Euphycophyta Phaeophyceae

Chapter VI EUPHYCOPHYTA PHAEOPHYCEAE ECTOCARPALES, SPHACELARIALES, TILOPTERIDALES CUTLERIALES, DICTYOTALES, CHORDARIALES, SPOROCHNALES * GENERAL The algae composing this class range from minute discs to 100 metres or more in length and are characterized by the presence of the brown pigment, fucoxanthin, which masks the green chlorophyll that is also present. The class can be divided into a number of orders and families, which can be treated independently (Fritsch, 1945), or the families may be placed into three groups as proposed by Kylin (1933). These groups are based upon the type ofalterna- tion of generations, though the classification involves difficulties so far as one family (Cutleriaceae - see, however, p. 142) is concerned. (a) Isogeneratae: Plants with two morphologically similar but cytologically different generations in the life cycle (e.g. Ectocarpaceae, Sphacel- ariaceae, Dictyotaceae, Tilopteridaceae, Cutleriaceae). (b) Heterogeneratae: Plants with two morphologically and cytologically dissimilar generations in the life cycle: (i) Haplostichineae: Plants with branched threads, which are often interwoven, and usually with trichothallic growth (e.g. Chordariaceae, Mesogloiaceae, Elachistaceae, Sper- matochnaceae, Sporochnaceae, Desmarestiaceae). (ii) Polystichineae: Plants built up by intercalary growth into a parenchymatous thallus (e.g. Punctariaceae, Dictyosi- phonaceae, Laminariales). 122 V. J. Chapman, The Algae © V. J. Chapman 1962 EUPHYCOPHYTA 123 (c) Cyclosporeae: Plants possessing a diploid generation only, (e.g. Fucales). In any consideration of phylogenetic problems there is really very little difference between the two methods of classification. The Phaeophyceae are extremely widespread and are confined almost entirely to salt water, being most luxuriant in the colder waters, though the genera Heribaudiella, Pleurocladia and Bodan- ella, six species of Ectocarpus and Sphacelaria ftuviatilis occur in fresh water. All of these except the Sphacelaria belong to the Ecto- carpales and many of the records come only from China. Some of the species of brown algae exhibit morphological varia- tions and it has been shown that these may depend on (a) season of the year, and (b) nature of the locality. Church (1920) has given us an elaborate account of the morphology of the Phaeophyceae, and he suggested that if a brown flagellate came to rest it could develop in one of three directions to give: (a) Uniseriate filaments which occupy a minimum area and obtain maximum light energy per unit of area, growth being either distal or intercalary. (b) A mono- or polystromatic thallus which occupies a maximum area and obtains a minimum light energy per unit ofarea. (c) Mass aggregation. A morphological examination of the brown algae will show that development has taken place along each of these directions, often resulting in plant bodies of a complex construction, and the follow- ing types can be recognized among the various species: (a) Simple filaments (e.g. Acinetospora). (b) Branched filaments (e.g. Pylaiella). (c) Erect filaments arising from a prostrate thallus (e.g. Myrio- nema). (d) Interwoven central filaments (cable type, e.g. Myriogloia). (e) Prostrate portion only (reduced filamentous or cable type, e.g. Phaeostroma). (f) Filaments uniting to form a sphere (hollow parenchymatous or modified cable type, e;g. Leathesia). (g) Multiseptation of primary cable type (e.g. Chorda). (h) Erect filaments with cortication (corticated type, e.g. Sphacelaria ). I24 THE ALGAE (i) Simple or laminate parenchymatous thallus (e.g. Punctaria). (j) Improved parenchymatous structure with internal differentiation of the tissues (e.g. Laminariales). Many of the simpler types, whether reduced or not, exhibit the condition of heterotrichy similar to that found in the Chaeto- phorales, but this is a feature that will be discussed elsewhere (cf. p. 3I5). The thalli may also reach a relatively large size and under these circumstances additional support is obtained as follows: (I) Increase in wall thickness (Stypocaulon) or the production of a firmer cellulose material (Sphacelaria). (2) Twisting and rolling of the threads together. (3) Development of root branches or haptera. (4) The appearance of descending and ascending corticating filaments. (5) Multiseptation takes place in a longitudinal direction. (6) Development of internal hyphae. The characteristic method of growth in the group is by means of an intercalary meristem at the base of a hair. This is termed trichothallic growth. In some of the more advanced orders trichothallic growth has been replaced by an apical cell or an apical or marginal group of cells. Branching may proceed from any cell, and it frequently takes the form of a regular or irregular dichotomy, though in the