Elements of Structural and Systematic Botany Part 5

CHAPTER VI.

THE BROWN ALGae (_Phaeophyceae_).

[Ill.u.s.tration: FIG. 24.--Forms of diatoms. _A_, _Pinnularia_. i, seen from above; ii, from the side. _B_, _Fragillaria_ (?). _C_, _Navicula_. _D_, _F_, _Eunotia_. _E_, _Gomphonema_. _G_, _Cocconeis_.

_H_, _Diatoma_. All 300.]

These plants are all characterized by the presence of a brown pigment, in addition to the chlorophyll, which almost entirely conceals the latter, giving the plants a brownish color, ranging from a light yellowish brown to nearly black. One order of plants that possibly belongs here (_Diatomaceae_) are single celled, but the others are for the most part large seaweeds. The diatoms, which are placed in this cla.s.s simply on account of the color, are probably not closely related to the other brown algae, but just where they should be placed is difficult to say. In some respects they approach quite closely the desmids, and are not infrequently regarded as related to them. They are among the commonest of organisms occurring everywhere in stagnant and running water, both fresh and salt, forming usually, slimy, yellowish coatings on stones, mud, aquatic plants, etc. Like the desmids they may be single or united into filaments, and not infrequently are attached by means of a delicate gelatinous stalk (Fig. 25).

[Ill.u.s.tration: FIG. 25.--Diatoms attached by a gelatinous stalk.

150]

They are at once distinguished from the desmids by their color, which is always some shade of yellowish or reddish brown. The commonest forms, _e.g._ _Navicula_ (Fig. 24, _C_), are boat-shaped when seen from above, but there is great variety in this respect.

The cell wall is always impregnated with large amounts of flint, so that after the cell dies its shape is perfectly preserved, the flint making a perfect cast of it, looking like gla.s.s. These flinty sh.e.l.ls exhibit wonderfully beautiful and delicate markings which are sometimes so fine as to test the best lenses to make them out.

This sh.e.l.l is composed of two parts, one shutting over the other like a pill box and its cover. This arrangement is best seen in such large forms as _Pinnularia_ (Fig. 24, _A_ ii).

Most of the diatoms show movements, swimming slowly or gliding over solid substances; but like the movements of _Oscillaria_ and the desmids, the movements are not satisfactorily understood, although several explanations have been offered.

They resemble somewhat the desmids in their reproduction.

THE TRUE BROWN ALGae.

These are all marine forms, many of great size, reaching a length in some cases of a hundred metres or more, and showing a good deal of differentiation in their tissues and organs.

[Ill.u.s.tration: FIG. 26.--_A_, a branch of common rock weed (_Fucus_), one-half natural size. _x_, end of a branch bearing conceptacles. _B_, section through a conceptacle containing oogonia (_og._), 25. _C_, _E_, successive stages in the development of the oogonium, 150. _F_, _G_, antheridia. In _G_, one of the antheridia has discharged the ma.s.s of spermatozoids (_an._), 150.]

One of the commonest forms is the ordinary rock weed (_Fucus_), which covers the rocks of our northeastern coast with a heavy drapery for several feet above low-water mark, so that the plants are completely exposed as the tide recedes. The commonest species, _F. vesiculosus_ (Fig. 26, _A_), is distinguished by the air sacs with which the stems are provided. The plant is attached to the rock by means of a sort of disc or root from which springs a stem of tough, leathery texture, and forking regularly at intervals, so that the ultimate branches are very numerous, and the plant may reach a length of a metre or more. The branches are flattened and leaf-like, the centre traversed by a thickened midrib. The end of the growing branches is occupied by a transversely elongated pit or depression. The growing point is at the bottom of this pit, and by a regular forking of the growing point the symmetrical branching of the plant is brought about. Scattered over the surface are little circular pits through whose openings protrude bunches of fine hairs. When wet the plant is flexible and leathery, but it may become quite dry and hard without suffering, as may be seen when the plants are exposed to the sun at low tide.

The air bladders are placed in pairs, for the most part, and buoy up the plant, bringing it up to the surface when covered with water.

