CLASSIFICATION OF ALGAE The various classes of algae are essentially distinguished by the pigments in their chromatophores, the kinds of reserve-foods stored after photosynthesis, and the nature of their reproduc tive structures. They are :—I. Isokontae (Chlorophyceae), the green algae; II. Charales, the stoneworts; III. Heterokontae, the yellow-green algae; IV. Bacillariales (q.v.), the diatoms; V. Phaeophyceae, the brown algae; VI. Rhodophyceae, the red algae; VII. Myxophyceae (Cyanophyceae), the blue-green algae. There are, however, various groups of Flagellata (e.g., Chryso monadineae, Peridinieae) which include some algal forms and should no doubt rank as algae (see PROTOPHYTA), but the scope of this article is restricted to the above classes, the majority of whose members are true algae.
The motile forms are classed as VOLVOCALES, which include the unicellular Chlamydomonas (fig. i, A) and various colonial types such as Pandorina (fig. i, B), Eudorina (with 32 spherical cells arranged round the edge of a mucilage-sphere), and Volvox (fig. 2, A), where the colonies are composed of several thousand cells, all four often abundant in small pools. In V olvox the pro duction of new colonies is restricted to special larger cells, and both this genus and Eudorina are oogamous. The Volvocales also include palmelloid forms, such as Tetraspora whose delicate green mucilage-masses, with the cells in groups of four, are common attached to water-plants in spring.
The motionless CHLOROCOCCALES (=Protococcales) comprise Chlorococcurn (on damp soil), Chlorochytrium with large cells inhabiting the air spaces of the duckweed, and Trebouxia (= Cystococcus) which is found in many lichens and possesses an axile chloroplast. These three unicellular forms reproduce by zoospores, but the latter are lacking in Chlorella (fig. I, c), which is common in contaminated waters (e.g., sewage) and also occurs as "green cells" in various animals (Hydra, etc.), and Micrac tinium (fig. 2, B), whose free-floating cells bear long delicate bristles. Many are colonial, e.g., Scenedesmus (fig. i, D), the flat plates of Pediastrum with marginal cells produced into short processes, and the "water-net" (Hydrodictyon, fig. 2, c).
The ULOTRICHALES include unbranched filamentous forms, such as Ulothrix (fig. I, r), not uncommon in streams, and Hormidium, forming a weft on damp soil, both with a curved parietal chloro plast in their cells, as well as Prasiola which is terrestrial and has an axile chloroplast. The filaments of the last commonly undergo extensive division by longitudinal walls, resulting in leafy expanses which may be very abundant where organic matter abounds (e.g., near the penguin rookeries in the Antarctic). Somewhat similar are the flat sheets of the sea lettuce (Ulna) which are common objects on the sea-shore; but here the chloroplast is parietal. The Clado phoraceae, which are usually branched, have large cells containing many nuclei and for this reason are often referred to the Si phonales. Cladophora, with thick cell-walls and an elaborate net like chloroplast is abundant in well aerated pools and streams, and also occurs on rocks in the sea.
In the CHAETOPHORALES the thallus is differentiated into pros trate and projecting systems, well seen in Stigeoclonium (fig. 2, D), where, as often in this group, the branches terminate in long hairs. Dra parnaldia, which like Stigeoclonium is frequent in slow-moving waters, differs in possessing distinct main axes with large clear cells having a scanty chloroplast, while the bunched laterals are deep green. Trentepohlia, found as orange tufts on rocks and trees especially in hilly country, has its cell-sap coloured by haema tochrome and possesses special zoosporangia. The group also includes many prostrate epiphytes, e.g., Aphanochaete (frequent on Cladophora or Oedogonium) which is markedly anisogamous, and Coleochaete (fig. 2, E), whose cells bear curious sheathed hairs and which is oogamous. The common unicellular Pleurococcus, found on tree-trunks, etc., is probably an extremely reduced member of this group.
The OEDOGONIALES with big cells containing a netlike chloro plast, reproduce abundantly by large zoospores which bear a ring of numerous cilia and are formed singly in the cells. They include the unbranched Oedogonium and the richly branched Bulbochaete, where each cell bears a long hair with an inflated base. These are oogamous forms, often producing their flat antheridia in special "dwarf-male" filaments which are found attached on (fig. i, M) or near the swollen oogonia.
