Med K5461 . [;/*<** J /Q2 °ell« showing the solitary r chromatophores ( x 410). From Tremethick Moor, Cornwall. 1 Chodat in Bull. L’Herb. Boiss. tom. v, no. 4, 1897, p. 302, t. 10, f. 8—12. W. A. 4 Class 3. CHLOROPHYCEAE. This group, which includes all the green Algae, attains its greatest development in fresh water, and the number of species exceeds the combined total of the freshwater species of all other Algae. The simpler forms of green Algae are unicellular (e.g. some of the Protococcoideae and Desmidiaceae), some are coenocytic (e.g. Vaucheriacese, Sphaeropleaceae, Pediastreae), some are incompletely septate (e.g. Cladophoraceae), and others are multicellular or com- pletely septate (e.g. CEdogoniales, Chaetophorales, Zygnemaceae). In other than the unicellular forms the thallus exhibits every degree of development from simple rounded cells to long, simple or branched filaments, flat expansions, or pulvinate masses of tissue. As a rule there is no differentiation of the ordinary vegeta- tive cells, but in some there is a marked distinction between the vegetative and reproductive cells. The cell-protoplasm (or cytoplasm) of the green Algae consists of a lining layer or ‘ primordial utricle ’ which adheres closely to the cell- wall1, and, in many Algae, of additional anastomosing strands and threads traversing the interior of the cell. It contains numerous granules of variable size which behave differently with staining reagents. Evidence goes to prove that there is no definite proto- plasmic continuity between the cells of multicellular green Alga?. A division of labour is rarely observed amongst this class of plants, and in the Conjugatse the cells of most of the filamentous forms arc under normal circumstances quite able to lead an independent existence. The vacuoles are much as in other plant-cells and they contain a fluid usually known as the cell-sap. In the Conjugatse the cell-sap is occasionally coloured violet or purple owing to the 1 Cf. Chodat et Boubier, ‘ Sur la Plasmolyse et la membrane plasmique,’ Journ. Bot. de Morot, Paris, 1898. Ch lorophycece 5 1 presence of a pigment termed by Lagerheim phy coporphyrin1. This violet colour occurs normally in Ancylonema Nordenslcioldii Berggr., Mesotcenium violascens De Bary, M. purpureum W. & G. S. West and Mougeotia capucina (Bory) A g., and under excep- tional circumstances it is found in various species of Zygnema, Spirogyra and Desmids. The Volvocacese and the zoogonidia and gametes of other green Algse possess vibratile cilia, which are very variable in their length, number, disposition, and symmetry ; and in certain of the same forms contractile vacuoles are present. In the genera Tetraspora and Apiocystis ‘ pseudocilia ’ are found, which do not possess any power of movement. A single nucleus is present in the cells of all the green Algae except the coenocytic and incompletely septate forms, and during the formation of asexual non-motile spores, zoogonidia, or gametes, it undergoes divisions corresponding to the divisions of the proto- plasm. In some green Algse mitotic division of a more or less complex character has been observed-’. The cell-iuall is very variable and its structure is often difficult of observation. In the formation of a cell-wall such as after the quiescence of a zoogonidium, it is developed on the outer surface of the protoplasm as the result of more or less complex processes. The young cell-wall usually consists of cellulose, but sometimes equally of pectose. Under the action of strong acids or other hydrating reagents an ordinary thick cell-wall will swell up and show traces of lamination. Each lamina represents successive layers of growth in thickness and in most plants consists of a mixture of cellulose and pectose constituents in variable propor- tions. In the Chlorophyceae these two constituents' of the cell- wall are differentiated while the wall is very young. They exhibit considerable differences in their behaviour with reagents, the cellulose constituents giving a violet colour with chlor-zinc-iodine (Schulze’s solution), whereas the pectose constituents do not. In many Algse the pectose constituents of the cell-wall are in the form of gelatinous layers on the outside of inner layers of cellulose. This mucilaginous material stains readily with aniline dyes such 1 Lagerheim in Vidensk.-Selsk. Skrift., I mathem.-natur. Kl., Kristiania, 1895, no. 5. 2 Observed in Spirogyra by Mitzkewitsch (Flora, lxxxv, 1898) and C. van Wisselingh (Bot. Zeitung, lvi, 1898 ; Flora, lxxxvii, 1900) ; in Chlamydomonas by Dangeard (Le Botaniste, vi, 1899); in Closterium by Klebahn ; also in Botry- dium, etc., etc. 4—2 52 Chlorophycece as fuchsin, safranin, methylene-blue and gentian-violet. The outer layers often become thick coats of mucilage by the formation of series of pectose constituents which exhibit all stages between insolubility and complete solubility in water. It is not merely a hydration but a molecular change, and successive increments are often added by the gelatinization of other layers of the cell-wall. In some of the unicells the increase in thickness of the cell-wall due to gelatinization is only on one side and elongated colonies such as those of Hormotila are formed. The gelatinous pectose compounds although sometimes forming a large proportion of the cell -wall, do not alternate with layers of cellulose, but there appears to be a continual exudation of them through the inner layers of cellulose, a mass of jelly being thus formed on the outside of the cellulose wall. This is best seen in some of the Protococcoidese and Conjugate. The mucilage in which filaments of Algae are so frequently embedded exhibits a distinct radiating fibrillar structure which is clearly brought out by various reagents. The radiating structure of the enveloping mucus has long been known in the Conjugatae and has at times given rise, especially in the Desmidiaceae, to grave morphological misconceptions. The cell-walls of CEclogonium and other Algae exhibit peculiari- ties of structure which will be described in their respective families. Whatever the nature of the cell-wall one of its primary func- tions is the regulation of osmotic changes. Hairs and bristles are developed by certain green Algae belong- ing to the Coleochaetaceae, Herposteiraceae, CEdogoniaceae, Chaeto- phoraceae and Chaetopeltidae. They are of many kinds, from slender articulate branches such as the piliferous apices of Chcvto- phora, to exceedingly fine inarticulate hairs such as the setae of Bulbocluete, Herposteiron, Chcutosphceridmm, or Conochaite. The thallus often develops special root-like organs of attach- ment or haptera (commonly termed rhizoids), but these are as a rule only found in the young plants, most of the older ones occur- ring as freely floaiting masses. The chromatophores are usually distinct and in the forms with a filamentous or expanded thallus they are frequently characteristic of the different families or genera. They are of a bright green colour due to the presence of chlorophyll and are therefore chloro- jjlasts. Sometimes they are very difficult to define, but at other 53 Chlorophycece times they stand out clearly, occupying only a relatively small portion of the cytoplasm. They may be solitary or very numerous, of infinite variety of form, central or parietal, and the edges may be entire or deeply incised. In some forms they are ribbon-like and wound spirally round the interior of the cell-wall (e.g. Spirogyra, Genicidaria, and some species of Spirotcenia), and in others they are central, spirally twisted masses (e.g. some species of Spirotcenia). Sometimes they are reduced and very pale in colour, and in the rhizoids and terminal cells of the hairs of some green Algae they are entirely wanting. The chloroplasts of most green Algae contain pyrenoids or proteid bodies which serve as a reserve of food-material. Much has been done towards the investigation of these bodies during the past few years and the presence or absence of pyrenoids has been regarded by some as a sufficient generic distinction. This is, however, attaching an importance to these proteid bodies which is scarcely borne out by facts. Although they frequently divide equally on the division of the cell, they also multiply without any cell-increase. They are likewise known to disappear during the development of certain species, and it has been clearly demon- strated that during certain stages of Tetraspora, Splicer ella and Eudorina they can arise spontaneously. Moreover, forms of An- kistrodesmus falcatus (Corda) Ralfs containing pyrenoids are sometimes met with in the same collection as others which have no pyrenoids. Similarly, the chromatophores of Debarya calosporct (Palla) W. & G. S. West may or may not contain pyrenoids1. Starvation causes a disappearance of pyrenoids and they frequently increase in numbers if the cell is well nourished. On the whole, there is little doubt that the presence or absence of pyrenoids depends largely upon external conditions and is a character to which a great deal too much importance has been attached in discriminating between the genera of green Algae. In the Chlorophyceae the stored product of assimilation is almost invariably starch. Exceptions to this are found commonly in Mesotcenium in the Desmidiaceae and in the Vaucheriacese. Cell-division generally takes place in all the cells of the thallus, but in a few instances there is a definite growing point which is usually an apical cell. 1 West & G. S. West in Ann. Bot. xx, March, 1898, p. 49; in Journ. Bot. Aug. 1900, p. 289. 54 Chlorophycece Multiplication by cell-division occurs in many of the lower forms of the Protococcoideae and in the Desmidiaceae. In the Zygnemaceae, and particularly in the smaller species of Spirogyra, the filaments often dissociate into solitar}^ cells each of which then divides and forms a new filament. Asexual reproduction by zoogonidia is general throughout the class, although there is a notable exception in the Conjugatae, in which motile reproductive cells are entirely wanting. In many of the Chaetophorales, Microsporales and Protococcoideae reproduction takes place by non-motile spores which may be either akinetes or aplanospores. Asexual spores are also more rarely found in the Conjugatae, having been observed in Zygnema , Spirogyra, and in a few Desmids, and they are formed normally in the rare genus Gonatonema. Sexual reproduction occurs in most of the families of green Algae, and may be either isogamous or heterogamous. In iso- gamous reproduction the sexual organs are gametangia, usually giving rise to planogametes which conjugate and produce a zygo- spore. In the Conjugatae only aplanogametes are found. Many planogametes are generally produced from one gametangium, but only one aplanogamete. In heterogamous reproduction the sexual organs are oogonia and antheridia, and the gametes consist of oospheres and antherozoids. In all cases with the exception of Herposteiron, the oosphere remains in situ in the parent plant, being fertilized within the oogonium, and the result of fertilization is an oospore. The oogonia are always unicellular, and, except in Cylindrocapsa and some species of (Edogonium , so are the anthe- ridia. Only one oosphere is produced in an oogonium, except in Splicer oplea, and one, two, or many antherozoids may arise from an antheridium. The gametophyte is the principal generation, the sporophyte being generally represented by the sexually-produced spore. In Goleochcete, in (Edogonium , and in Mougeotia very rudimentary sporophyte generations consisting of several cells do exist. Of all freshwater Algse the Chlorophycese have the most varied habitats. They are found in every possible damp or wet situation and some are epiphytes, others endophytes, and a few are even parasites1. 1 Phyllosiphon Arisari Kuhn (in Sitzungsber. d. naturf. Ges. in Halle, 1878) is a parasitic Alga observed only on the leaves of Arisarum vulgare in Italy and the Chlorophycece 55 Richter1 and Comere2 have conducted experiments with a view to ascertaining if certain of the freshwater Alga?, especially Chloro- phycete, can exist in salt water. Richter states that the lower the organization of the Alga the better its power of adaptation, but Comere finds that only those Algae with a robust structure and with large chloroplasts can successfully withstand immersion in salt water. Some species of Edogonium and Cladophora can live in water containing 3‘5 °/0 of sodium chloride, Vauclieria sessilis in water containing 2 °/Q, and some of the large species of Spirogyra in water containing from 1’8 — 2°/0* Richter affirms that (Edogonium , Spirogyra, or Vaucheria have less power of adaptation to life in salt water than Stichococcus or Tetraspora. In all cases the salinity of the water caused the cells to increase in size and when the concentration was high malformation of the cells invariably occurred. Starch at first disappeared from the cells, but reappeared when the adaptation was more complete. Not- withstanding the somewhat contradictory nature of these two sets of experiments, it appears that certain of the freshwater Chloro- phycese can adapt themselves to an increasing salinity of the water in a manner comparable with the adaptation of a few forms of the green Algse to a life in hot water3. The class Chlorophycese can be conveniently subdivided into nine orders, all of which are found abundantly in the British Islands. Order I. Edogoniales. Thallus filamentous, simple or branched. Cells uninucleate, with a large, parietab anastomosing chloroplast containing one or several pyrenoids. Cell-division characterized by the inter- calation of a new piece of cell-wall between the mother-cell and the distal end of the daughter-cell. Sexual reproduction by heterogamous gametes. Zoogonidia with an anterior circle of cilia. Ex- clusively freshwater. south of France; P. maximus Lagerh., P. Philodendri Lagerh. and P. Alocasice Lagerh. are parasiteson the leaves of species of Arisarum, Philodendrum and Alocasia in Ecuador (vide Lagerheim in Nuova Notarisia, 1892, pp. 120 — 124). Trichophilus Weber is a genus of Alg® parasitic on the hairs of Brady pus (the Three-toed Sloth) ; another species has also been found on species of Nenia (Clausilia) ; cf. Lagerheim in Berieht. der Deutsch. Bot. Gesellsch. 