_, a coa.r.s.e-branching alga, growing commonly in running water, where it forms tufts, sometimes a metre or more in length. By floating out a little of it in a saucer, it is easy to see that it is made up of branching filaments.

The microscope shows (Fig. 13, _A_) that these filaments are rows of cylindrical cells with thick walls showing evident stratification.

At intervals branches are given off, which may in turn branch, giving rise to a complicated branching system. These branches begin as little protuberances of the cell wall at the top of the cell.

They increase rapidly in length, and becoming slightly contracted at the base, a wall is formed across at this point, shutting it off from the mother cell.

The protoplasm lines the wall of the cell, and extends in the form of thin plates across the cavity of the cell, dividing it up into a number of irregular chambers. Imbedded in the protoplasm are numerous flattened chloroplasts, which are so close together as to make the protoplasm appear almost uniformly green. Within the chloroplasts are globular, glistening bodies, called "pyrenoids."

The cell has several nuclei, but they are scarcely evident in the living cell. By placing the cells for a few hours in a one per cent watery solution of chromic acid, then washing thoroughly and staining with borax carmine, the nuclei will be made very evident (Fig. 13, _B_). Such preparations may be kept permanently in dilute glycerine.

[Ill.u.s.tration: FIG. 13.--_Cladophora._ _A_, a fragment of a plant, 50. _B_, a single cell treated with chromic acid, and stained with alum cochineal. _n_, nucleus. _py._ pyrenoid, 150. _C_, three stages in the division of a cell. i, 1.45 p.m.; ii, 2.55 p.m.; iii, 4.15 p.m., 150. _D_, a zoospore 350.]

If a ma.s.s of actively growing filaments is examined, some of the cells will probably be found in process of fission. The process is very simple, and may be easily followed (Fig. 13, _C_). A ridge of cellulose is formed around the cell wall, projecting inward, and pushing in the protoplasm as it grows. The process is continued until the ring closes in the middle, cutting the protoplasmic body completely in two, and forms a firm membrane across the middle of the cell. The protoplasm at this stage (_C_ iii.) is somewhat contracted, but soon becomes closely applied to the new wall. The whole process lasts, at ordinary temperatures (20-25 C.), from three to four hours.

At certain times, but unfortunately not often to be met with, the contents of some of the cells form, by internal division, a large number of small, naked cells (zoospores) (Fig. 13, _D_), which escape and swim about actively for a time, and afterwards become invested with a cell wall, and grow into a new filament. These cells are called zoospores, from their animal-like movements. They are provided with two cilia, closely resembling the motile cells of the _Protococcaceae_ and _Volvocineae_.

There are very many examples of these simple _Confervaceae_, some like _Conferva_ being simple rows of cells, others like _Stigeoclonium_ (Fig. 14, _A_), _Chaetophora_ and _Draparnaldia_ (Fig. 14, _B_, _C_), very much branched. The two latter forms are surrounded by ma.s.ses of transparent jelly, which sometimes reach a length of several centimetres.

[Ill.u.s.tration: FIG. 14.--_Confervaceae_. _A_, _Stigeoclonium_. _B_, _Draparnaldia_, 50. _C_, a piece of _Draparnaldia_, 2. _D_, part of a filament of _Conferva_, 300.]

Among the marine forms related to these may be mentioned the sea lettuce (_Ulva_), shown in Figure 15. The thin, bright-green, leaf-like fronds of this plant are familiar to every seaside student.

[Ill.u.s.tration: FIG. 15.--A plant of sea lettuce (_Ulva_). One-half natural size.]

Somewhat higher than _Cladophora_ and its allies, especially in the differentiation of the reproductive parts, are the various species of _dogonium_ and its relatives. There are numerous species of _dogonium_ not uncommon in stagnant water growing in company with other algae, but seldom forming ma.s.ses by themselves of sufficient size to be recognizable to the naked eye.

The plant is in structure much like _Cladophora_, except that it is unbranched, and the cells have but a single nucleus (Fig. 16, _E_).

Even when not fruiting the filaments may usually be recognized by peculiar cap-shaped structures at the top of some of the cells.

These arise as the result of certain peculiarities in the process of cell division, which are too complicated to be explained here.

