Algae - Structure and Reproduction

ALGAE - STRUCTURE AND REPRODUCTION The algae exhibit a diversity of form not found in any other group of plants, ranging from one-celled organisms through mani fold colonial types to simple or branched rows of cells (filaments) and onwards to elaborate massive growths attaining dimensions and complexities of structure that sometimes vie with those of flowering plants. The group is of great interest to the evolutionist, because it affords all possible stages in the evolution of a plant body.

Unicellular and Colonial Forms.

Simplestare the uni cellular individuals which are commonly spherical or oblong and usually enveloped by a firm cell-wall. The protoplasm of such cells resembles that of other plants in possessing a nucleus and one or more chromatophores containing chlorophyll and other pigments. Many unicellular algae (e.g., Chlamydomonas, fig. I, A) move freely with the help of delicate prolongations of the protoplasm, the cilia, which arise usually at the front end and push the cell through the water by a movement somewhat like that of the arms of a human being whilst swimming. Motile individuals may be aggregated into colonies of diverse shape (Pandorina, fig. I, B; Volvox, fig. 2, A), moving by the united action of all the cilia. The cells of these algae are often provided with a red streak or dot, the eye-spot, and two or more pulsating vacuoles near the front end, structures which they have in corn mon with many unicellular animals.

Multiplication is effected by successive divisions of the proto plasm into 2, 4, 8, etc., parts, which secrete new membranes and gradually acquire all the parent's characteristics; in the colonial types each cell may divide in this way to produce a new colony. Reproduction is often preceded by withdrawal of the cilia and cessation of movement. In numerous unicellular and colonial algae this motionless state is the usual one. Naked motile cells (zoo spores, see below) may be formed as a means of reproduction (Chlorococcum) or the power of movement is altogether sup pressed and the new individuals are motionless from the first (e.g., Chlorella, fig. r, c and the colonial Scenedesmus, fig. I, D). At times the progeny of a motile unicell (e.g., Chlamydomonas) fail to acquire cilia and remain encased within the mucilaginous membrane of the parent ; this may happen to successive genera tions and result in the formation of large jelly-like masses, from which the contained cells may ultimately escape as swarmers. In some algae (Tetraspora, Gloeocapsa, fig. I, E) this palmelloid state is the normal one. There is little doubt that the motionless have in most cases evolved from motile types in one or other of the ways just indicated.

Filamentous Forms.

Manypond-scums and small seaweeds are filamentous, consisting sometimes of a simple row of similar cells (Ulothrix, fig. F; Tribonerna, fig. 2, p), although in most there is repeated branching; all the branches may be alike (Ecto carpus, fig. H) or some may stand out as main axes bearing smaller lateral branches, which often arise in bunches (e.g., Batra chospermum, fig. 4, A). Filamentous algae commonly reproduce by zoospores, i.e., naked ciliated bodies formed (with or without previous division) from the protoplasm of ordinary (fig. I, a) or specially enlarged (fig. 3, A) cells and liberated through a hole in the wall. The close similarity of these zoospores to the motile individuals of the same class warrants the assumption that the filamentous algae have evolved from such motile unicells, the method of their origin being illustrated by the way in which the zoospores proceed to produce new filaments. Their movements may continue for half an hour up to two or three days ; but ulti mately they settle down on some firm substratum, withdraw the cilia, and secrete a membrane. The end in touch with the sub stratum spreads out into a lobed holdfast (fig. I, P), whilst the other rapidly lengthens and divides to form the thread, which in quiet water soon breaks away from its holdfast and floats freely. In rivers or in the sea the attachment is more permanent. In all filamentous forms, however, propagation by detached pieces of the threads (fragmentation) is frequent.

In many filamentous algae the thallus is composed of two branched systems, the one creeping upon some substratum and bearing the other which floats out into the water (e.g., Stigeo clonium, fig. 2, D, and many of the simpler seaweeds). By suppres sion of the projecting system purely prostrate forms result, in some of which the branching is so dense that the threads fit together to form one-layered discs (fig. 2, E). All these and many other smaller algae are commonly found attached to larger ones or to aquatic flowering plants; i.e., they are epiphytes.

Every cell of the simpler filamentous algae can enlarge and divide to form new cells, thus leading to a lengthening of the thread, but in the branched types such growth is often restricted to definite regions (e.g., at the bases of the hair-like tips in many brown seaweeds, fig. I, H) or confined to the end cell of each branch (Cladophora).

Structure of Seaweeds.