Fucales a spiral arrangement may be found. The cells vary greatly in size but they always have distinct walls, which are usually composed of cellulose on the inside and pectin outside, and although they are commonly uninucleate occasionally they become multinucleate. Plastids are also present and when the brown pigment has been removed by boiling, the thallus then be- comes green. Usually each cell contains several plastids, which are usually parietal and discoid though there is some diversity of form. Although pyrenoids have been described for some species they do not appear to be wholly comparable with those in other algae. The pigments are not the same as those in the higher plants because chlorophyll b is replaced by chlorophyll c and there are a number of accessory xanthophylls (see p. 2). The products of assimilation are sugar, alcohols, fats and complex polysaccha- rides but not starch. A characteristic feature of the Phaeophyceae is the presence of colourless, highly refractive vesicles known as fucosan vesicles or physodes. These contain a material known as EUPHYCOPHYTA 125 fucosan, and they are particularly abundant in tissues where active metabolism or division is taking place. Hyaline hairs occur in many forms and their function has been variously ascribed as: (I) shock absorbers, (2) respiratory and absorptive organs, (3) protection against intense illumination, (4) protection against epiphytes, (5) protection against covering by sand or silt, (6) mucilage organs. None of the evidence for any of these suggestions is entirely satisfactory, and the whole problem demands further investigation. Vegetative reproduction may take place by splitting ofthe thallus or else by the development of special propagules (Sphacelaria). Asexual reproduction is commonly secUred by means ofuni- or bi- flagellate zoospores which are normally produced in specialized cells or sporangia. In one group (Dictyotales) tetraspores replace the zoospores, these bodies being produced in groups of four in each sporangium on plants that do not normally bear sexual organs. In yet another group (Tilopteridales) asexual reproduction is by means of uni- to quadrinudeate monospores. The homologies of these monospores have been subject to much speculation and they have been variously regarded as equivalent to (a) propagules of Sphacelaria, (b) simple forerunners oftetraspores, (c) degenerate tetraspores, (d) parthenogenetic ova. The second suggestion is perhaps the most satisfactory in our present state of knowledge, especially when considered in relation to the vegetative characters. Sexual reproduction ranges from isogamy, with both gametes motile and characteristically bearing two flagellae inserted laterally, through a series in which differentiation first to anisogamy and finally to oogamy can be traced. The ova are not normally retained on the parent plant so that fertilization takes place in the water, though the ovum may remain attached by a long mucilaginous stalk (cf. p. 193). The change from isogamy to anisogamy is also accompanied by a corresponding differentiation of the gametangia. Electron microscope studies of the antherozoids of several species (Manton, 1951, 1952) has revealed the fact that the long anterior flagellum is composed of eleven 126 THE ALGAE fibrils and the shorter posterior one of nine fibrils which seem to be of a rather different nature. Down the sides of the long flagellum can be found small hairs which presumably make it a more efficient locomotory organ. Both unilocular and plurilocular sporangia are commonly found, but the fate of their products varies considerably. Plurilocular sporangia can be borne on both diploid and haploid plants, whereas unilocular sporangia are commonly confined to diploid plants. The plurilocular sporangia on the diploid plants represent an accessory means of reproduction, whilst those of the haploid generation are gametangia. In isogamous forms gametes are normally produced from plurilocular sporangia, so that in anisogamous forms the antheridia and oogooia must be regarded as modified plurilocular sporangia. On this basis the plants can be divided into two groups as in Table 2. TABLE 2 Sporangia in Phaeophyceae I. One kind of plurilocular sporangium. (i) Un,i- and plurilocular sporangia on the same individuals, e.g. Ectocarpus spp. (ii) Uni- and plurilocular sporangia on different individuals, e.g. SphaceZaria bipinnata, CZadostephus. II. Two kinds of plurilocular sporangia. (i) Meio- and megasporangia, e.g. E. virescens tsee p. 130). (ii) Micro- and megasporangia: (a) Unilocular sporangia only on separate plants, e.g. Sphace- Zaria hystrix, H alopteris filicina. (b) Unilocular sporangia and both types of gametangia all on separate plants, e.g. Sphacelaria

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