There are two forms of reproduction, non-s.e.xual and s.e.xual. In the first the contents of certain cells escape in the form of large zoospores (Fig. 16, _C_), of oval form, having the smaller end colorless and surrounded by a crown of cilia. After a short period of active motion, the zoospore comes to rest, secretes a cell wall about itself, and the transparent end becomes flattened out into a disc (_E_, _d_), by which it fastens itself to some object in the water. The upper part now rapidly elongates, and dividing repeatedly by cross walls, develops into a filament like the original one. In many species special zoospores are formed, smaller than the ordinary ones, that attach themselves to the filaments bearing the female reproductive organ (oogonium), and grow into small plants bearing the male organ (antheridium), (Fig. 16, _B_).

[Ill.u.s.tration: FIG. 16.--_A_, portion of a filament of _dogonium_, with two oogonia (_og._). The lower one shows the opening. _B_, a similar filament, to which is attached a small male plant with an antheridium (_an._). _C_, a zoospore of _dogonium_. _D_, a similar spore germinating. _E_, base of a filament showing the disc (_d_) by which it is attached. _F_, another species of _dogonium_ with a ripe spore (_sp._). _G_, part of a plant of _Bulbochaete_. _C_, _D_, 300; the others 150.]

The s.e.xual reproduction takes place as follows: Certain cells of a filament become distinguished by their denser contents and by an increase in size, becoming oval or nearly globular in form (Fig. 16, _A_, _B_). When fully grown, the contents contract and form a naked cell, which sometimes shows a clear area at one point on the surface. This globular ma.s.s of protoplasm is the egg cell, or female cell, and the cell containing it is called the "oogonium." When the egg cell is ripe, the oogonium opens by means of a little pore at one side (Fig. 16, _A_).

In other cells, either of the same filament or else of the small male plants already mentioned, small motile cells, called spermatozoids, are formed. These are much smaller than the egg cell, and resemble the zoospores in form, but are much smaller, and without chlorophyll. When ripe they are discharged from the cells in which they were formed, and enter the oogonium. By careful observation the student may possibly be able to follow the spermatozoid into the oogonium, where it enters the egg cell at the clear spot on its surface. As a result of the entrance of the spermatozoid (fertilization), the egg cell becomes surrounded by a thick brown wall, and becomes a resting spore. The spore loses its green color, and the wall becomes dark colored and differentiated into several layers, the outer one often provided with spines (Fig. 16, _F_). As these spores do not germinate for a long time, the process is only known in a comparatively small number of species, and can hardly be followed by the ordinary student.

[Ill.u.s.tration: FIG. 17.--_A_, plant of _Coleochaete_, 50. _B_, a few cells from the margin, with one of the hairs.]

Much like _dogonium_, but differing in being branched, is the genus _Bulbochaete_, characterized also by hairs swollen at the base, and prolonged into a delicate filament (Fig. 16, _G_).

The highest members of the _Confervaceae_ are those of the genus _Coleochaete_ (Fig. 17), of which there are several species found in the United States. These show some striking resemblances to the red seaweeds, and possibly form a transition from the green algae to the red. The commonest species form bright-green discs, adhering firmly to the stems and floating leaves of water lilies and other aquatics.

In aquaria they sometimes attach themselves in large numbers to the gla.s.s sides of the vessel.

Growing from the upper surface are numerous hairs, consisting of a short, sheath-like base, including a very long and delicate filament (Fig. 17, _B_). In their methods of reproduction they resemble _dogonium_, but the reproductive organs are more specialized.

CHAPTER V.

GREEN ALGae--_Continued_.

ORDER III.--POND Sc.u.mS (_Conjugatae_).

The _Conjugatae_, while in some respects approaching the _Confervaceae_ in structure, yet differ from them to such an extent in some respects that their close relationship is doubtful. They are very common and familiar plants, some of them forming great floating ma.s.ses upon the surface of every stagnant pond and ditch, being commonly known as "pond sc.u.m." The commonest of these pond sc.u.ms belong to the genus _Spirogyra_, and one of these will ill.u.s.trate the characteristics of the order. When in active growth these ma.s.ses are of a vivid green, and owing to the presence of a gelatinous coating feel slimy, slipping through the hands when one attempts to lift them from the water.

Spread out in water, the ma.s.ses are seen to be composed of slender threads, often many centimetres in length, and showing no sign of branching.