The CONJUGATAE, which are widely distributed in freshwater, are distinguished by their elaborate chloroplasts with prominent pyrenoids, by lacking asexual reproduction, and by a peculiar sex ual process (conjugation) in which the gametes are constituted by the contents of the ordinary cells. They comprise a filamentous series (Zygnemales), including Spirogyra (with spiral chloro plasts) (fig. 2, F) and Zygnema (with two star-shaped chloroplasts in each cell), but are more extensively represented by the unicel lular Desmids. The cells of the latter exhibit a great variety of beautiful and symmetrical shapes, and are commonly divided by a median constriction into two semicells, each harbouring one or two complex chloroplasts ; examples are furnished by Closterium with semilunar cells, Cosmarium (fig. 2, G), and Staurastrum (fig. 2, I), where the semicells are produced in two or more arms. A few Desmids form threadlike colonies. During conjugation in the Zygnemales, opposite cells of two parallel threads become connected by tubes (fig. 2, F), whereupon the protoplasm of the one glides over into that of the other and fuses with it to form a zygospore (Spirogyra), or the protoplasmic masses meet in the tube and form the zygospore there (often in Zygnema). In Desmids the protoplasts escape from two adjacent individuals and fuse to form a zygospore which lies between the empty halves of the parent cells (fig. 2, H). Both Oedogoniales and Conjugatae are highly specialized.
The SIPHONALES are coenocytic, i.e., their large multinucleate bodies possess few or no septa. The only freshwater form, V au cheria, shows this clearly, its branched, rather coarse filaments containing numerous nuclei and lens-shaped chloroplasts in the lin ing protoplasm which surrounds a huge and continuous vacuole. The marine forms mostly show a more elaborate structure. Thus, the Caulerpas of warmer seas have a thallus differentiated into a creeping stem (up to a foot long) anchored by branched rootlets and bearing leaflike outgrowths, which in their varied form and arrangement simulate the shoots of diverse higher plants (fig. 2, K). Other Siphonales (Codiaceae) resemble certain brown and red seaweeds in having a body built up of numerous closely placed and often intertwined threads like those of V aucheria, e.g., Codium and Halimeda, the latter with a flat segmented thallus encrusted with lime. In another family (Dasycladaceae) an erect main axis bears compact whorls of branches between which there is often extensive deposition of carbonate of lime; one of the most striking forms is the Mediterranean Acetabularia (fig. 2, L), where the whole plant is encased in lime. Many similar calcareous forms are known as fossils. The Siphonales are mostly isogamous or anisogamous, producing their gametes in special gametangia; but V aucheria is oogamous with antheridia and oogonia situated side by side on the filament (fig. 2, J).
Stoneworts.—II.The CHARALES (stoneworts), which grow submerged in fresh and brackish waters, are so complex, espe cially in their reproductive organs, that they can have but a very remote relationship with the green algae. The slender cylindrical axes bear whorls of short branches (often called leaves), which are separated by long internodes and produce occasional branches in their axils. The popular name is due to the considerable deposition of lime in the superficial membranes. All the cells contain numerous small chloroplasts. The segments cut off from the apical cell divide into an upper half forming a node and a lower half forming an internode ; the latter lengthens greatly but remains undivided, while the node becomes segmented by vertical walls, the peripheral cells growing out into "leaves" which, except for their limited growth, show the same structure as the "stems." In Chara threads sprout out from the lowest nodes of the leaves and closely invest the stem, but in Nitella this cortex is lacking.
In spite of prolific vegetative reproduction sexual organs are formed abundantly. They usually arise close together at a node (fig. 2, m). The large oogonia are completely invested by five green spiral threads whose tips form a symmetrical group (corona) at the apex. The spherical antheridium has a wall composed of eight somewhat curved "shields" which at maturity acquire a yellow or red colour ; to the middle of the inner (concave) surface of each shield is attached a rod-shaped manubrium carrying a number of long coiled threads, in each compartment of which a spermatozoid is formed. At maturity the whole structure falls to pieces and the spermatozoids are set free. The fertilized ovum acquires a thick wall and rests prior to its germination, which is complex.
The Heterokontae, though represented by far fewer genera and species than the Isokontae, exhibit much the same range of form. The motile unicell is illustrated by the rare Chloramoeba (fig. 2, N), whose cells are without a wall, but there are no motile colonies. Various palmelloid types are known, the commonest being Botry ococcus, an abundant plankton form, in which the mucilage of the rather irregular colonies is often so dense that the contained cells are difficult to detect. Motionless unicells are seen in Chloro botrys (a rare moorland form), Halosphaera, and the multinu cleate Ophiocytium (fig. 2, o), which is common in stagnant pools. Its curved cells have a wall composed of two very unequal pieces, the smaller becoming detached as a lid when zoospores or aplano spores are to be liberated. Tribonema (= Conferva, fig. 2, P), a common freshwater form, has filaments with long narrow cells which have a wall composed of two overlapping pieces and many disc-shaped yellowish green chloroplasts. The only siptioneous form is Botrydium (fig. 2, Q), not uncommon on damp mud ; when the latter dries, all the protoplasm withdraws into the underground rhizoid and divides into a number of thick-walled cysts.