1892, Bd x, Heft 8, pp. 514—517. 1 Richter in Flora, lxxv, 1892. 2 J. Comere in Nuova Notarisia, xiv, 1903, pp. 18 — 21. 3 G. S. West in Journ. Bot. July, 1902, pp. 242 — 243. 56 Chlorophycece Order II. Ghcetophorales. Thallus filamentous, sometimes simple, but more often branched. Cells uninucleate; chloroplasts parietal, generally single and with pyre- noids. Sexual reproduction either isogamous or heterogamous. Mostly freshwater. Order III. Ulvales. Thallus expanded, membranous, paren- chymatous, attached when young. Cells uninu- cleate ; chloroplasts single, parietal, with one pyre- noid. Sexual reproduction isogamous. Mostly marine. Order IV. Schizogoniales. Thallus filamentous, sometimes parenchymatous, or expanded by fusion of filaments in one plane. Chloroplast single, central and substellate, with one pyrenoid. Mostly subaerial. Order V. Microsporales. Thallus filamentous, unbranched. Cells uninucleate, with a large, parietal, reticulated or band-like chloroplast, destitute of pyrcnoids. Exclusively freshwater. Order VI. Cladophorales. Thallus filamentous, simple or branched, incompletely septate. Segments large with numerous parietal chloroplasts each with a pyrenoid. Sexual reproduction isogamous or hetero- gamous. Marine or freshwater. Order VII. Siphonece. Thallus filamentous and coenocytic, unseptate, consisting of one large branched cell with many nuclei. Chloroplasts numerous, without pyrenoids. Sexual reproduction heterogamous. Mostly marine. Order VIII. Conjugatce. Thallus unicellular or filamentous. Cells uninucleate ; chloroplasts single or several, usually large and of some definite shape, with pyrenoids. Sexual reproduction by isogamous aplanogametes. Exclusively freshwater. Order IX. Protococcoidece. Small unicellular, multicellular or colonial Algae. Cells uninucleate or coenocytic ; chloroplasts very variable in form, size and disposi- tion, with or without pyrenoids. Sexual reproduc- tion of an isogamous or heterogamous character is known in some. Almost exclusively freshwater. CEdogoniacetv 57 Order I. GEDOGONIALES. In this order the thallus consists of fixed, simple or branched filaments. The cells possess a single nucleus and the chloroplast is a parietal, more or less cylindrical, anastomosing mass of chloro- phyll, containing one or more pyrenoids. The vegetative division, in which a curious interpolation of new pieces of cell-wall takes place, is peculiar to the order. The zoogonidia are also anomalous, being characterized by a circlet of numerous cilia round the anterior end. In the autumn, plants of this order frequently have their cells packed with starch. The sexual organs are well-dif- ferentiated oogonia and antheridia, and the sexual reproduction is greatly specialized. There is only one family which includes three genera, two of which are abundantly found in the British Islands. Family 1. CEDOGONIACE^E. This family is represented in the British Isles by numerous species of the two widely distributed genera (Edogonium and Bulbochcete. The young stages of these plants possess well-de- veloped organs of attachment, but most of the species of CEdogo- nium float freely in the water when adult. The thallus is simple or branched and some of the cells exhibit a peculiar transverse striation at their upper extremities. This is particularly notice- able in the large species of (Edogonium and is the result of inter- calary surface growth. Beneath one of the transverse cell-walls an annular cushion of cellulose is deposited, and after each division a circular split is formed in the cell-wall opposite this cushion, the two parts remaining very slightly separated by a new piece of cell- wall derived from the cushion of cellulose. The rings or cushions of cellulose were investigated by Him1, who found that they consisted of a central mucilaginous mass, surrounded by a coating of cellulose formed as an inner cell-wall layer, which becomes intimately concrescent with the old membrane above and below the ring. After each division another slit is formed beneath and close to the first one, the process being repeated until the upper end of the cell frequently presents the appearance of having a number of ‘caps’ placed one over the other (figs. 13 B and C; 14 A), each ‘cap’ indicating a division of the mother-cell. 1 Hirn in Acta Soc. Scient. Fennicac, xxvii, 1900. 58 Chlorophycece Most of the cells in Bulbochcete are furnished with long tubular bristles and the terminal cell of the filament in one or two species of CEdogoniuvi also ends in a long bristle (fig. 14 C). There is one large chloroplast in each cell disposed in the form of a cylindrical net-work, a large proportion of it forming anastomosing cushions on the inner surface of the cell-wall. The pyrenoids vary from one to several according to the species, and sometimes the number varies in different cells of the same plant. There is usually one nucleus with a prominent nucleolus (fig. 10 J n), situated in a more or less central position. The nucleus occasionally divides without a corresponding division of the cell. Growth of the filaments takes place by the transverse division of any of the vegetative cells. Fig. 10. A— I, (Edogonium sp., from Frizinghall, W. Yorkshire, showing stages of one type of development from a zoogonidium in which the basal cell does not become greatly swollen ( x 4G0). p, pyrenoid. J, (Edogonium sp., from Shipley Glen, W. Yorkshire, after treatment with Acetic Acid and Hsematoxylin, show- ing nuclei (n), x460. 59 (Edogoniacece Asexual reproduction takes place by means of zoogonidia, which are formed singly from ordinary vegetative cells. There is a re- juvenescence of the entire cell-contents, a large rounded mass being formed, which ultimately escapes. In CEdogonium this process may take place in any of the vegetative cells of the filament, whether terminal or not, and it sometimes occurs in a young plant consisting only of one cell. The cell-wall splits into two halves by a transverse slit near its upper extremity and the rounded mass of rej uvenized protoplasm makes its exit in a delicate hyaline vesicle. This mass assumes a pyriform shape and at the narrower end a small colourless protuberance is formed, round the base of which arises a circle of numerous cilia (fig. 11 z). This striking zoogonidium, which may or may not possess a red pig- ment spot, quickly swims away, the entire process lasting only a few minutes. On coming to rest it attaches itself by its anterior hyaline end, loses its cilia, and develops a 8ium- A> CEdogonium jjoscu (ne 1 Cl.) Wittr., from near Senens, Corn- cell-'wall, inis cell ultimately forms wall. B, (E . Himii Gutw., from a new filament by transverse cell- Churchill, Donegal, Ireland ( x 460). division (fig. 10 E — I). The basal cell may be rounded and swollen or it may develop a hapteron or organ of attachment (fig. 10 A — D). Wille has observed resting-spores in some species of CEdogonium1. The sexual reproduction in this family of Algae presents a greater specialization of the male and female organs than is found in any other family of the green Algae. The oogonia may be developed from any of the ordinary vegetative cells, and most frequently arise from cells which exhibit intercalary surface growth at their upper extremities. They are usually spherical or ovoidal in form and occur singly or in series of from 2 to 10. The contents of each oogonium become rounded off, forming a single oosphere 1 Vide Bot. Centralbl. xvi, 1883. Fig. 11. The escape of the zoo- gonidium (z) from its zoogonidan- 60 Chlorophycece which contains much chlorophyll. The antheridia may be developed m the same filament as the oogonia, as in the i nonoecious species (fig. 12), or they may arise in separate male filaments, as in the dioe- cious species (figs. 13 and 14). The antheridia are sometimes unicellular, consisting of a short cell rather narrower than the ordinary vegetative cell and containing less chlo- rophyll. More frequently, however, they consist of more than one cell, and occasionally of a dozen or more, the contents of each antheridial cell dividing into two masses each of which becomes an anther- ozoid. Rarely only one antherozoid is produced in an antheridial cell. The antherozoids are similar in form to the zoogonidia and are ciliated in the same way, but they are smaller and contain less chlorophyll. Dioecious species in which the male filaments are large and but little inferior in size to the female filaments are said to be dioecious macran- drous (fig. 13). There is, however, another type of dioecious species in which the male plants are very small and are attached to the female plants ; these are said to be dioecious nanncindrous (fig. 14). This type requires a further description. Certain short cells are produced in Fig. 12. Monoecious species of (22 dogonium. A, a form of (E. obsoletum Wittr., from near Goring, Oxfordshire. B, (22. zig-zag Cleve var. robustum West & G. S. West, from Harefield, Middlesex. C, <22. Itzigsohnii De Bary var. minor West, from the Orkney Is. D, (22. Ahlstrandii Wittr., from Pilmoor, N. Yorkshire (x460). oo, oogonium ; a, antheridium. (Edogoniacece Cl the female filaments either singly or in chains, each cell being larger than the antheridial cells of the monoecious or dioecious macrandrous species, and known as an androsporangium. The androsporangium is usually produced in the neighbourhood of an oogonium and becomes the mother-cell of a motile ciliated spore known as an androspore, intermediate in size between an antherozoid and a zoogo- nidium. Each androspore swims about for a time and then attaches itself to the female plant, either actually on the oogonium or on some adjacent cell. It then sur- rounds itself with a cell-wall and grows into a very small male plant known as a ‘dwarf- male’ or a nannandrium. The dwarf-male usually con- sists of a basal vegetative cell which supports one or more antheridial cells, but occasionally it is reduced to one antheridial cell only. Two antherozoids arise in each antheridial cell as in the ordinary monoecious and dioecious species, and they are set free by the splitting , . ° Fig. 13. Dioecious macrandrous species of off of a cap if there be only (Edogonium. A, male plant of (E. rufescens one antheridial cell, or by Wittr- £om ^illy Is. B, female plant of J same. C, female plant of CE . lautummarum the general dismemberment Wittr., from Welsh Harp, Middlesex. D, male of the antheridium if there plant same ( x 460). oo, oogonium; a, an- thendium. are several antheridial cells. When the oosphere is ready for fertilization a hyaline receptive spot appears in it at a point opposite that part of the wall of the oogonium which will open. The oogonium opens in many ways but the method of opening is constant for any one species. Some- times a circular crack is formed, which may be median, superior, or inferior ; sometimes a pore arises either in a superior or inferior 62 Chlorophycece Fig. 14. Dioecious narmandrous species of ( Edogonium . A, a form of CE. undula- tum (Breb.) A. Br., from Pilmoor, N. Yorkshire. B, in a thick, cylindrical, mucous in- vestment. This outer gelatinous coat varies in its relative size, and is always hyaline and homo- geneous. The cells are commonly oblong-cylindrical with broadly- rounded extremities, and they contain a single parietal chloro- plast usually disposed as an equatorial band. One pyrenoid is generally present in each chlo- roplast, although rarely pyrenoids are quite absent. The genus is very closely allied to Ulothrix, but is distinguished by its thick gelatinous coat and by the con- stant separation of the cells after division. Cienkowski and others have regarded the genus as a mere state or condition of Ulothrix, but I think that is open to much doubt. The cell-wall is extremely thin and delicate, and the formation of zoogonidia has not been observed. Fig. 22. A, Hormospora mutabilis Br6b., from near Mullion, Cornwall. B, H. ordinata West & G. S. West, from Cam Fell, W. Yorkshire. C — E, Gloeotila prntopenita Kiitz., from Pil- moor, N. Yorkshire. ( x 440.) The most frequent British species is 11 . mutabilis Breb. (fig. 22 A), which occurs principally in bogs, especially amongst Sphagnum , and in such localities