Suchapical growth is usual in many filamentous seaweeds, most of which possess a more elaborate structure than the freshwater forms. The segments cut off from the apical cell often divide lengthwise as well as crosswise, so that the filament consists of several longitudinal series of cells which commonly show a definite arrangement in tiers (e.g., Sphacelaria, fig. 1, I). In the older parts of many filamentous seaweeds a further complication of structure results from the outgrowth of cortical threads, either from the basal cells of the branches (Batrachosperrnum, fig. 4, A') or merely from the superficial cells of the thread itself ; these apply themselves closely to its surface and form a dense small-celled investment which may become many layers thick and completely hide the original thread.

The thallus of some seaweeds is built up by the close juxta position of many filaments. In such there is a central cord or axis consisting of numerous longitudinal threads which are often more or less intertwined and which bear large numbers of short branches, projecting approximately at right angles and uniting to form a surface of varying degrees of compactness (fig. 1, j). Many of these forms appear to the naked eye as rather thick little branched threads (Nemalion, Castagnea), but seaweeds of quite a different outward shape may possess the same construction.

Many seaweeds are flattened and leafy, but such forms have doubtless originated from filamentous types in the course of evolution. Some in fact begin life as a simple filament and the derivation from a branched thread may even be decipherable in sections of the mature alga. In Ulva (sea lettuce) and Porphyra the thalli are thin flat sheets composed of uniform cells without localized growth, but in carrageen (fig. 4, F) and the brown Dictyota there are special apical cells at the tips of the numerous flattened branches and these have a firmer texture, being composed of three or more layers of cells, the inner ones usually much larger than the outer (see fig. 3, j).

The most complex structure is found in the large brown sea weeds (Laminariales and Fucales). Here it is usually possible to distinguish stalk and blade (e.g., Laminaria, fig. 1, x) and the latter may show a central thickened region (midrib) as in the Bladderwrack. In Lessonia (fig. 3, G) the stalk may have the thickness of a man's thigh and bears at its apex numerous long blades, the whole resembling a small tree. The bulky thalli, which are often several yards from base to apex, are attached by elab orate holdfasts, fingerlike in Laminariales, suckerlike in Fucales, and often clinging tenaciously to the rocks. In sections of the stalks it is easy to distinguish a central medulla of very irregularly arranged and prevalently elongate cells serving as a strengthening cord and for the conduction of food-substance, a wide cortex of broader cells storing food-reserves, and a superficial assimilatory region of small cells with many chromatophores. The surface cells at times divide abundantly and thus lead to the progressive increase in thickness of the stalk. The most striking of these large forms is Sargassum (fig. 1, L), where the cylindrical stalks bear numerous leafy outgrowths provided with a midrib and bearing in their axils small branch-systems, either terminating in air bladders or containing reproductive organs.

Most algae possess a slimy feeling owing to the mucilaginous quality of the cell-walls. This feature is specially pronounced in the larger seaweeds and particularly in those inhabiting the stretch of shore between tidelevels, where the capacity of mucilage to absorb and retain moisture no doubt constitutes a protection against drying.

Chromatophores.—Thechlorophyll and other pigments that give different algae their distinctive colours are (except Myxophy ceae, see below) contained in special chromatophores, which are of diverse shape and size and in some groups are of great impor tance for the identification of genera and species. They are usually parietal (i.e., located in the protoplasm just beneath the cell-walls, fig. 1, c and F), but in some green and red forms they are axile (i.e., in the centre of the cell, fig. 2, G). In green algae the cells commonly contain only one or a few large chromatophores, but in the other classes they are usually more numerous and simpler in form, often appearing as little lens-shaped bodies (fig. 2, P). Embedded in the substance of the chromatophore are often found conspicuous rounded bodies, the pyrenoids (fig. 1, A and F), which consist of protein. Their exact purpose is not yet clear. In the green algae the pyrenoids are usually surrounded by starch, so that treatment with iodine renders them very conspicuous.

Asexual Reproduction.

Thepurely vegetative multiplica tion of algae by fragmentation of the threads has already been mentioned and fragments of the thallus may even, in the more elaborate forms under suitable conditions, give rise to a new organism. In most algae, however, a more specialized method (asexual reproduction), involving the liberation of spores formed from the entire or subdivided protoplasm of the cells, occurs at certain times. Many produce ciliated zoospores (cf. above) which are always without a wall and vary in character in the different classes; in more advanced types (e.g., Ectocarpus, fig. 3, A) they are produced within larger cells (sporangia), often differing in shape from the others and sometimes restricted to definite parts of the body. The zoospores may at times fail to develop cilia, secreting a thin wall already prior to their liberation. Such aplan ospores are clearly zoospores which have lost the power of move ment, and possibly illustrate the mode of origin of the motion spores found in many algae lacking zoospores altogether. Thus, in most red seaweeds the sporangia produce four such motionless spores (tetraspores, fig. 4, c).