Diatoms.—IV. BACILLARIALES, which are abundant in all kinds of situations, are a specialized class of unicellular and colonial forms whose silicified wall is composed of two overlapping pieces. They are dealt with in a separate article.
The simplest Phaeophyceae are the ECTOCARPALES, the palmel loid forms previously included in this class being now referred to the Chrysophyceae (see PROTOPHYTA). Ectocarpus, whose brown tufts or tresses are commonly seen in rock-pools, is like a brown Stigeoclonium having prostrate and projecting systems; each branch of the latter grows by the division of a series of flat cells situated below the terminal hair (fig. 1, H). Numerous zoospores (fig. 3, B) are produced in large (unilocular) sporangia (fig. 3, A), while the gametes which are quite similar arise singly in the com partments of elongate gametangia (plurilocular sporangia, fig. 1, H). In Pilayella sporangia and gametangia are interspersed be tween the ordinary cells. The gametes, though alike in form, may differ in behaviour, some soon ceasing to move and being sought out by others which retain their activity. In Giffordia, with two kinds of gametangia, there is marked anisogamy. The sexual indi viduals of Pilayella (see Life-cycle, p. 592) are found in spring, the asexual in summer ; both bear plurilocular sporangia, those of the latter liberating zoospores which give rise to a new asexual generation without the intervention of a sexual one.
Many Ectocarpales are far more complex, though all can be related to Ectocarpus. A few examples must suffice. Desmarestia (fig. 3, c) has main axes, from 1-1-4 feet long, bearing numerous short stiff branches which in winter and spring end in feathery tufts, but in summer are bare. The large cells of the axis (fig. 3, n) each produce a pair of short branches, from the lowest cells of which threads grow out in profusion and become matted together to form a wide "cortex" which grows very thick in the older parts. The branched gelatinous threads of Castagnea and the irregular jellylike lumps of Leathesia, common on larger seaweeds, are built up of numerous filaments in dense juxtaposition at the surface, but more loosely arranged in the interior. Of a different type are the bladderlike thalli of Asperococcus and the long occasionally constricted tubes of Scytosiphon, which arise by longitudinal division in the cells of the primary thread followed by the develop ment of a central air-cavity. Many Ectocarpales (Elachista, Myriotrichia) form small epiphytic cushions or tufts.
The cylindrical threads of the SPHACELARIALES possess a prom inent apical cell which cuts off segments parallel to its base (fig. I, 1). These, without further elongation, divide abundantly and often very regularly by transverse and longitudinal walls ; the older parts are, however, often covered by a dense small-celled cortex. The numerous branches mostly remain quite short and by their arrangement determine the habit of the plant. Sphacelaria is a bushy form, rarely exceeding one or two inches in height with feathery arrangement of the branches. Cladostephus is larger and has whorled branches. The former multiplies abundantly by means of triangular or three-armed propagules. The reproduction of Sphacelariales resembles that of Ectocarpales; some are anisogamous like Giffordia.
Cutleria, the principal genus of CUTLERIALES has a branched ribbon-like thallus, several layers of cells thick, the ultimate segments ending in a fringe of hairs, each exhibiting at its base a dividing zone like that of Ectocarpus. The dark dots on the uni sexual thalli correspond to groups of hairs bearing gametangia (fig. 3, E), the female having large deep brown compartments, the male small pale yellow ones. Both gametes are motile (fig. I, o). After fusion they produce a flat crustlike thallus (Aglaozonia stage, fig. 3, F), several layers of cells thick and bearing a dense row of sporangia on the upper surface. The Zoospores ordinarily give rise to the Cutleria-plant, but this regular alternation does not always occur, either generation being capable of reproducing itself.
The complex internal structure has already been described; of special interest are the sieve-tubes, closely resembling those of higher plants, found in the inner cortex of Macrocystis and Nereocystis. The medulla in all cases contains numerous "trumpet hyphae" (fig. 3, I) with a marked dilation at the septa which show a sievelike pitting.
The gametophytes are microscopic filaments bearing either antheridia producing a single, almost colourless, spermatozoid (fig. 3, H), or oogonia with a single ovum.