species of Ulothrix do not usually exist. The cells are 16 — 19 p in diameter and 1-|- — If times longer than broad. II. ordinata West & G. S. West (fig. 22 B) is a smaller and much rarer species with cells 5-8 p in diameter. H. plena Breb. is the only other British species. Genus Glceotila Kutz., 1843. This is a genus of small Algae intermediate in character between Hormospora and Ulothrix. The cells are very small, oblong or elliptical and more or less moniliform, but they are not so completely separated as those of Hormospora ; they are arranged in a single series in a delicate mucous envelope, and each one contains a parietal chloroplast of small size which is disposed as in Hormospora. I have not 7 8 Cliloropliyceai observed any pyrenoids in the chloroplasts of this genus. Borzi1 has recently described the formation of zoogonidia. The cells of G. protogenita Kiitz. (fig. 22 C — E), which is the typical species of the genus, only reach a diameter of 3 — 4'5 g. The plants are very rare and occur in bogs or boggy pools. Genus Geminella Turp., 18282; Lagerh., 18833. [? Planctonema Schmidle, 1903.] This genus is scarcely to be distinguished from Hormospora Breb. except for the arrangement of the cells in pairs. The cells are fairly remote, are enveloped in a thick mucous coat, and after division the daughter-cells separate only very slightly. The chloroplast is exactly as in Hormospora. The cells of some of the filaments occasionally develop thick brown cell-walls and become resting akinetes. G. interrupta Turpin (fig. 23 A — C) is the only known species and it is rarely found in the British Isles. The cells are 6‘5 — 7 g in diameter. It would perhaps be more correct to unite the genera Geminella and Hormospora, It is impossible to find any generic characters sufficient to separate Planctonema Schmidle from Geminella or Gloeotila. Genus Radiofilum Schmidle, 1894. The filaments are simple, some times short and fragile, some- times long and flexuose, and they are enclosed in a considerable mucous sheath, which exhibits a more or less distinct radiating fibrillar structure. The cells are globose, ellipsoid, or sublenti- cular, free and distant or joined by a narrow hyaline bridge, always forming moniliform filaments after the manner of those of the Nostocaceae. In each cell there is one chloroplast containing a single pyrenoid. In one species the cell-wall is composed of two 2 Turpin in Mfim.Tu Mus. d’hist. nat. 1828, tom. xvi, p. 329, t. 13, f. 24. 3 Lagerh. in Ofvers. af K. Vet.-Akad. Forb. 1883, no. 2. the former having priority. ^ - m * , Fig. 23. A— C, Geminella interrupta Turp. ; A aud B, from near the Lizard, Cornwall ( x 440) ; C, two resting a,ki- netes from Glen Tummel, Perthshire, Scotland ( x 350). D, Radiofilum flaves- cens G. S. West, from Wicken Fen, Cambridgeshire ( x 440). 1 ■Rr.viri < SOirli Alffolocrioi II.’ Ulotrichacecv 79 equal halves, but in others it is not. The cells multiply by division which is preceded by a division of the chloroplasts and pyrenoids. The type species of the genus, It. conjunctivum Schmidle1, has not been observed from Britain. It. Jlavescens G. S. West (tig. 23 D) is a larger species, with much longer flexuose filaments and broad elliptical cells ; the diameter of the cells is 7 '5 — 10 -5 jx and the chromatophores are of a yellowish- green colour. Genus Stichococcus Nag., 1849. [Hormococcus Chodat, 1902.] The filaments are entirely or very largely aerial, and are composed of cylindrical cells. They readily become disarticulated into frag- ments composed of a few cells, the extremities of the terminal cells being broadly rounded. There is one parietal chloroplast in each cell, plate-like or more or less irregular in form, and occupying as a rule only a portion of the cell-wall. A small pyrenoid is present in each. In some of the aquatic forms of Stichococcus flaccidus I have observed a fragmenta- tion of the chloroplast (cf. fig. 24 B). The plants are propagated by division of the cells and fragmen- tation of the filaments, by akinetes, and by zoogonidia. The latter are bi- ciliated, have no pigment spot, and arise singly from cells which are under- going rapid division. This genusmuch resembles Ulothrix, but its adaptation to an aerial existence has caused a multiplication by dis- articulation of the filaments and a reduction almost to a unicellular con- dition. This disarticulation often takes place first on one side and then on the other, giving a zig-zag appearance to the disarticulated filaments. Klebs has shown that it is facilitated either by too much or too little nourishment. The genus was well studied by Gay2. There appears to be no justification whatever for Chodat’s name “Hormo- coccus.” *ii**«f % J> Big. 24. A, Stichococcus bacillaris Nag., from Saltaire, W. Yorkshire. B, S. flaccidus (Kiitz.) Gay, from Barnes Com- mon, Surrey. C, S. dissectus Gay, from damp walls, London. D, S. variabilis West & G. S. West, from Bradford, W. York- shire ( x 440). 1 Schmidle in Flora, 1894, Heft 1, p. 47, t. vii, f. 4 5. Paris, 1891. EeCherCheS SUr 16 d4ve1, et leS classif- de /x). They are all very delicate, with thin cell-walls, and most of them can only be identified with certainty from living specimens. The zygospores of few of them are known and they are rarely met with ; the outer layer of the wall of the zygospore is usually ornamented. Most of the species occur in peat-bogs. Genus Mesotsenium Nag., 1849. The cells are cylindrical or subcylindrical, generally straight or slightly curved, and are with- out any trace of a median constriction. The apices are, as a rule, broadly rounded. The chloroplast, of which there is usually only one in a cell, is a flattened, axile plate extending from end to end of the cell, and there may be one or several pyrenoids. Sometimes there are two flattened chloroplasts. The cells often contain numerous globules of an oily nature. There are ten British species of the genus, seven of which occur as mucilaginous masses amongst mosses and hepatics, generally on wet rocks. The largest of these is M. Be Greyi Turn. (fig. 54 D; length 74 — 125 p; breadth 15 '5 — 30 p) and the smallest is M. macrococcum (Kiitz.) Roy & Biss, var. micrococcum (Kiitz.) W. & G. S. West (length 13 '5 — 15 ‘3 p ; breadth Desmidiaceai 155 8-6 p). M. caldarioruin (Lagerh.) Hansg. is an attenuated species which occurs in greenhouses, forming a thin mucilaginous stratum on damp walls, etc. (length 27—46 p ; breadth 10-5— 11*5 p). M. Endlicherianum Nag. is the most frequent of the free-floating forms (length 25 — 27 p ; breadth 8 5 9 o p), and M. purpureum W. & G. S. West and M. violascens De Bary contain phycoporphyrin. The zygospores of species of this genus are generally poly- hedral with thick brown walls. Fig. 54. A, SpirotcBnia condevsata Breb., from Rhiconick, Sutherland ( x 334), showing parietal chloroplast. B, Sp. obscura Ralfs, from Terrington, N. Yorks. ( x 435), showing the axile, cristate chloroplast. C, zygospore of Sp. truncata Arch. ( x 250, after Archer). D, Mesotcenium De Greyi Turner, from near Settle, W. Yorks. ( x 435). E and F, M. maerococcum (Kiitz.) Roy & Biss., from near Giggleswick, W. Yorks. ( x 334). G, zygospore of M. chlamydosporum De Bary, from Mayo, Ireland ( x 334). H and I, Cylindrocystis Brebissonii Menegh., from Helvellyu, Westmoreland; H, vegetative cell; I, zygospore ( x 435). J, Cyl. diplospora Lund., from Galway, Ireland ( x 435). K, Netrium Digitus (Ehrenb.) Itzigsh. & Rothe, from Moel Siabod, N. Wales (x435). Genus Cylindrocystis Menegh., 1838. The cells are straight and cylindrical, averaging twice longer than the diameter, and they may or may not possess a slight median constriction. The 150 Chlorophycece apices of the cells are generally rounded or truncately rounded. There is a single axile, substellate chloroplast in each semicell and in the centre of each chloroplast is a large pyrenoid. The radiating prolongations of the chloroplast vary in number in the different species, and often become flattened against the interior of the cell-wall. The most abundant species is Cyl. Brebissonii Menegh. (length 43 — 55 p ; breadth 15 — 18 p, fig. 54 H and I) which occurs in quantity in upland Sphagnum-bogs and in peaty pools. It sometimes occurs in pure masses both amongst Sphagnum and on wet peat. Cyl. crassa De Bary also occurs in Sphagnum-bogs and amongst other Algae and mosses on wet rocks. Cyl. diplospora Lund, prefers the .waters of lakes and quiet pools, and is a much rarer plant ; one form of it — var. major West — is the largest member of the genus; length 102 — 114 p; breadth 48 — 54 p. Genus Netrium (Nag., 1849). A genus with straight, cylin- drical, oblong-cylindrical or fusiform cells, and without any median constriction. The chloroplasts are two (in one species four) in number, one (in one species two) in each semicell ; each chloro- plast is axile with about six radiating longitudinal plates. These plates are deeply notched along their free edges in all except Netrium inter r upturn. There are several pyrenoids in each chloro- plast, generally small and scattered. The plants placed in this genus are excluded from the Placoderm Desmids, and therefore from the genus Penium, on account of the structure of their cell- wall, which is unsegmented, without pores, destitute of a differen- tiated outer layer, and quite smooth. jY. Digitus (Ehrenb.) Itzigsh. & Rothe is one of the most abundant Desmids of elevated boggy moorlands and varies much in form and size; length 130 — 387 p ; breadth 40 — 82 p ; fig. 54 K. JY. oiblongum (De Bary) Liitkem. occurs in similar situations but is not quite so frequent ; length 96 — 135 p ; breadth 32 — 33 p. JY. interruptum (Breb.) Liitkem. is cylindrical with obtusely conical apices, and each chloroplast is transversely segmented, so that there are four chloroplasts in each cell arranged in an axile series. In this species the free edges of the plates are not notched ; length 220- — 320 p; breadth 37 — 64 p. Sub-family II. PLACODERMJS. This group includes the great majority of Desmids and is characterized by the segmented cell-wall with its differentiated outer layer. The cell-wall usually possesses pores, but this is not invariably the case. There is always a very distinct line of demarca- tion between the old and new semicells, the younger portions of Desmidiacece 157 the cell-wall not being continuous with the older portions, but joined to them by a narrow, oblique or bevelled surface. The cell- division is of a fixed type, taking place strictly in the manner of previous cell-divisions, and the younger semicells are interpolated between the older ones. The sub-family can be divided into two sections, one of which includes those Desmids in which cell-division does not always take place at a fixed point and those in which it does always take place at a fixed point known as the isthmus. A. Point of division of cells variable or sometimes fixed at the isthmus. Tribe 3. Peniece. This tribe only includes one genus, in which the cells are solitary, of moderate length, straight and generally cylindrical. Sometimes, but not always, there is a slight median constriction. The points of division are often variable, although the actual cell- division is of the same type. The cell-wall may be with or without pores, and the cell often grows periodically until it becomes adult. Genus Penium Breb., 1844. This genus is difficult to strictly define, as it undoubtedly contains many species which will ulti- mately have to find a resting-place elsewhere. The cells are straight, cylindrical, subcylindrical, ellipsoidal, or fusiform, and the apices may be rounded or truncate. The vertical view is always circular. The inner layer of the cell-wall is in many forms orna- mented with striations, punctulations or granulations, but in others the cell-wall is quite smooth. There is one axile chloroplast in each semicell, with radiating longitudinal plates which have the free margin entire. The pyrenoids are one or many and uniseriate. Sometimes there is a prominent vacuole near each extremity of the cell containing moving granules. There are about 27 British species of Penium, of which P. Libellula (Focke) Nordst. (length 250 — 354 /x ; breadth 38 — 51 g ; fig. 55 D) is one of the largest and most striking. P. mcirgaritaceum (Ehrenb.) Breb., P. Cylindrus (Ehrenb.) Breb. (fig. 55 A and B) and others, are granulated species, and P. spirostriolatum Barker (length 123 — 274 g ; breadth 23 — 26 g ; fig. 55 C) is a species with spiral striations on the cell-wall, which often anastomose or become broken up into dotdike thickenings. P. polymorphum Perty (length 55—58 g ; breadth 25 — 27 /x) is one of the most general of the upland, moorland species, and some of the smallest species of the genus are P. inconspicuum West (length 14-5 — 19 fi ; breadth 5 — 5’7 g), P. minuiissimum Nordst. (length 12'2 — 13 g\ breadth 158 Chlorophycece 6'8— 7'3 fi) and P. suboctangulare West (length 10‘7— 11\5 M ; breadth 6'8— 7 /x ; fig. 55 E). The cell-wall of many of the species is of a yellow or reddish- brown colour. P. minutum (Haifa) C'leve is relatively the longest species of the genus, but should, perhaps, be rele- gated elsewhere. Tribe 4. Closteriece. The cells are elongate, generally curved, and usu- ally attenuated