The interior of the plant is very soft and gelatinous, while the outer part forms a sort of tough rind of much firmer consistence. The ends of some of the branches (Fig. 26, _A_, _x_) are usually much swollen, and the surface covered with little elevations from which may often be seen protruding cl.u.s.ters of hairs like those arising from the other parts of the plant. A section through one of these enlarged ends shows that each elevation corresponds to a cavity situated below it. On some of the plants these cavities are filled with an orange-yellow ma.s.s; in others there are a number of roundish olive-brown bodies large enough to be easily seen. The yellow ma.s.ses are ma.s.ses of antheridia; the round bodies, the oogonia.

If the plants are gathered while wet, and packed so as to prevent evaporation of the water, they will keep perfectly for several days, and may readily be s.h.i.+pped for long distances. If they are to be studied away from the seash.o.r.e, sections for microscopic examination should be mounted in salt water (about 3 parts in weight of common salt to 100 of water). If fresh material is not to be had, dried specimens or alcoholic material will answer pretty well.

To study the minute structure of the plant, make a thin cross-section, and mount in salt water. The inner part or pith is composed of loosely arranged, elongated cells, placed end to end, and forming an irregular network, the large s.p.a.ces between filled with the mucilaginous substance derived from the altered outer walls of these cells. This mucilage is hard when dry, but swells up enormously in water, especially fresh water. The cells grow smaller and more compact toward the outside of the section, until there are no s.p.a.ces of any size between those of the outside or rind. The cells contain small chloroplasts like those of the higher plants, but owing to the presence of the brown pigment found in all of the cla.s.s, in addition to the chlorophyll, they appear golden brown instead of green.

No non-s.e.xual reproductive bodies are known in the rock weeds, beyond small branches that occur in cl.u.s.ters on the margins of the main branches, and probably become detached, forming new plants. In some of the lower forms, however, _e.g._ _Ectocarpus_ and _Laminaria_ (Fig. 28, _A_, _C_), zoospores are formed.

The s.e.xual organs of the rock weed, as we have already seen, are borne in special cavities (conceptacles) in the enlarged ends of some of the branches. In the species here figured, _F. vesiculosus_, the antheridia and oogonia are borne on separate plants; but in others, _e.g._ _F. platycarpus_, they are both in the same conceptacle.

The walls of the conceptacle (Fig. 26, _B_) are composed of closely interwoven filaments, from which grow inward numerous hairs, filling up the s.p.a.ce within, and often extending out through the opening at the top.

The reproductive bodies arise from the base of these hairs. The oogonia (Fig. 26, _C_, _E_) arise as nearly colorless cells, that early become divided into two cells, a short basal cell or stalk and a larger terminal one, the oogonium proper. The latter enlarges rapidly, and its contents divide into eight parts. The division is at first indicated by a division of the central portion, which includes the nucleus, and is colored brown, into two, four, and finally eight parts, after which walls are formed between these. The brown color spreads until the whole oogonium is of a nearly uniform olive-brown tint.

When ripe, the upper part of the oogonium dissolves, allowing the eight cells, still enclosed in a delicate membrane, to escape (Fig. 27, _H_). Finally, the walls separating the inner cells of the oogonium become also absorbed, as well as the surrounding membrane, and the eight egg cells escape into the water (Fig. 27, _I_) as naked b.a.l.l.s of protoplasm, in which a central nucleus may be dimly seen.

The antheridia (Fig. 26, _F_, _G_) are small oblong cells, at first colorless, but when ripe containing numerous glistening, reddish brown dots, each of which is part of a spermatozoid. When ripe, the contents of the antheridium are forced out into the water (_G_), leaving the empty outer wall behind, but still surrounded by a thin membrane. After a few minutes this membrane is dissolved, and the spermatozoids are set free. These (Fig. 27, _K_) are oval in form, with two long cilia attached to the side where the brown speck, seen while still within the antheridium, is conspicuous.