[Ill.u.s.tration: FIG. 18.--_A_, a filament of a common pond sc.u.m (_Spirogyra_) separating into two parts. _B_, a cell undergoing division. The cell is seen in optical section, and the chlorophyll bands are omitted, _n_, _n'_, the two nuclei. _C_, a complete cell.

_n_, nucleus. _py._ pyrenoid. _D_, _E_, successive stages in the process of conjugation. _G_, a ripe spore. _H_, a form in which conjugation takes place between the cells of the same filament. All 150.]

For microscopical examination the larger species are preferable.

When one of these is magnified (Fig. 18, _A_, _C_), the unbranched filament is shown to be made up of perfectly cylindrical cells, with rather delicate walls. The protoplasm is confined to a thin layer lining the walls, except for numerous fine filaments that radiate from the centrally placed nucleus (_n_), which thus appears suspended in the middle of the cell. The nucleus is large and distinct in the larger species, and has a noticeably large and conspicuous nucleolus. The most noticeable thing about the cell is the green spiral bands running around it. These are the chloroplasts, which in all the _Conjugatae_ are of very peculiar forms. The number of these bands varies much in different species of _Spirogyra_, but is commonly two or three. These chloroplasts, like those of other plants, are not noticeably different in structure from the ordinary protoplasm, as is shown by extracting the chlorophyll, which may be done by placing the plants in alcohol for a short time. This extracts the chlorophyll, but a microscopic examination of the decolored cells shows that the bands remain unchanged, except for the absence of color. These bands are flattened, with irregularly scalloped margins, and at intervals have rounded bodies (pyrenoids) imbedded in them (Fig. 18, _C_, _py._).

The pyrenoids, especially when the plant has been exposed to the light for some time, are surrounded by a circle of small granules, which become bluish when iodine is applied, showing them to be starch. (To show the effect of iodine on starch on a large scale, mix a little flour, which is nearly all starch, with water, and add a little iodine. The starch will immediately become colored blue, varying in intensity with the amount of iodine.) The cells divide much as in _Cladophora_, but the nucleus here takes part in the process. The division naturally occurs only at night, but by reducing the temperature at night to near the freezing point (4 C., or a little lower), the process may be checked. The experiment is most conveniently made when the temperature out of doors approaches the freezing point. Then it is only necessary to keep the plants in a warm room until about 10 P.M., when they may be put out of doors for the night. On bringing them in in the morning, the division will begin almost at once, and may be easily studied. The nucleus divides into two parts, which remain for a time connected by delicate threads (Fig. 18, _B_), that finally disappear. At first no nucleoli are present in the daughter nuclei, but they appear before the division is complete.

New filaments are formed by the breaking up of the old ones, this sometimes being very rapid. As the cells break apart, the free ends bulge strongly, showing the pressure exerted upon the cell wall by the contents (Fig. 18, _A_).

Spores like those of _dogonium_ are formed, but the process is somewhat different. It occurs in most species late in the spring, but may sometimes be met with at other times. The ma.s.ses of fruiting plants usually appear brownish colored. If spores have been formed they can, in the larger species at least, be seen with a hand lens, appearing as rows of dark-colored specks.

Two filaments lying side by side send out protuberances of the cell wall that grow toward each other until they touch (Fig. 18, _D_). At the point of contact, the wall is absorbed, forming a continuous channel from one cell to the other. This process usually takes place in all the cells of the two filaments, so that the two filaments, connected by tubes at regular intervals, have the form of a ladder.

In some species adjoining cells of the same filament become connected, the tubes being formed at the end of the cells (Fig. 18, _H_), and the cell in which the spore is formed enlarges.

Soon after the channel is completed, the contents of one cell flow slowly through it into the neighboring cell, and the protoplasm of the two fuses into one ma.s.s. (The union of the nuclei has also been observed.) The young spore thus formed contracts somewhat, becoming oval in form, and soon secretes a thick wall, colorless at first, but afterwards becoming brown and more or less opaque. The chlorophyll bands, although much crowded, are at first distinguishable, but later lose the chlorophyll, and become unrecognizable. Like the resting spores of _dogonium_ these require a long period of rest before germinating.

[Ill.u.s.tration: FIG. 19.--Forms of _Zygnemaceae_. _A_, _Zygnema_. _B_, _C_, _D_, _Mesocarpus_. All 150.]