towards each extremity. There is no trace of a median con- striction and the plants are circular in transverse sec- tion. The points of division are always in the middle region of the cell and the cell-wall usually possesses pores. The cells are only symmetrical in one longi- tudinal plane. Genus Roya West & G. S. West, 1896. This genus was named after the late Dr John Roy, who first pointed out the differences between those plants which are now included in it and Fig. 55. A and B, Fenium Gylindrus (Ehrenb.) Br6b. ; A, from Loch Minnoch, Kirkcudbright; B, zygospore from Thursley Common, Surrey ( x 474). C, P. spirostrio- latuvi Barker, from Kerry, Ireland (x474). D, P. Libellula (Focke) Nordst., from Bowness, Westmoreland (x200). E, P. suboctangulare West, with zygospore, from Killarney, Kerry, Ireland ( x 365). F, P. curtum Br6b., from Grimspound, Devonshire ( x 474). G and H, Roya obtusa (Br6b.) W. & G. S. West var. montana W. & G. S. West, from Baildon Moor, W. Yorks. ( x 570). I, R. Cambrica W. & G. S. West, from Llyn Ogwen, N. Wales (x474). J and K, R. Pseudoclosterium (Boy) W. & G. S. West, from Pilmoor, N. Yorks. ( x 474). species of Closterium. The cells are elongate, cylindri- cal, very slightly curved and scarcely atten uated towards the poles. There is only one chloroplast which occu- pies almost the entire cell- cavity. It extends to within a very short distance of each pole and its extremi- ties are convex. In the Desmidiacece 159 median portion of the cell there is a slight lateral excavation in the chloroplast for the lodgement of the nucleus, which is thus asymmetrical. There are from four to thirteen pyrenoids in the chloroplast, arranged in a single series. The cell-wall is relatively thin and without pores. The genus is readily distinguished from Glosterium by the absence of any marked attenuation towards the apices and by the single chloroplast with convex extremities, the latter being in such close proximity to the ends of the cell that there is no room for an apical vacuole. The lateral position of the nucleus is also characteristic. There are only three species of the genus, all of which occur in Britain. R. obtusa (Breb.) W. & G. S. West (and its var. montana; tig. 55 G and H) is not an uncommon Desmid in mountainous districts; length 48 — 117 y.; breadth 5*5 — 12-5 y. R. Pseudoclosterium (Roy) W. & G. S. West is a very narrow species of rare occurrence ; length 98 — 192 y ; breadth 2'6 — 3 y ; fig. 55 J and K. R. Cambrica W. & G. S. West is only known from N. Wales; length 173 — 177 y, breadth 6'2 — 6-7 y.; fig. 55 I. Genus Closterium Nitzsch, 1817 \ The cells are elongate, generally distinctly curved, and often markedly lunate or arcuate. In most cases they are strongly attenuated towards the poles, the latter being obtuse, truncate, rostrate, or drawn out into long needle-like points. In most of these plants the cell-wall possesses pores and in a large number of species it is striated, the striations being internal thickenings of the cell-wall running from pole to pole and disposed at regular intervals. The number and strength of the striations varies very much in different species of the genus. There are two chloroplasts, one in each semicell, and there may be one or many pyrenoids in each chloroplast. The extremities of the chloroplasts are concave and they do not reach the apices of the cell, thus affording room for prominent apical vacuoles which contain one or many moving granules of variable size and shape. In those species in which the apices are greatly produced the chloroplasts do not extend into the prolongations, 1 The name “ Arthrodia ” (Raf. in Desv. Journ. 1813, i, p. 235) cannot possibly be utilized as a generic name in place of Closterium. Rafiuesque’s diagnosis applies equally to Ankistrodesmus, Penium, Bocidium, Pleurotcenium , Cylindrocystis, and Tetmemorus. Nordstedt (in Hedwigia 1893, Heft 3, p. 148) has clearly pointed out that “ Arthrodia ” must always remain a “ genus ignotum ” and a “ nomen delen- dum.” The same author’s remarks should also be consulted concerning “Gy yes Ehrenb.,” “ T-Ielierella Bory,” “ Ursinella Turpin,” “ Prolifer a Vaueh.” and “ Con - jugata Vauch.” 160 Chlorophycece and the apical vacuoles are often correspondingly lengthened out. The chloroplasts are similar in form to those of the genus Penium, consisting of an axile mass with more or less distinct longitudinal Fig. 56. A, Closterium acerosum (Schrank) Ehrenb., from Glen Shee, Perthshire ( x 200). B— D, Cl. striolatum Ehrenb., from Frensham, Surrey; B and C, outlines, x 125 ; D, x 260. E, Cl. acuturn Bret., from Sligachan, Skye ( x 474). F, Ol.parvulvm Nag., zygospore, from Esher West-end Common, Surrey ( x 474). G, Cl. rostratum Ehrenb. var. brevirostratum West, zygospore, from Wimbledon Common, Surrey ( x 200). ridges. In Cl. acuturn Breb.‘ and several others there are no longitudinal ridges. The curvature of many of the species of this genus is very constant and may be utilized as a specific character. In stating Desmidiacecv 161 the measurements of a species, the diameter across the middle of the cell should be given, the distance between the apices, and the curvature of the outer margin expressed in degrees of arc. The zygospores are generally globose and smooth, but the spherical zygospore of Cl. calosporum Wittr. is furnished with conical papillations. The zygospores of certain species, such as Cl. rostratum Ehrenb. (fig. 56 G) and Cl. Kutzingii Breb., are flattened and quadrate with truncate angles; that of Cl. Venus Ktitz. is also angular and sometimes twisted. There are about 60 British species of the genus, of which Cl. turgidum Ehrenb. (length 476—940 g ; breadth 43 — 80 \x) is the largest and Cl. pusillum Hantzsch var. monolithum Wittr. (length 29'8 — 40 '4 g; breadth 7 '5 — 8'6 g) the smallest. Cl. aciculare Tuffen West and C. pronum Breb. are the most elongate species of the genus, and certain forms of Cl. ucutum Breb. (fig. 56 E) are the narrowest. The commonest and most widely distributed species are Cl. parvulum Nag. (fig. 56 F), Cl. Venus Kiitz., Cl. Leibleinii Kiitz. (fig. 51 B), Cl. moniliferum (Bory) Ehrenb., Cl. Elirenbergii Menegli. (fig. 51 A) and Cl. acerosum (Schrank) Ehrenb. Cl. gracile Breb. is often abundant in Sphagnum bogs. The commonest striated species are Cl. striolatum Ehrenb. and Cl. rostratum Ehrenb. Liitkemiiller has divided the genus — and I think quite correctly — into two groups, the first one being characterized by the possession of an elongated median girdle, which is an intercalation of a cylindrical piece of cell-wall resulting from a growth to maturity subsequent to cell-division. The second group is characterized by the absence of this girdle, the two daughter-cells becoming adult immediately after cell-division. B. Point of division of cells always fixed a,t the isthmus. Tribe 5. Cosrnariece. The great majority of Desmids are included in this tribe and amongst them there is great diversity of form and size. They are solitary or filamentous, or grouped in spherical colonies, and most of them possess at least three planes of symmetry. The cell-wall consists of two thin, very firm layers, with pores, and is frequently ornamented with spines, warts and excrescences of all kinds. There is no periodical growth, the cell becoming adult immediately after division by the mature growth of the younger semicell. Series a. The oblique junction of the new and old halves of the cell-wall (at the region of the isthmus) remains quite plane during division. The plants are solitary or colonial. W. A. 11 102 Chlorophycece Genus Docidium Breb., 1844 ; em. Lundell, 1871. The cells are comparatively small, elongate, straight and slightly constricted. They are subcylindrical with an inflation on each side of the constric- tion, or nodulose from pole to pole. The base of each semicell is plicated and there is a small basal granule under each plication. The basal plication of the semicells is the principal point of distinction between this genus and Pleurotcenium. The apices of the cells are always truncate and smooth. There is a central chloroplast in each semicell, very irregular in form, and containing an axile row of pyrenoids. The zygospores are unknown. There are three British species, of which I). Baculum Breb. is the only one that is widely distributed, and even it is distinctly rare ; length 167—262 p ; breadth at basal inflation of semicells 12 — 13 p ; tig. 57 A — C. D. undulatum Bail, is restricted to the western districts of Ireland and Scotland, and the semicells have a nodulose appearance caused by repeated shallow constrictions from base to apex; length 187 — 262 p; breadth at basal inflation of semicells 13 — 16 p ; fig. 57 D and E. Genus Pleurotsenium Nag., 1849. The cells are elongate, straight, and slightly con- stricted. They are cylindrical, with or without inflations on either side of the constriction, or they may be nodulose along their entire length. They are slightly attenuated towards each pole and the apices are usually truncate, often being furnished with a ring of conical, tooth-like projections. There is no basal plication of the semi- cells. The chloroplasts are disposed as longitudinal, parietal bands, several in each semicell, and are furnished with a number of pyrenoids. Sometimes they become broken up into numerous parietal pieces, each containing a single pyrenoid. In most of the species the central portions of the cell contain large vacuoles and occasionally numerous moving granules are observed in the terminal or subterminal ones. These granules are of a yellow colour and black when seen in mass, differing in this respect from the moving granules normally present in the genus Glosterium. They are Fig. 57. A — C, Do- ci diumB aculum Br6b . , from Bowness, West- moreland ; A, x 315 ; B, base of semicell, x 630 ; C, basal view of semicell, x 630. I) and E, D. undu- latum Bail., from near Oughterard, Gal- way, Ireland ; D, x 315 ; E, base of semicell, x 630. Desmidiacece 163 produced mostly by pathological conditions. Zygospores of few species are known; they are globose and smooth. Fig. 58. A, Pleurotcenium coronation (BiAb.) Rabenh., from Helvellyn, West- moreland ( x 236). B, zygospore of PI. Ehreribergii (Breb.) De Bary, from Thursley Common, Surrey (x315). (Breb) Ralfs, from Lough Anna, Donegal, Ireland. A, front view of cell ; B, zygospore ( x 315). Species of this genus are much more abundant in the tropics than in temperate regions, and the tropical species frequently assume a secondary filamentous condition. Only nine species are known as British, of which PL maximum (Reinsch) Lund, is the largest, reaching a length of 852 /x and a diameter of 54 /i. The most abundant species is PL Ehrenbergii (Breb.) De Bary ; length 240 — 400 p. ; breadth 18 — 24 p ; fig. 58 B. PL Trabecula (Ehrenb.) Nag. and PI. truncatum (Breb.) Nag. are each widely distributed. PI. nodosum (Bail.) Lund, is the most striking species and is confined to the western districts of Wales, Ireland and Scotland. Genus Tetmemorus Ralfs, 1844. This is a well-marked genus with straight cells of moderate length, slightly constricted in the 11—2 164 CMorophycece middle and deeply cleft at each apex. The cells are usually attenuated towards each pole and the apical cleft or incision is tightly closed. There is a central chloroplast in each semicell with a single axile row of pyrenoids. The zygospores are globose and smooth, or subquadrate with rounded angles. There are only four British species, T. granulatus (Breb.) Ralfs (length 138 — 238 g ; breadth 30 — 45 /x; fig. 59) and T. Icevis (Kiitz.) Ralfs (length 57 — 76 g ; breadth 19 — 25 /x) being the most widely distributed. The former, which possesses a sparsely scrobiculated cell-wall, is one of the most ubiqui- tous of Desmids and the latter has a distinct preference for mountainous districts. T. Brebissonii (Menegh.) Ralfs is a more or less frequent Desmid in bogs and pools containing submerged Sphagnum. Genus Euastrum Ehrenb., 1832. In this genus the cells are relatively shorter than in any of the preceding genera of the Fig. 60. A, Euastrum elegans (Breb.) Kiitz. (a form), from Cupel Curig, N. Wales ( x 474). B, zygospore of E. elegans, from New Forest, Hants. ( x 474). C, E. binale (Turp.) Ehrenb., from Thursley Common, Surrey (x474). D, zygo- spore of E. oblongum (Grev.) Ralfs, from Pilmoor, N. Yorks. ( x 200). /, front view; s, side or lateral view; v, vertical view. Cosmariete and they are always distinctly flattened. Seen from the front view they are elliptical, or narrowly elliptical, and they possess a median constriction of considerable depth. The lateral margins may be entire or lobed, and the apices are usually deeply notched. In the lobed species there is always an odd number of lobes to each semicell, the apical lobe (which bears the notch) being termed the ‘ polar lobe.’ In the vertical and side views Desmidiaceca 165 there is generally a well-marked protuberance in the middle of each side of the semicells. There is one axile chloroplast in each semicell, which is often very irregular ; occasionally it possesses irregularly disposed, longitudinal plates. There is a single central pyrenoid in the small species and several scattered ones in the large species. The zygospores are globose or ellipsoid, and are furnished either with numerous simple spines or with conical papillfe. There are 43 British species of the genus, about half of which are widely distributed. The smallest and perhaps the most abundant species of the genus is E. binale (Turp.) Ralfs ; length 10 — 20 p; breadth 9 — 16 p; thickness 5-5 — 7 p ; fig. 60 C. E. elegans Breb. (fig. 60 A and B), E. ansatum Ehrenb., E. Didelta (Turp.) Ralfs and E. oblongum (Grev.) Ralfs (fig. 60 D) are widely distributed all over the country, but in some districts they are by no means common. E. insigne Hass, (length 98 — 110 p; breadth 49 — 60 p) is an upland form found abundantly amongst Sphagnum. E. verrucosum Ehrenb. and E. gemmatum Breb. are two of the prettiest species, and E. crassum (Breb.) Kiitz. (length 167 — 185 p ; breadth 87 — 97 p) and E. oblongum (Grev.) Ralfs (length 144 — 167 p ; breadth 80 — 83 p) are the largest species found in Britain. E. pectinatum Breb. is more frequently found with zygospores than any other species of the genus. Genus Micrasterias Ag., 1827. [ Holocystis Hass., 1845 ; Tetrachastrum Dixon, 1859.] This genus contains some of the largest and prettiest of Desmids. The cells are flattened, in many cases almost disc-shaped, and they are circular or broadly elliptical in outline. The semicells are divided by deep incisions into either three or five lobes, of which the polar lobe may be entire or furnished with a wide excavation at the apex. The lateral lobes are sometimes narrow and attenuated, but more frequently they widen from the base outwards and are divided by incisions of variable depth into lobules. The flat surfaces of the cells are occasionally furnished with spines or wart-like projections, and in side or vertical view the cells are generally narrowly fusiform. There is a central, plate-like chloroplast in each semicell, often exhibiting irregular thickenings or ridges, and containing many scattered pyrenoids. The zygospores are globose and furnished with strong spines, simple or furcate at the apex. There are 16 British species of the genus, none of which are really abundant, although M. denticulata Breb. (length 205— 350 p; breadth 185— 276 p; fig. 61 C) and M. truncata (Corda) Breb. (length 100—138 p ; breadth 90— 129 p; fig. 61 B) are widely distributed. M. rotata (Grev.) Ralfs (length 220 240 p ; breadth 195 — 220 p) and M. papillifera Breb. (length 135 145 ^ ; 166 Chlorophycece breadth 115—145 p) are not uncommon in permanent boggy pools and lakes, whilst M. oscitans Ralfs var. mucronata (Dixon) Wille and M. Jenneri Ralfs are frequently found in the Sphagnum-bogs of mountainous areas. M. furcata Ag., M. Crux-Melitensis (Ehrenb.) Hass. (fig. 61 A) and M. pinnatifida (Kiitz.) Ralfs are amongst the rarest and most elegant species of the genus. Fig. 61. A, Micrasterias Crux-Melitensis (Ehrenb.) Hass., from Bowness, West- moreland ( x 365). B, M. truncata (Corda) Breb., from Thursley Common, Surrey (x200). C, zygospore of M. denticulata Breb., from Halgavor Moor, Cornwall ( x 110). Genus Cosmarium Corda, 1834. [. Dysphinctium Nag., 1849 ; Galocylindrus (Nag.) Kirchn., 1878; Cosmaridium Gay, 1884; Pleurotceniopsis (Lund.) Lagerh., 1887.] This is the largest genus of Desmids, embracing several hundreds of species, which although exhibiting many varied characters, all conform to a common type of structure. The cells are relatively short and the length is rarely more than one-and-a-half times the breadth. There is a median constriction of variable depth, in some cases very slight, but in others exceedingly deep. The semicells may be circular, elliptical, semicircular, ovate, pyramidate, or cuneiform in general outline, and the apex, which may be rounded or broadly truncate, possesses no apical notch. The cell-wall may be smooth, scrobiculate, granulate, verrucose, or clothed with minute spines, the orna- mentation in almost all cases being arranged upon some definite plan. There is frequently a protuberance of some considerable Desm/idiacecB 1G7 size in the middle of each face of the semicell, so that the vertical view, which is elliptical in some species, may in others be furnished on each side with a more or less prominent swelling. In the majority of species there is one central chloroplast in each semicell, possessing four somewhat curved longitudinal plates and either one or two large pyrenoids. In a few species there are Fig. 62. A, Gosmarium renifoime (Ealfs) Arch, (a form), from Wicken Fen, Cambs. ( x 473). B, C. granatum Br6b., from Chippenham Fen, Cambs. ( x 473). C and D, C. granatum var. subgranatuvi Nordst., from Hornsey Mere, E. Yorks. ( x 473). E — G, C. Meneghinii Br6b.; E and F, from Hornsey Mere, E. Yorks.; G, zygospore of a form from Bowness, Westmoreland ( x 473). H, C. prce- morsum Br6b. , from Carrantuohill, Kerry, Ireland (x473). I and J, C. bioculatum Br4b. ; I, from Boswell Pits, Cambs.; J, zygospore from Puttenham Common, Surrey (x473). K and L, forms of C. Regnesii Eeinsch; L shows one stage of cell-division ( x 1170). M, G. isthmium West, from Harris, Outer Hebrides ( x 473). N, C. pseudoconnatum Nordst., from Capel Curig, N. Wales (x473). /, front view; s, side or lateral view; v, vertical view. several (from four to eight) parietal chloroplasts in each semicell, each containing one or more pyrenoids. Some investigators of these plants have attempted to establish Nageli’s genus Dysphinctium (= Calocylindrus Kirchn.) in order to include certain species which are best left in the old genus 168 Chlorophyceai Gosmarium. The genus Dysphinctium can never be recognized in a sound system of classification, as opinion must always remain divided even upon many of the commonest forms that different authors have included in it. Its characters are too indefinite and artificial to be of any real systematic value. Similarly, Fleur o- tceniopsis can never be established as a genus, as it would be a small, polyphyletic assemblage, including a few strangely incon- gruous species which occupy places far apart in the evolutionary scheme of the genus Gosmarium. It must be remembered that the primitive chloroplast of the Desmids is an axile one and that the parietal condition has been independently acquired in Gosmarium by a few scattered members of the genus. I have already stated1 that if the large genus Gosmarium is ultimately split up, the sections will not have to be based upon single characters alone, but upon combinations of characters of which the external form will be the most paramount. Until more is known concerning the distribution of species of Gosmarium, the genus is best left in its entirety. The zygospores may be globose, angular-globose, cubical, or almost of any outward form ; they may be smooth, scrobiculate, furnished with simple or furcate spines of variable length, or adorned with conical papilbe. In a few species, such as G. melano- sporum Arch., the spore-wall becomes perfectly black. As there are 250 British species of Cosmarium it is not easy to point out the chief peculiarities of the genus. The largest species which occurs in Britain, and also one of the rarest, is C. ovale Ralfs (length 182 — 188 p; breadth 100 — 107 p). The smallest is C. subretusiforme W. & G. S. West (length 7-8—8 p ; breadth 6’2— 6'5 p). The commonest species found in the ponds and ditches of lowland districts are C. Botrytis (Bory) Menegh., C. prce- morsum Breb. (fig. 62 H), C. subcostatum Nordst., 0. humile Gay, C. granatum Breb. var. subgranatum Nordst. (fig. 62 C and D), C. abbreviature, Racib. and several forms of C. Meneghinii Breb. In the bogs of moorland districts C. Cucurbita Breb. is usually abundant, and in upland Sphagnum areas C. Ralfsii Breb., C. pyramidatum Breb., C. subtumidum Nordst., and others, are fairly general. Some species, such as C. Holiniense Lund., G. anceps Lund., C. subspeciosum Nordst. and C. Pokornyanum (Grun.) W. & G. S. West, are usually found on dripping rocks, and C. Dovrense Nordst., C. microsphinctum Nordst., and several others, prefer wet calcareous rocks. C. pygmceum Arch, sometimes occurs in myriads amongst the leaves of submerged Sphagnum. Genus Xanthidium Ehrenb., 1834. The cells of this genus are somewhat flattened as in most species of Cosmarium, and the 1 G. S. West, ‘Alga-flora of Cambridgeshire,’ Journ. Bot. 1899, p. 115. Desmidiacece 169 median constriction is invariably deep. The semicells may be elliptical, trapeziform, hexagonal or octagonal in outline, and the angles are furnished with simple or furcate spines. The presence of these spines constitutes the primary distinction between Xctiithi- dium and Cosmarium, and they are usually arranged in one plane or in two parallel planes. In the centre of each semicell is a thickened, scrobiculate area, or a protuberance of variable size, and this character is the sole distinguishing feature between Xanthidium and Artlirodesmus. In a few species the spines are Fig. 63. A, Xanthidium armatum (Breb.) Babenh., from Sligachan, Skye ( x 365). B, zygospore of X. antilop eeum (Br6b.) Kiitz., from Ballynahinch, Galway, Ireland ( x 365). reduced to small apiculations at the angles. The chloroplasts are parietal in most of the species and are arranged in the form of cushions, four or six in each semicell, each with a pyrenoid. In some of the minute species there is a single, central chloroplast in each semicell, furnished with one pyrenoid. The zygospores are globose and adorned with blunt papillae or long slender spines, simple or furcate at their extremities. The genus was divided by Lundell in 1871 1 into two sub-genera; sub-genus Holocanthum, in which the spines are entire ; sub-genus Schizacanthum, in which the spines are forked at the apex. These 1 Lundell in Nova Acta Beg. Soc. Scient. Upsala, 1871, ser. 3, vol. viii, p. 74. 170 Chlorophycece were put forward as genera by Wille1, but upon insufficient grounds. Even in the most typical species which would fall under Schizacanthum, namely X. armatum, the spines are sometimes simple2, and in some of the tropical species there are numerous intermediate stages between simple and much-forked spines. There are 14 British species, of which a few are general but not abundant, and the rest are very scarce. X. armatum (Breb.) Babenh. is a large and handsome Desmid occurring in moderate quantity in the bogs of the hilly districts of the British Isles; length with spines 137 — 200 /x; breadth with spines 88 — 110 /x ; fig. 63 A. X. antilopceum (Breb.) Kiitz. (fig. 63 B) and X. cristatum Breb. are not uncommon in certain areas, and X. concinnum Arch., which is the smallest species of the genus (length 9 — 9'5 fx ; breadth without the apiculations 9’5 — 10-5 /a), sometimes occurs abundantly amongst Sphagnum. Species of this genus are regular constituents of the freshwater plankton. Genus Arthrodesmus Ehrenb., 1838. This genus is primarily distinguished from Xanthidium by the absence of the protu- berance or scrobiculated area in the centre of the semicells. As a general rule the spines are fewer in number than in Xan- thidium, and they are all disposed in one plane. The median constriction is deep and the semi- cells, which may be elliptical, trapeziform, or subhexagonal in shape, are furnished at the lateral angles with one or two spines of variable size and strength. In the verti- cal view the cells are always elliptical and the poles are furnished with spines. There is a single central chloroplast in each semicell contain- 1 Wille in Engler & Prantl’s Natiirl. Pflanzenfam. 1890, p. 11. 2 West in Jonrn. Linn. Soc. Bot. xxix, 1892, p. 164, t. xxii, f. 1. Fig. 64. A — C, Arthrodesmus Incus (Breb. ) Hass., from Harrop Tarn, Cumberland; C, zygo- spore ( x 365). D, A. Incus var. Ralfsii W. & G-. S. West, from Capel Curig, N. Wales ( x365). E, A. Incus var. validus W. &'G. S. West (a form), from near Glenties, Donegal, Ireland ( x 474). F and G, A. octocornis Ehrenb.; F, from Llyn Idwal, N. Wales; G, zygospore from Puttenham Common, Surrey ( x 474). H — J, A. bifidus Breb. var. truncatus West; H, from Strensall Common, N. Yorks.; I and J, from Keston Common, Kent ( x474). /, front view ; v, vertical view. Desmidiacecti 171 ing one or two pyrenoids. The zygospores are globular, and the outer surface may be smooth or clothed with simple, conical or subulate spines. Of the eleven British species, A. Incus (Breb.) Hass, is the only one that is general and abundant. It is a small species, frequently met with in the con- jugating state, and is one of the most variable of Desmids ; length without spines 13 — 33 fx ; breadth without spines 13 — 28 g; length of spines 5 — 32 g ; fig. 64 A — C. A. convergens Ehrenb. and A. octocornis Ehrenb. (fig. 64 F and G) are not uncommon, but the other species are all rare. Genus Staurastrum Meyen, 1829. This is the second largest genus of Desmids and contains some hundreds of species of very varied character, the majority of which possess a deep median constriction. The semicells are elliptical, semicircular, oblong, or cyathiform in outline, and the vertical view is generally triangular, but may be quadrangular or polygonal. The angles are sometimes rounded, sometimes acute, or they may be produced into processes of considerable length. In many cases the angles of one semicell alternate with those of the other. The cell-wall is occasionally smooth, but it is more frequently adorned with a variety of spines or wart-like excrescences, usually symmetrically disposed. In those species in which the angles are produced into processes the latter are generally furnished with two or three strong diverging spines at their extremities. There is usually one chloroplast in each semicell, consisting of a central mass with a number of radiating plates, varying from five to eight (usually six; two in each angle). One pyrenoid is present in each chloroplast. In a few species there are several chloroplasts in each semicell disposed in the manner of parietal cushions, but intermediate states between this condition and a central mass are not infrequent. This indefinite character was utilized by Lundell1 for the formation of a sub-genus, Pleurenterium, which has recently been proposed as a genus2. The absurdity of such a genus is realized on considering the few incongruous forms it would have to include. It should be borne in mind that the parietal condition of the chloroplasts has been arrived at quite independently by a few widely different species of the genus Staurastrum. The zygospores are globose or angular, rarely winged, but more commonly clothed with long spines, which are simple or furcate at 1 Lundell in Nov. Acta Beg. Soc. Scient. Upsala, 1871, ser. 3, vol. viii, p. 72. 