The act of fertilization may be easily observed by laying fresh antheridia into a drop of water containing recently discharged egg cells. To obtain these, all that is necessary is to allow freshly gathered plants to remain in the air until they are somewhat dry, when the ripe s.e.xual cells will be discharged from the openings of the conceptacles, exuding as little drops, those with antheridia being orange-yellow; the ma.s.ses of oogonia, olive. Within a few minutes after putting the oogonia into water, the egg cells may be seen to escape into the water, when some of the antheridia may be added. The spermatozoids will be quickly discharged, and collect immediately in great numbers about the egg cells, to which they apply themselves closely, often setting them in rotation by the movements of their cilia, and presenting a most extraordinary spectacle (_J_). Owing to the small size of the spermatozoids, and the opacity of the eggs, it is impossible to see whether more than one spermatozoid penetrates it; but from what is known in other cases it is not likely. The egg now secretes a wall about itself, and within a short time begins to grow. It becomes pear-shaped, the narrow portion becoming attached to the parent plant or to some other object by means of rootlets, and the upper part grows into the body of the young plant (Fig. 27, _M_).

[Ill.u.s.tration: FIG. 27.--_H_, the eight egg cells still surrounded by the inner membrane of the oogonium. _I_, the egg cells escaping into the water. _J_, a single egg cell surrounded by spermatozoids. _K_, ma.s.s of spermatozoids surrounded by the inner membrane of the antheridium. _L_, spermatozoids. _M_, young plant. _r_, the roots.

_K_, 300; _L_, 600; the others, 150.]

The simpler brown seaweeds, so far as known, multiply only by means of zoospores, which may grow directly into new plants, or, as has been observed in some species, two zoospores will first unite. A few, like _Ectocarpus_ (Fig. 28, _A_), are simple, branched filaments, but most are large plants with complex tissues. Of the latter, a familiar example is the common kelp, "devil's ap.r.o.n" (_Laminaria_), often three to four metres in length, with a stout stalk, provided with root-like organs, by which it is firmly fastened. Above, it expands into a broad, leaf-like frond, which in some species is divided into strips.

Related to the kelps is the giant kelp of the Pacific (_Macrocystis_), which is said sometimes to reach a length of three hundred metres.

[Ill.u.s.tration: FIG. 28.--Forms of brown seaweeds. _A_, _Ectocarpus_, 50. Sporangia (_sp._). _B_, a single sporangium, 150. _C_, kelp (_Laminaria_), ?. _D_, _E_, gulf weed (_Sarga.s.sum_). _D_, one-half natural size. _E_, natural size. _v_, air bladders. _x_, conceptacle bearing branches.]

The highest of the cla.s.s are the gulf weeds (_Sarga.s.sum_), plants of the warmer seas, but one species of which is found from Cape Cod southward (Fig. 28, _D_, _E_). These plants possess distinct stems and leaves, and there are stalked air bladders, looking like berries, giving the plant a striking resemblance to the higher land plants.

CHAPTER VII.

CLa.s.s III.--THE RED ALGae (_Rhodophyceae_).

These are among the most beautiful and interesting members of the plant kingdom, both on account of their beautiful colors and the exquisitely graceful forms exhibited by many of them. Unfortunately for inland students they are, with few exceptions, confined to salt water, and consequently fresh material is not available. Nevertheless, enough can be done with dried material to get a good idea of their general appearance, and the fruiting plants can be readily preserved in strong alcohol. Specimens, simply dried, may be kept for an indefinite period, and on being placed in water will a.s.sume perfectly the appearance of the living plants. Prolonged exposure, however, to the action of fresh water extracts the red pigment that gives them their characteristic color. This pigment is found in the chlorophyll bodies, and usually quite conceals the chlorophyll, which, however, becomes evident so soon as the red pigment is removed.

The red seaweeds differ much in the complexity of the plant body, but all agree in the presence of the red pigment, and, at least in the main, in their reproduction. The simpler ones consist of rows of cells, usually branching like _Cladophora_; others form cell plates comparable to _Ulva_ (Fig. 30, _C_, _D_); while others, among which is the well-known Irish moss (_Chondrus_), form plants of considerable size, with pretty well differentiated tissues. In such forms the outer cells are smaller and firmer, const.i.tuting a sort of rind; while the inner portions are made up of larger and looser cells, and may be called the pith. Between these extremes are all intermediate forms.

They usually grow attached to rocks, sh.e.l.ls, wood, or other plants, such as the kelps and even the larger red seaweeds. They are most abundant in the warmer seas, but still a considerable number may be found in all parts of the ocean, even extending into the Arctic regions.