Dictyota, which is typical of the DICTYOTALES, has a forked band-shaped thallus growing by an apical cell. The asexual indi viduals bear at intervals spherical sporangia, each producing four motionless spores. These give rise to male or female individuals. Oogonia and antheridia are formed in compact groups on the surface, the former (fig. 3, j) unicellular and producing each one ovum, the latter of the usual septate type; the male cells have only a single apical cilium. In Padina, which is rarer, the thalli are fan-shaped with an inrolled margin and exhibit a prominent con centric zoning.
The FUCALES, although not nearly so large, are quite as complex as the Laminariales. There is no asexual propagation, but sexual reproduction is very prolific. The sexual organs are borne amidst numerous hairs on the inner surface of round cavities (concep tacles) (fig. 3, x), which appear as swellings on the thalli and open to the exterior by a small aperture. The large oogonia (fig. I, N) produce eight (Focus), four (Ascopliyllum), two (Pelvetia), or only one ovum (Himanthalia), while the antheridia, borne in large numbers on branched hairs (fig. 3, L), form minute sperma tozoids. Male and female organs occur on distinct individuals or, more rarely (Pelvetia) in the same conceptacle. The gametes are extruded from the conceptacles in a drop of mucilage during low tide, and fertilization takes place when the sea returns. The fertilized ovum grows direct into a new plant. The concctptacles are generally restricted to the tips, although Fucus and others have similar, but smaller, barren conceptacles scattered over the whole thallus.
In the bladderwrack (Fucus) the flat segments have a thick midrib which persists as a stalk in the older parts of the plant ; several species have paired air-bladders. Pelvetia, which is much smaller, has narrow branches which are deeply channelled, while the irregularly swollen tips render fertile plants very conspicuous. The long straplike thalli of Ascophyllum, dilated at intervals by air-bladders, bear numerous short branches which arise from mar ginal slits and in the upper parts of the plant contain the concep tacles. In Himanthalia the thallus is roughly of the shape of a top and about an inch across, but periodically the upper surface gives rise to a forked straplike outgrowth, as much as a yard long and studded with conceptacles. The distribution of these forms on the sea-shore is dealt with below. Sargassum (fig. I, L) has already been described. The internal structure of Fucales is similar to that of Laminariales, though somewhat simpler.
A seventh group of brown algae, the Tilopteridales, are rare and too incompletely known to be considered here.
The simple BANCIALES, whose cells have an axile star-shaped chromatophore and are without pits, include Bangia, with un branched threads whose cells are longitudinally divided, and Por pliyra, with a flat Ulva-like thallus, both marine. Their relation to other red algae is not clear. They reproduce asexually by libera tion of the contents (monospores) of ordinary cells, while their sexual reproduction is not unlike that of the Nemalionales (see below).
The large remainder of the red algae, styled FLORIDEAE, are classed in four groups distinguished by the events following upon fertilization. Their thallus exhibits diverse structure, but growth is usually apical. Callithamnion, one of the simplest forms, con sists merely of richly branched threads with elongate cells. Many other Florideae are filamentous, though with a more elaborate structure. Thus in Ceramium the threads show a characteristic banding (fig. 4, D), due to the formation of a small-celled cortex around the upper end of each principal cell. In Polysiphonia the segments of the apical cell divide with great regularity by longi tudinal walls, forming characteristic tiers with a deeply coloured central cell shimmering through the peripheral ones. Batracho spermum, the "frog-spawn alga," found in slowly moving streams, has soft threads resembling a string of beads, each constituted by a whorl of branches (fig. 4, A) ; the large cells of the main axes are overgrown by cortical threads.
In Nemalion or Furcellaria the threads are coarser and, like Codium or Castagnea, composed of numerous filaments in more or less dense aggregation (fig. I, j). Corallina shows the same structure, but here the pink barrel-shaped segments are densely encrusted with lime and separated by uncalcified joints, where the central threads bear no lateral branches. Closely related are Lithothamnion and Melobesia, with strongly calcified encrusting thalli which are specially common in the warmer seas and often assist in the building of coral-reefs. Such forms are also known as fossils.
Many Florideae have a flattened thallus which may be exten sively branched, e.g., carrageen (Chondrus crispus, fig. 4, F) and Gigartina, the latter with numerous teatlike outgrowths which harbour the fruits. Some of the most beautiful forms belong to Delesseria (fig. 4, G).