2 Wille in Engler & Prantl’s Natiirl. Pflanzenfam. 1890, p. 11. 1 i 2 Chlorophycece their extremities, and often situated each at the apex of a mamillate or obtusely conical protuberance. Fig. 65. A and B, Staurastrum anatinum Cooke & Wills, from Llyn-y-ewm-ffynon, N. Wales (x473). C, St. punctulatum Breb., from Esher Common, Surrey ( x 473). D, St. polytrichum Perty, from Galway, Ireland ( x 365). E, St. elongatum Barker, from Rhiconich, Sutherland (x473). F, St. brachiatum Balfs, from Down, Ireland ( x 473). G, zygospore of St. furcigerum Breb., from Pilmoor, N. Yorks. (x473). /, front view; v, vertical view. There are more than 160 species of this genus known to occur in the British Islands, but few of them are abundant. The most frequent species in low-lying districts are St. pygmceum Breb., St. punctulatum Breb. (fig. 65 C) and St. hexacerum (Ehrenb.) Wittr. In moorland areas St. margaritaceum (Ehrenb.) Menegh. is general. The most abundant of the spiny species is St. teliferum Ralfs. One of the largest British species of the genus is St. tumidum, Breb. (length 112 — 132 g; breadth 91 — 103 g) and the smallest is St. iotanum Wolle. Some species, such as St. capitulum Breb., St. pileolatum Desmidiacece 173 Breb., St. Kjellmanii Wille (fig. 51 E), St. acarides Nordst. and St. Amelin Boldt, are principally confined to mountainous regions ; others, such as St. pelagicum W. & G. S. West, St. pseudopelugicum W. & G. S. West, St. jaculiferum West , St. paradoxuvi Meyen var. longipes Nordst., St. brevi- spinum Breb. and forms of St. anatinum Cke. & Wills (fig. 65 A and B), are abundant in the plankton of lakes. A few of the most beautiful species of the genus, amongst which may be mentioned St. Ophiura Lund., St. Cerastes Lund., St. Arctiscon (Ehrenb.) Lund., St. verticillatum Arch, and St. longi- spinum (Bail.) Arch., are confined to the extreme western districts of Scotland, Wales and Ireland, and are most abundant in the plankton of those areas. Genus Cosmocladium Breb., 1856. The cells of this genus are similar to those of some of the smooth species of Cosmariuvi, but the individual cells are united by relatively thin mucilaginous threads into branched colonies. Sometimes the entire colony is Fig. 66. A, Cosmocladium constrictum (Arch.) Josh., from Pilmoor, N. Yorks. ( x 475). B, C. pulchellum Breb., from near Tarbert, Harris, Outer Hebrides ( x 475). C, zygospore of C. perissum Roy & Biss., from the Clova Mts., Forfar ( x 475). B — F, Oocardium stratum Nag., after Liitkemiiller ( x 730). enveloped in a mucilaginous mass of much less density than the connecting threads. 1 here is one chloroplast in each semicell containing a single pyrenoid. The zygospores are globose and smooth, or they may be lobed (as in C. perissum Roy & Biss.). 174 Cliloropliycecu There are five British species of this genus all of which are very rare. C. constrictum (Arch.) Josh. (fig. 66 A) and C. pulchellum Br6b. (fig. 66 B) are perhaps more often observed than the others. All the species are very small, the largest being G. Saxonicum De Bary; length 15—17 p ; breadth 13-5— 14-5 jx. Genus Oocardium Nag., 1849. This is the most extra- ordinary of all the genera of Desmids and usually occurs in large colonies. The cells are small, slightly constricted and much depressed, being considerably broader than their length. The semicells are unequally depressed on the two sides, so that the plant is symmetrical in one plane only. The vertical view is broadly elliptical. There is one chloroplast in each semicell, con- sisting of somewhat irregular plates radiating from a central mass containing one pyrenoid. The colony is generally hemispherical in shape, 1 — 2 mm. in diameter, and occurs attached to calcareous rocks, not unfrequently being itself encrusted with calcium carbonate. It consists of a number of more or less parallel, radiating strands of mucus of considerable thickness, each strand widening out towards the surface of the colony and occasionally branching. In the free end of each mucous strand is lodged a single cell, disposed with its longitudinal axis at right angles to the axis of the mucous strand. The zygospores are unknown. The only known species is Oocardium stratum Nag. It is extremely rare and I have only observed it in the limestone districts of West Yorkshire, attached to rocks and stones in the beds of several mountain streams. Length 13—16-5 fx; breadth 18 — 19'5 fx ; fig. 66 D— F. Genus Sphserozosma Corda, 1835. The cells are small and attached to form long filamentous colonies, often twisted and sometimes enveloped in a mucous investment. The median con- striction may be deep and narrow or it may be widely open, and the semicells may be elliptical, oblong, or subrectangular in form. The attachment of the cells is apical and is effected by small rounded tubercles or short capitate processes. The vertical view is elliptical. There is one axile chloroplast in each semicell, furnished with a single pyrenoid. The zygospores, which are globose or oblong, are either smooth or furnished with subulate spines. There are only five British species of the genus and none of them is abundant. S. vertebratum Ralfs (length of cells 19 fx; breadth 21 — 24 p ; Desmidiacece 175 fig. 67 C) is the largest, and S. excavatum Ralfs (fig. 67 D— F) and S. granu- latum Roy & Biss, are the most widely distributed. Genus Onychonema Wallich, 1860. The cells are small and form simple filamentous colonies. The median con- striction is deep and narrow, and the semicells are ellip- tical or reniform, sometimes with strong lateral spines at each side. There are two capitate processes of con- siderable length attached to each apex and disposed asym- metrically. The cells are united into long flexible filaments by the overlapping of these processes over the adjacent cells. There is one axile chloroplast in each semicell, with a single pyre- noid. The zygospores are globose and furnished with simple spines. There are three British species of the genus, all of which are distinctly rare. The one most generally observed is 0. filiformis (Ehrenb.) Roy & Biss, (length of cells 14 — 15 p; breadth 14-5 — F Fig. 67. A, Spondylosium pulchellum Arch., from Glen Shee, Perthshire (x 365). B, S. papillatum W. & G. S. West, from Skipwith Common, E. Yorks. (x475). C, Spluero- zosma vertebratum Ralfs, from near Crowan, Cornwall (x475). D— F, Sph. excavatum Ralfs ; D, from Llyn Idwal, N. Wales ( x 475) ; E , zygospore from Puttenham Common, Surrey ( x 475) ; F, zygospore from New Forest, Hants. ( x 475). G — H, Onychonema Nordstedtiana Turner, from Strensall Common, N. Yorks. (G, x 475 ; H, x730). 16/4 Genus Spondylosium Breb., 1844. [Leuronema Wallich, I860.] The cells are small or of medium size and are united by their apices to form filamentous colonies, occasionally twisted and often enveloped in a copious mucus. The median constriction is usually deep and linear and the semicells are of very variable form. The apices are flat or concave and the cells are joined merely by the close apposition of their apices, this being the sole distinguishing feature between Spondylosium and Sphcerozosma. The vertical view is elliptical, triangular, or trilobed. The chloroplasts are as in Spiutirozosvia, and the zygospores are globose and smooth. 176 Chlorophycece There are seven species of the genus known to occur in the British Islands none of which is abundant. S. papillatum W. & Q. S. West (length of cells 8-9-5 p ; breadth 9-5-10-5 p; fig. 67 B) and S. pulchellum Arch, (length ^ 5 15 breadth 11 — 12-5 p ; fig. 67 A) are the most widely distributed. Genus Hyalotheca Ehrenb., 1841. The cells are more or less cylindrical and are connected by their broadly truncate apices into filamentous colo- nies. The median constric- tion is very slight and the semicells are trapezoid, sub- quadrate or oblong in form, with straight or convex lateral margins. The filaments are usually twisted and always enveloped in a thick coat of mucus. There are frequently several slight swellings at the base of each semicell near the constriction, causing the cir- cular vertical view to possess two or three nipple-like pro- jections at equal intervals round the margin. There is one chloroplast in each semi- cell, consisting of a central mass with a number of radiat- ing plates, and containing one pyrenoid. The zygospores are globose and smooth, and in H. dissiliens the four empty semicells unite to form a cruciform structure which surrounds each spore. There are four British species of the genus, of which H. dissiliens (Sru.) Breb. is general and often abundant; length 15 — 25 p ; breadth 21 — .33 p; fig. 68 A — C. This Desrnid is more frequently found with zygospores than any other. II. mucosa (Dillw.) Ehrenb. is a much scarcer plant, although widely distributed. II. undidata Nordst. (length 13 '5 — 17-5 p ; breadth 7'5 — 9 p) and II. neglecta Racib. (length 28 — -34-5 p ; breadth 1 1 '5 — 13 p ; fig. 68 E — H) are amongst the rarest of British Desmids. Series b. The oblique junction of the new and old halves of the cell-wall (at the region of the isthmus) develops an internal Fig. 68. A — C, Hyalotheca dissiliens (Sm.) Br6b. ; A and B, from Capel Curig, N. Wales (x365); C and D, zygospores from Galway, Ireland ( x 365). E — H, H. neglecta Racib., from the New Forest, Hants. ( x 475) ; E — G, showing aplano- spores (a) ; H, zygospore (z). Desmidiacece 177 girdle-like thickening which projects into the old semicells during the earlier stages of division. The cells are united to form thread- like colonies. Genus Desmidium Ag., 1824. [Didymoprium Kiitz., 1843; Aptogonum Ralfs, 1848.] The cells are united to form twisted filamentous colonies, often enveloped in a wide mucous coat. The Fig. 69. A, Desmidium Swartzii Ag., from near Preston, Lancashire (x365). B, D. quadratum Nordst., showing cell-division (x475). C, zygospore of D. cylindricum Grev., from Donegal, Ireland (x350). D, zygospores of D. aptogonum Breb. ( x 475). E and F, Gymnozyga moniliformis Ehrenb., from Bhiconich, Sutherland (x475); F, showing cell-division. G, zygospores of G. moniliformis var. gracilescens Nordst. ( x 475). median constriction is moderately deep and the semicells are much depressed, so that the cells are generally much broader than their length. The attachment of the cells is either by the close apposition of their flat apices or by the apposition of corresponding truncate apical projections. In the latter case there is a space of variable width visible between the actual apices of two adjacent cells. In vertical view the cells may be triangular, quadrangular, or elliptical with mamillate poles. There is a single central chloroplast in each semicell containing as many pyrenoids as there are angles in the vertical view, and there are two longitudinal plates diverging from each pyrenoid into the angle. The zygo- 12 W. A. 178 CJiloi 'ophycece spores are ellipsoidal, smooth or furnished with somewhat flattened or conical papillae. There are six British species of the genus, nono of which is abundant. I). Swartzii Ag. (length 14— 19 p; breadth 37— 43 p; fig. 69 A) and D. cylin- clncum Grev. (fig. 69 C) are the two most general species. 