[Ill.u.s.tration: FIG. 29.--_A_, a red seaweed (_Callithamnion_), of the natural size. _B_, a piece of the same, 50. _t_, tetraspores. _C_ i-v, successive stages in the development of the tetraspores, 150.

_D_ I, II young procarps. _tr._ trichogyne. iii, young; iv, ripe spore fruit. I, III, 150. iv, 50. _E_, an antheridium, 150. _F_, spore fruit of _Polysiphonia_. The spores are here surrounded by a case, 50.]

The methods of reproduction may be best ill.u.s.trated by a specific example, and preferably one of the simpler ones, as these are most readily studied microscopically.

The form here ill.u.s.trated (_Callithamnion_) grows attached to wharves, etc., below low-water mark, and is extremely delicate, collapsing completely when removed from the water. The color is a bright rosy red, and with its graceful form and extreme delicacy it makes one of the most beautiful of the group.

If alcoholic material is used, it may be mounted for examination either in water or very dilute glycerine.

The plant is composed of much-branched, slender filaments, closely resembling _Cladophora_ in structure, but with smaller cells (Fig. 29, _B_). The non-s.e.xual reproduction is by means of special spores, which from being formed in groups of four, are known as tetraspores. In the species under consideration the mother cell of the tetraspores arises as a small bud near the upper end of one of the ordinary cells (Fig. 29, _C_ i). This bud rapidly increases in size, a.s.suming an oval form, and becoming cut off from the cell of the stem (Fig. 29, _C_ ii). The contents now divide into four equal parts, arranged like the quadrants of a sphere. When ripe, the wall of the mother cell gives way, and the four spores escape into the water and give rise to new plants. These spores, it will be noticed, differ in one important particular from corresponding spores in most algae, in being unprovided with cilia, and incapable of spontaneous movement.

Occasionally in the same plant that bears tetraspores, but more commonly in special ones, there are produced the s.e.xual organs, and subsequently the sporocarps, or fruits, developed from them. The plants that bear them are usually stouter that the non-s.e.xual ones, and the ma.s.ses of ripe carpospores are large enough to be readily seen with the naked eye.

If a plant bearing ripe spores is selected, the young stages of the female organ (procarp) may generally be found by examining the younger parts of the plant. The procarp arises from a single cell of the filament. This cell undergoes division by a series of longitudinal walls into a central cell and about four peripheral ones (Fig. 29, _D_ i). One of the latter divides next into an upper and a lower cell, the former growing out into a long, colorless appendage known as a trichogyne (Fig. 29, _D_, _tr._).

The antheridia (Fig. 29, _E_) are hemispherical ma.s.ses of closely set colorless cells, each of which develops a single spermatozoid which, like the tetraspores, is dest.i.tute of cilia, and is dependent upon the movement of the water to convey it to the neighborhood of the procarp. Occasionally one of these spermatozoids may be found attached to the trichogyne, and in this way fertilization is effected. Curiously enough, neither the cell which is immediately fertilized, nor the one beneath it, undergo any further change; but two of the other peripheral cells on opposite sides of the filament grow rapidly and develop into large, irregular ma.s.ses of spores (Fig. 29, _D_ III, IV).

While the plant here described may be taken as a type of the group, it must be borne in mind that many of them differ widely, not only in the structure of the plant body, but in the complexity of the s.e.xual organs and spores as well. The tetraspores are often imbedded in the tissues of the plant, or may be in special receptacles, nor are they always arranged in the same way as here described, and the same is true of the carpospores. These latter are in some of the higher forms, _e.g._ _Polysiphonia_ (Fig. 29, _F_), contained in urn-shaped receptacles, or they may be buried within the tissues of the plant.

[Ill.u.s.tration: FIG. 30.--Marine red seaweeds. _A_, _Dasya_. _B_, _Rhodymenia_ (with smaller algae attached). _C_, _Grinnellia_. _D_, _Delesseria_. _A_, _B_, natural size; the others reduced one-half.]

The fresh-water forms are not common, but may occasionally be met with in mill streams and other running water, attached to stones and woodwork, but are much inferior in size and beauty to the marine species. The red color is not so p.r.o.nounced, and they are, as a rule, somewhat dull colored.