In the NEMALIONALES (incl. Batrachospermum and Nemalion) numerous short sporogenous threads sprout from the carpogonium (cf. fig. 4, A'), their end-cells liberating naked carpospores which produce a new individual. These sporogenous threads are some times regarded as a sporophyte (see above). Asexual reproduction in the Nemalionales is either lacking or effected by monospores formed singly in spherical sporangia.
The remaining Florideae reproduce asexually by tetraspores developed in tetrasporangia which either project freely (fig. 4, c), or, more commonly, are embedded among the superficial cells of the thallus (fig. 4, D) ; they are sometimes confined to special branches. The tetrasporangia usually occur on individuals devoid of sexual organs, although in other respects similar to the sexual plants. A regular alternation of the two has been established in a considerable number of cases (cf. above).
In the more advanced groups the fertilized carpogonium de velops filamentous outgrowths (gonimoblasts, fig. 4, H) which fuse with one or more auxiliary cells distinguished by rich proto plasmic content. At the points of fusion there arise bunches of sporogenous threads forming the carpospores, which produce the individuals bearing tetraspores. Only the protoplasm of gonimo blast and auxiliary cell fuse, their nuclei remaining far apart, so that the process gives rather the impression of a parasitism of the gonimoblast on the auxiliary cell than of a second process of fertil ization. In the CRYPTONEMIALES the two or three gonimoblasts are long threads fusing repeatedly with auxiliary cells which are here located at the ends of neighbouring branches (fig. 4, H) . In the remaining groups (GIGARTINALES, RHODYMENIALES) the auxiliary cells are associated with the carpogonia to form so-called pro carps, and the gonimoblasts are quite short. Thus, in Callitham nion, there are two auxiliary cells flanking a cell of the main axis (fig. 4, I, a and b) and one bears the four-celled branch which ends in the carpogonium ; two masses of carpospores are formed. In Polysiphonia and many other cases the single auxiliary cell and the carpogonial branch become enclosed, already prior to fertili zation, by a cuplike outgrowth from the adjacent cells (fig. 4, E). The enlarged envelope subsequently forms an investment around the single mass of carpospores, the whole structure being termed a cystocarp. In forms with a compact thallus the cystocarps are often embedded in the surface layers (fig. 4, F) . Corallina and its allies produce all their reproductive organs in flask-shaped cavities (conceptacles).
The simpler members are colonial and grouped as CHROOCOC CALES, most of which are palmelloid with the cells embedded in abundant soft mucus (e.g., Gloeocapsa, fig. i, E) ; they form jelly like masses on damp walls, rocks, etc. A different type is seen in Merismopedia (fig. 4, j) whose plates of deep blue cells are com mon in freshwater plankton.
The remaining Myxophyceae are filamentous (HoRMOGO NEALES) . Oscillatoria, forming vivid blue sheets at the bottom of ponds in autumn, has simple threads (fig. 4, K), which may glide slowly through the water or exhibit slow pendulumlike move ments to which the genus owes its name. Many of its allies have their threads encased in a firm mucilage sheath (e.g., Lyngbya). In Pliormidium huge numbers of threads are agglutinated by their soft sheaths to form leathery or papery sheets which often cover rocks, etc., over considerable stretches. In Scytonema (fig. 4, N) the filament breaks at certain points and the two pieces push out of the sheath sideways to form paired branches. This "false branching" is common in a number of Myxophyceae. The threads of Scytonema and its allies exhibit occasional larger cells (hetero cysts), which have a firm cellulose wall and whose contents are at first yellow, but ultimately disappear completely.
Heterocysts are also found in the unbranched filaments of Nostoc (fig. 4, L) and Anabaena (fig. 4, M). The coiled threads of the former are embedded in large numbers in soft mucilage to form spheres or leafy expanses on damp ground or in water. In Rivularia and its allies the heterocysts are always situated at one end of the filament, whilst the opposite one tapers to a point or ends in a long hair. Stigonema (fig. 4, o) is a more robust form with several rows of cells within the sheath and showing true branching.
In a very few cases the contents of heterocysts have been found to produce a new thread, but for the most part they do not fulfil any recognizable function. Reproduction in the Myxophyceae is usually accomplished vegetatively, by fission in the colonial forms and by the detachment of short lengths of thread called hormo gones in the filamentous types. The hormogones perform slow, gliding movements, whose mechanism is not fully understood. Many Hormogoneales produce resting spores, enlarged thick walled cells with abundant granules, which may arise from any cell of a thread, though commonly located near a heterocyst (fig. 4, M)• The Myxophyceae, though so simply constructed, are successful in many habitats (e.g., hot springs, the frozen Antarctic lakes) where few other algae (except diatoms) can exist.