1). quadratum Nordst. (fig. 69 B) and D. graciliceps (Nordst.) Lagerh. are two species very rarely met with in the British Isles. Genus Gymnozyga Ehrenb., 1840. [Bambusina Katz., 1845.] The cells are cylindrical or barrel-shaped and united by their flat ends into slightly twisted filamentous colonies. There is a slight median constriction and at the base of each semicell is a swelling of variable size. The vertical view is circular, sometimes with two opposite papillae. The cell-wall frequently possesses delicate longitudinal grooves. There is one central chloroplast in each semicell with six radiating longitudinal plates and one pyrenoid. The zygospores are ellipsoidal and smooth. The only British species is G. moniliformis Ehrenb. (length 25 — 30 p ; breadth 17 '5 — 22’5 p ; fig. 69 E), which is generally distributed in boggy districts, particularly in elevated localities. Order IX. PROTOCOCCOIDEvE. This order includes a large number of green Algae which are mostly unicellular in character. The cells are commonly aggregated to form loose irregular colonies and are often embedded in a copious mucilage. In a few of the forms a small multicellular expansion is developed, and in others a definite coenobium which is sometimes coenocytic in character. In some the cells are normally ciliated and the plants are either motile unicells or more or less complex motile colonies. The order includes the lowest and most primitive of the green Algae, forms through which most of the other Chlorophyceae have been evolved along divergent lines. They are wonderfully varied in character and are found in almost all possible situations. The cell-walls may be extremely delicate or firm and thick, and there is often a great development of the gelatinous pectose compounds. The number and disposition of the chloroplasts vary greatly in the different genera, and pyrenoids may or may not be present. P rotococcoidece 179 Most of the Protococcoidere multiply by simple cell-division, which takes place in one, two, three, or sometimes in many directions of space. Asexual reproduction is generally accomplished by the pro- duction of one or many bici Hated zoogonidia from a mother-cell, and in some families this is the sole method of reproduction. There are many different types of asexual spores found in the genera of the Protococcoidese. Some of these are akinetes and others are aplanospores. One type, which is worthy of special note, is the autospore. A number of autospores are generally produced in a cell, each spore having on liberation the general form and appearance of the mother-cell. In some of the ccenobic forms the plants are reproduced by the formation of autocolonies. Most of the methods of sexual reproduction, both isogamous and heterogamous, which are exhibited by the various Chlorophycese, are found in the order Protococcoidese. The order seems to me to be best subdivided into the following- eight families, of which the first one is doubtfully placed in the Protococcoidese as the plants included in it have certain relation- ships with the Chaetophoraceae. Family 1. Chcetopeltidece. Unicellular or multicellular, sometimes pseudoparenchymatous. Some or all of the cells furnished with hairs or bristles, either simple or sheathed and often mucous. Multiplication by division of cells in two directions. Reproduction by 2- or 4-ciliated zoogonidia and by 2-ciliated gametes. Family 2. Volvocacece. Unicellular or consisting of a definite coenobium of cells, which are either united by protoplasmic processes or enclosed within the swollen mucous mother-cell-wall. All the cells are ciliated and motile in the vegetative state. Family 3. Endosphceracece. Unicellular or slightly branched and ccenoey tic ; cells solitary, generally rounded, often with button-like excrescences of cellulose ; chloroplasts with numerous pyrenoids. No vegetative division. Reproduction by spores, zoogonidia and gametes. All the genera are endophytic. Family 4. Characiew. Unicellular; cells solitary, differentiated into base and apex, epiphytic on other Algas ; chloroplast parietal with one pyrenoid. No vegetative division. Reproduction solely by zoo- gonidia formed by successive divisions of the contents of a mother-cell. Family 5. Pleurococcaceai. Unicellular and globular, or of short, ramified, few-celled filaments, never attenuated into hairs; often pseudoparenchymatous ; chloroplasts one or several, parietal, with or without pyrenoids. Multiplication by division in two or three directions, and more rarely by zoogonidia. Cell-walls very firm. 12—2 180 Chtorophycece Family 6. Hydrodictyaccce. Thallus consisting of a ccenobium of ccenocytes, non-motile, and formed by the apposition of quiescent zoo* gonidia, which may or may not have escaped from the mother-ccenocyte. Reproduction sometimes by resting-spores. Family 7. Protococcacece {or A utosporacece). Cells solitary, free- swimming, or colonial and associated in minute, more or less definite colonies, easily dissociated or persistent. Multiplication by successive division of contents forming autospores or autocolonies. Zoogonidia rarely developed. Family 8. Palmellacece. Microscopic or macroscopic, gelatinous and indefinite. Cells embedded in a copious gelatinous envelope. Multiplication by division in every direction ; cells often grouped in twos or fours, sometimes with pseudocilia. Colonies free-floating or attached. Zoogonidia with two cilia. Family 1. CIEETOPELTTDE.E. The Alg® included in this family consist of several genera of very obscure affinities, all of which can be distinguished from other members of the Protococcoidese by the presence of set* or bristles. At present they are but little known and the true nature of the bristles has not yet been thoroughly worked out. They are uni- cellular, or aggregates of loose cells, sometimes forming short filaments or flat, pseudoparenchymatous expansions, which in some instances appear to have arisen by a concrescence of short dicho- tomous branch -systems. The flat thallus of Ghmtopeltis is similar to that found in the Ulvacese, except from point of view of size and the fact that it is attached by the whole of one surface. Choitosphcvridiwm is a genus which may, perhaps, owing to the short creeping filaments which it sometimes develops and the sheathed bristles, have some relationship to the Herposteirace®, but its characters are so widely different from those of Herposteiron that it is best kept apart from the Cheetophorales until our knowledge of the genus is augmented. It may be that the resemblances between the Ch®topeltide® and certain of the Ch®tophorales, such as they are, are due to a parallelism of modification rather than to a direct affinity. This has been illustrated in the table I have given of the phylogeny of the green Algae on page 30. In genera such as Cluetosphceridiuvi and Conochazte multipli- cation certainly takes place by the division of the cells in two directions. 181 Chcetopeltidece Reproduction has been observed to take place in seveial of the genera by zoogonidia with two or four cilia,, and by biciliated isogamous gametes. Throughout the entire family there is a marked dorsiventrality such as appears elsewhere in the Coleochaetacese, Trentepohliaceae, etc. The following British genera are known : — A. Plants with a disciform thallus furnished with scattered bristles Chcetopeltis. B. Plants consisting of a loose aggregate of cells in one stratum. * Each cell with one seta or bristle. Bristles with a basal sheath ; chloroplasts parietal Choetosphceridium. ** Each cell with several setae or bristles. t Bristles few, with basal sheaths Conoclicete. ft Bristles many, without basal sheaths Polychcetophora. Genus Chsetopeltis Berth., 1878. The thallus is a flat plate, almost circular in outline, and consists of a single layer of compact cells, more or less radiating from the centre. The growth of the thallus is peripheral as in Goleochcete, but the cell-walls are much more gelatinous. From the upper surface of the thallus a number of scattered mucous setae arise, which are unseptate, simple, and of considerable length. Each cell contains one parietal chloroplast, which is often much lobed and perforated, and is furnished with a single pyrenoid. Reproduction takes place by zoogonidia of which 2 — 8 arise from a mother-cell, each one possessing four cilia. Isogamous gametes are also produced, similar in appearance to the zoogonidia but with two cilia. Oh. orbicularis Berth, is not an uncommon plant in this country, occurring as an epiphyte on the stems and leaves of various aquatic Phanerogams. The thallus varies greatly in size and sometimes reaches a diameter of 1 mm. Genus Chaetosphaeridium Klebahn, 1892 \ The cells are small and spherical, generally occurring in loose aggregates, epiphytic on larger Algae and other aquatic plants. They are sometimes enveloped in mucus, but more often quite destitute of a gelatinous investment. Klebahn states that the cells are com- monly joined in a short filament by means of empty cylindrical utricles, but I have not observed the pi’esence of these structures 1 Klebahn in Jakrbiich. wissensch. Bot. xxiv, 1892, pp. 268 — 282, pi. 4. 182 Chlorophycece in any British specimens. Each cell possesses at its upper pole a small conical projection which forms a basal sheath for a long, delicate seta or bristle. This bristle is extremely fine and fragile, and is commonly broken off at the apex of the basal sheath. In most specimens it is impossible to see any sheath-like structure in the conical projec- tion, the bristle appearing to arise from its apex. The chloro- plast is very variable, but in some specimens it is distinctly parietal with one pyrenoid. The division of the cells sometimes takes place by the formation of a horizontal division-plane, the lower daughter-cell migrating to the side. Asexual reproduc- tion occurs by zoogonidia, pro- duced four (or more ?) in a cell. There is some little confusion between this genus and one described by Borzi in 1892 as Nordstedtia1. The latter was supposed to be founded on a plant described by Nordstedt as Herposteiron globosa 2, and since referred by Wolle to the genus ‘Aphanochcete.’ Klebahn, who compared the original specimens of Nordstedt with Borzi’s drawings of N ordstedtia, affirmed that the latter represented an entirely different plant. This being the case, one is compelled to accept the genus Ohcetosphceridium for the plants described by Nordstedt as “ Herposteiron globosa.” Ch. globosum (Nordst.) Klebahn is widely distributed in the British Islands, occurring chiefly in the Sphagnum-pools of permanent bogs. The cells are 1 1 — 18 p in diameter, loosely associated, and occasionally surrounded by a gelatinous envelope (fig. 70 A and B). There is a curious var. depressum of this species with the cells much depressed (fig. 70 C). The plant described as Ohcetosphceridium Pringsheimii by Klebahn appears to be identical with Aphanochcete globosa var. minor Hansgirg, and if it is distinct from Gh. globosum it should be known as Ch. minus Hansg. Most 1 Borzi in Nuova Notarisia, iii, 1892, p. 50. 2 Nordstedt, Alg. aq. dulc. et Char. Sandvic. 1878, p. 23, t. ii, f. 22, 23. Fig. 70. AandB, Ohcetosphceridium globosum (Nordst.) Klebahn; A, from Esher Common, Surrey ; B, from Bow- ness, Westmoreland. C, Ch. globosum var. depressum (West & G. S. West), from the New Forest, Hants. (All x 370.) Chcetopeltidece 183 probably, however, it is but a small variety of Ch. globosum with the cells 9 — 12 fx in diameter1. Genus Conochaete Ivlebahn, 1893. The cells are comparatively small, loosely aggregated, and embedded in a small amount of mucus. Each cell is sub- globose, often depressed, and possesses a number of delicate bristles which radiate in all directions. Each of the latter prises either from the apex of a mamillate protuberance of the cell-wall or from the base of an elongated sheath. There are one or two chloroplasts in each cell, each furnished with one pyrenoid. The cells are markedly dorsiventral and sometimes a prominent oil-globule is present in the basal half of the cell. The plants multiply by cell-division in two directions and by zoogonidia, of which four or eight are formed in each cell3. Two British species are known, both of which are exceedingly rare. C. comosa Klebahn possesses cells 13 — 26 p in diameter and the bristles, which are 3 — 5 in number, are sheathed at the base (fig. 71). C. polytricha (Nordst.) Kleb. possesses cells 10 — 16 p in diameter and the bristles arise from the apices of a number of mamillate protuberances of the cell- wall, the latter exhibiting a well-marked stratification. Genus Polychsetophora West & G. S. West, 1903. The plants are unicellular or sometimes composed of short, loosely connected filaments of six or eight cells. The cells are subglobose or ovoid, and the cell-walls are exceedingly thick and lamellose. From the outer layers of this lamellose cell- wall from 8 to 12 long flexuose bristles are given off. The bristles are very delicate and attenuated to fine points ; they are quite simple, without any trace of a basal sheath or any basal swelling, and they radiate in every direction. Sometimes the cell-wall is unequally developed, a large stratified outgrowth having been developed on one side. Such a cell has a stalked appearance which presents many points of 1 This small form is known from N. Yorkshire ; vide West & G. S. West in Trans. Yorks. Nat. Union, 1900, vol. v, p. 22. 2 Schmidle in Hedwigia, 1899, p. 162, t. vi, f. 16—19. Klebahn, from the New Forest, Hants. ( x 370). ch, chloroplast; o, oil-globule. 