Sexual Reproduction.

Alarge number of algae exhibit sexual reproduction and all stages in its evolution may be found within the group. In the simplest cases (e.g. Ulothrix, Ectocarpus) the sexual cells or gametes, except for smaller dimensions, essen tially resemble the zoospores and are formed in the same way. After a period of movement they meet in pairs and gradually fuse (fig. 1, Q) their nuclei fusing at the same time. Often the fusing gametes are identical in all respects (isogamy), although probably always derived from distinct individuals. In other cases there are two kinds of gametes (anisogany) which may either merely differ in behaviour, one coming to rest before fusion occurs (e.g., Ecto carpus), or more commonly the passive gamete is larger and more liberally supplied with chromatophores than the other (e.g., Cutleria, fig. 1, o).

The advanced algae exhibit greater differences between the sexual cells (oogamy) , the female or ovum being large, motionless, and provided with abundant chromatophores, the ciliated male or spermatozoid minute, actively motile, and consisting mainly of a nucleus with a thin covering of protoplasm (e.g., Vaucheria, Fucus) . Ova and spermatozoids are formed within special cells (sexual organs) of diverse types; the oogonia (fig. 1, M and N; 2, J) harbouring one or, more rarely, several ova, are larger than the antheridia, which mostly produce many spermatozoids of a pale colour (often yellow). As a general rule the ovum remains inside the oogonium whose wall develops an aperture through which the spermatozoid swims and fuses with (i.e., fertilizes) the ovum. The red seaweeds have motionless male cells which are drifted by currents to the female organs (fig. 4, B) . The unicellular sexual organs of most algae offer a sharp contrast to those of Bryophyta (liverworts and mosses).

The sexual cells are always devoid of a membrane, but after fusion the zygote soon secretes a thin wall and in seaweeds immediately proceeds to divide to form a new individual ; in fresh water algae, however, the membrane is usually thick and a resting spore, often with yellow or red contents, results. Sexual cells may occasionally develop into new individuals without fusion, but in the oogamous types this is only rarely observed (Cutleria, Chara).

The Life-cycle.

Themarked polymorphism, formerly ascribed especially to the simpler algae, is now disproved. A few occasion ally assume forms differing considerably from the normal. Thus, various filamentous algae at times pass into a palmelloid condition, resembling that described for Chlamydomonas.

The alternation between sexual (gametophyte) and asexual (sporophyte) individuals, which is such a prominent feature of the life-cycle of the higher plants, is exhibited in varied ways among the algae. The green forms mostly show no clear alter nation, the sexually formed spores, on germination, usually pro ducing a few (often four) zoospores or aplanospores which form the new individuals. In Coleoc/iaete there is more extensive division of the spore, a mass of 16 or 32 cells being formed, each of which can produce a new plant. This mass of cells is sometimes called a sporophyte, the ordinary Coleochaete-disc (fig. 2, E) bearing the sexual organs, being the gametophyte ; but since reduc tion in chromosome-number (see CYTOLOGY) occurs at the first division in the fertilized ovum (as in all green algae), the sporo phyte lacks the double chromosome-number usually regarded as characteristic of it. The simpler red seaweeds (Nemalionales) show the same features, the sporophyte here being represented by a bunch of threads that sprout out from the female organ after fertilization.

Among brown seaweeds many instances of alternation are known, where the asexual and sexual individuals differ in their chromosome-number in the same way as in higher plants. In some (Pilayella, Dictyota) the two generations are identical, except for this and for the kinds of reproductive structures they bear (homologous alternation). In Laminaria and its allies, however, they are sharply distinct (antithetic alternation), the sporophyte being an elaborate plant (fig. i, K), the gametophyte a filament of microscopic dimensions (fig. 3, H). In Cutleria, on the other hand, the ribbon-like thalli of the sexual individuals are more conspicuous than the flat crusts of the asexual (fig. 3, r). Certain facts appear to indicate that, even in these cases, the two genera tions may originally have been alike. Most red seaweeds (except Nemalionales) show a true homologous alternation, the sporophyte with tetraspores (fig. 4, c) and the gametophyte bearing sexual organs closely resembling one another ; but there is the curious complication that the fertilized ovum gives rise to bunches of threads which produce carpospores, and it is from these that the tetrasporic plants arise. In these forms there are thus two different sporophytes arising from one another, the second produc ing the gametophyte.