184 Chlorophycece resemblance to the cells in Borzi’s genus Hormotila. Each cell contains a single chloroplast, sometimes distinctly parietal, but frequently subcentral and filled with highly refractive oil-globules. The presence of pyrenoids has not been definitely determined. Multiplication takes place by division of the cells in two directions. Fig. 72. Polychcetopliora lamellosa West & G. S. West, from Cirencester, Gloucestershire ( x 370). P. lamellosa West & G. S. West is the only species of the genus and has been found in Gloucestershire amongst Tolypotkrix pygmcea.. Diam. of cells 19 — 35 /x ; thickness of cell-wall 2'8 — 10'5 p; length of bristles 86 — 183 g (fig- "2). Family 2. VOLVOCACEiE. The plants contained in this family are either unicellular or they consist of definite coenobia of cells. They are distinguished from all other Protococcoidese by being ciliated and motile in their vegetative condition. The coenobia consist of a definite group of cells either united together by protoplasmic processes or super- ficially arranged within the swollen wall of a mother-cell. The number of cells in any coenobium remains constant so long as the individual exists, and they all ai'ose by cell-division while the plant was still an embryo within the wall of the original mother- The cells are rounded, angular, or ovoidal in form, generally with a narrower anterior end to which are attached two, or rarely four, cilia. The protoplasm of the anterior region of the cells is cell. Volvocacea ? 185 hyaline in character and often contains a prominent pigment spot and two (sometimes more) contractile vacuoles. The pulsation of these vacuoles is alternate. There is a single nucleus, usually occupying a central position in the cell. One chloroplast is present containing one or more pyrenoids, and although its form is very variable, it is generally more or less confined to the broader posterior end of the cell. Multiplication takes place by the division of the contents of a mother-cell into 2, 4, or 8 daughter-cells, and, as the latter are new motile individuals, the vegetative division in this family is strictly homologous with the asexual reproduction by zoogonidia which is found in the other groups of the Chlorophycese. In all the repro- ductive processes in this family the plane of the first cell-division is usually at right angles to the longitudinal axis of the cell. Sexual reproduction occurs in most of the forms by the union of isogamous planogametes formed in a similar manner to the vegetative daughter-cells, but in greater numbers. In some of the higher forms there is a marked heterogamy, sexual reproduc- tion being brought about either by the union of heterogamous planogametes or by the fusion of antherozoids and oospheres. In the zygospores or oospores the pigment spots of the two gametes disappear, but the chloroplasts remain distinct and the cell-wall becomes cuticularized. Germination takes place after a period of repose. As one passes from the lower to the higher members of the Volvocacese there is a more striking progressive evolution of forms than is exhibited in any other family of Algae. There is a gradual replacement of isogamy by heterogamy, ultimately reaching the highest condition of oogamy, and associated with this is an increase in the number of cells and size of the ccenobium, accompanied by a differentiation of vegetative from reproductive cells. The plants of this family are of great interest on account of the fact that they are the connecting links between the lower forms of the green Algae and the Flagellata, a Protozoan group which exhibits a mixture of animal and vegetable characters, and from which it is now generally recognized that the Chlorophycese have been evolved. There can be no doubt, however, in the mind of anyone who has carefully studied the Volvocacese, that their characters are such as to clearly separate them from the Flagellata and place them as a family of the green Algae. 18(3 Chlorophycece Plants of this family sometimes occur in prodigious quantity in stagnant water, giving it a pale-green colour and a somewhat unpleasant odour. They are occasionally the cause of foulness of drinking-water, imparting to it a distinct oily taste1. Excluding the Polyblepharidese, which exhibit a mixture of Flagellate and Volvocine characters, the British Volvocaceso are divided into the three fol- lowing sub-families2: — Sub-family I. Chlamydomonadece. Unicellular ; globose or ovoidal, with a distinct but thin cell-wall. Cilia (or flagella) two, rarely four. With one chloroplast of very variable form, usually including a single pyrenoid. Sub-family II. Phacotece. Unicellular, with the cells as in the Chlamydomonadese, but with a thick solid cell-wall which separates into two halves on the escape of the daughter-cells. Sub-family III. Volvocece. Motile coenobia of Chlamydomonadine cells, usually embedded in a common mucilaginous investment ; more rarely united by protoplasmic processes. All the cells are capable of reproducing the plant or there is a differentiation into vegetative and reproductive cells. Vegetative multiplication by division of some or all of the cells to form daughter coenobia. Isogamous or heterogamous sexual reproduction. Sub-family I. CHLAMYDOMONADECE. This division of the Volvocacese includes a number of unicellular Algae which are spherical, ovoid, subcylindrical, or rarely fusiform in shape, and are provided with a thin cell-wall and two, or more I’arely four, cilia. The chloroplast is of variable form, but is typically cup-shaped and occupies the posterior region of the cell, more or less surrounding a central mass of protoplasm which contains the nucleus. There is usually one pyrenoid, and a lateral pigment spot is also generally present in the anterior region of the cell. Reproduction takes place by the division of the contents of a cell which has come to rest into 2, 4, or 8 daughter-cells, each of which soon acquires the characters of the mother-cell. The successive division-planes are at right angles to each other and the daughter-cells are ciliated motile individuals. Sometimes the vegetative cells assume a palmelloid condition, in which they 1 Whipple in Trans. Amer. Micr. Soc. xxi, 1900. 2 This classification is after Dill, ‘Die Gattung Chlamydomonas und ihre nachsten Verwandten.’ Jahrbiicher fur wissenschaftliche Botanik. Berlin, 1895, Bd xxviii. Volvocacecv 187 become rounded .and repose in a copious mass of mucilage. Akinetes are also known to occur. Sexual reproduction occurs by the conjugation of isogam ous or heterogamous planogametes, which are smaller but otherwise similar to the vegetative individuals. They arise as do the latter by the division of the contents of a mother-cell, but as many as 64 may be produced in one cell. A. Vegetative cells with 4 cilia Carteria. B. Vegetative cells with 2 cilia. * Cell-wall thin, closely adherent. t Cells spherical, ovoid, or ellipsoid, rarely fusiform; chloroplast definite, with one pyrenoid (Jlilamyd/Omoucis . tt Cells fusiform, 3 or more times longer than the diameter; chloroplast indefinite, with two or more pyrenoids Cklorogonmm. ** Cell-wall thin, outstanding and connected by protoplasmic threads Sphcerella. Genus Carteria Diesing, 1868. [Pithiscus Dang., 1888 ; Cor- biera Dang., 1888.] The cells are spherical, ellipsoid, or cordiform, with a bell-shaped chloroplast containing a single large pyrenoid. There is a prominent pigment-spot towards the anterior end of the cell, and there are four cilia. The only distinction between this genus and Ghlamydomonas is the presence of four cilia instead of two. C. multifilis (Fresen.) Dill is a fairly abundant species in small pools, more particularly of rain-water. Diam. vegetative cells 10 — 16 g (fig. 73 A — G). Genus Chlamydomonas Ehrenb., 1833. The vegetative cells ai’e of variable size, spherical, ovoid, oblong-ellipsoid, or pyriform, and the anterior end is often slightly produced to form a small beak or rostrum to which are attached two cilia. The cell- wall is hyaline, often very indistinct, and is closely adherent to the cell- body. The chloroplast is very variable in form, and is usually furnished with a single pyrenoid (rarely entirely without or with several pyrenoids). There are two (rarely more) contractile vacuoles in the anterior region of the cell, and generally a pigment-spot. The reproduction is typically that described for the sub-family, and the wall of the zygospore may be smooth or asperulate. There are about 29 known species of the genus, but the characters of some of them are not clearly evident. Little work has been done at the British 188 Chloropliyceoi species of this genus, but Chi. Kleinii Schmidle (length of cell 28—32 u ; breadth 8 12 p. ; fig. 73 J and K), Chi. De Baryana Gorosch. (breadth 12 20 /x ; fig. 73 H and 1 ) and Chi. pulvisculus Ehrenb. ( = Chl . Ehrenbergii Gorosch. ; breadth 14 — 26 jx) are not uncommon in ponds, ditches and rain- pools. Concerning Chlamydomo- nus Blackman and Tansley1 write, — “This genus holds a unique position among the Green Algae, and indeed among the whole of the Green Plants. It may be regarded as the phylogenetic starting point of the various lines of Chloro- phyceous descent. The history of these is a history of the intercalation of a vegetative phase between two successive motile (Chlamydomonadine) generations, these motile phases being retained for re- productive purposes as zoo- spores and gametes; in the oogamous types the male gamete alone remains motile, and constitutes in the Arche- goniate series the last remaining representative of the Chlamydomonadine cell. “ The co-existence within the limits of an undoubtedly natural genus of the most primitive form of gamogenesis (the conjugation of equal clothed gametes) with a gamogenesis which has the essential characteristics of true oogamy is also a feature of unique interest.” Genus Chlorogonium Ehrenb., 1830. The vegetative cells are fusiform, three or more times longer than the diameter, with a thin cell-wall closely adhering to the body. There are two cilia attached to the anterior extremity and a number of contractile vacuoles scattered through the protoplasm. The chloroplast is ill- defined, spongy and anastomosing in character, and contains four or five, or sometimes many pyrenoids. France describes the chloro- plast as forming a regular or irregular annular band, which may split into a single or double spiral. Reproduction takes place by 1 Blackman & Tansley in The New Phytologist, 1902, vol. 1, pp. 23, 24. vegetative cells; D, gametes; E, conjugating gametes; F, zygospore. H and I, Chlamydo- vionas Be Baryana Gorosch., from St Just, Cornwall. J and K, Chi. Kleinii Schmidle, from Uxbridge, Middlesex (All x 475). cv, con- tractile vacuoles ; n, nucleus ; p , pyrenoid ; zy, gamete; z, zygospore. Volvocacea >, 189 zoogonidia, four of which arise in a mother-cell by the double transverse division of the contents. Sexual reproduction also occurs by the fusion of isogamous or heterogamous gametes. Chi. euclilorum Ehrenb. (length up to 50 /x; breadth 8 — 12 p) is apparently a very scarce British Alga of which I have examined a few specimens from stagnant ditches near the Lizard, Cornwall. Cercidium elongation Dang. (1888) would be more correctly placed as another species of this genus. Genus Sphserella Soramerfeldt, 1824. [ Hcematococcus A g., 1828; Ghlamydococcus A. Br., 1849.] The cells are ovoid and very similar to those of Ghlamydomonas, but the cell-wall stands out away from the cell-body, being connected to it by delicate strands of protoplasm. The chloroplast is bell-shaped and contains one or several pyrenoids. The genus is scarcely separable from Ghlamydomonas, only differing in the outstanding cell-wall and the absence of contractile vacuoles, the zoogonidia being exactly of the Chlamydomonadine type. Sph. lacustris (Girod.) Wittr. [Ghlamydococcus pluvialis (Flot.) A. Br.] is abundant all over the country in ditches, rain -pools and bog-pools. The cells, which frequently become brick-red in colour owing to the presence of hrematochromin, vary from 8 to 30 fj. in diameter ; they often become encysted, and the reproduction is by zoogonidia and isogamous gametes (fig. 74). The curious phenomenon known as “Bed Rain” owes its colour in a few instances to the presence of this Alga. Sph. nivalis Sommerf., which can scarcely be specifically distin- guished from Sph. lacustris, is the “ Red Snow ” plant. Wille1 has recently attempted to show that Hcematococcus is the correct name of this genus, but I fail to see the reason for such a change. The remarks made by Hazen2 on the nomenclature of this genus should also be consulted. Fig. 74. A— H. Sphcc- rella lacustris (Girod.) Wittr., from Bradford, W. Yorks. (x475). Sub-family II. PHACOTEiE. This sub-family is only distinguished from the Chlamydomo- nadese by the thick, solid cell-wall, which on the escape of the daughter-cells during reproduction separates into two portions. 1 Wille, ‘ Algologische Notizen X,’ Nyt Magazin f. Naturvidenslcab, Bd 41, H. 1, Kristiania, 1903. 2 Hazen, ‘The Life-History of Spharella lacustris Mem. Torr. Bot. Club, vi, 1899, p. 236. 190 Chlorophycece The two component parts of the cell-wall are sometimes evident in the ordinary vegetative condition. The reproduction is similar to that of the Chlamydomonadese. * Genus Phacotus Pei’ty, 1852. The cell-body is ovoid in form and considerably flattened, so that when viewed from the side it is relatively narrow and biconvex. The cell- wall is thick, dark in colour, with a rough or rugulose exterior, and consists of two valve-like pieces. The chloroplast is large and parietal, with one or many pyrenoids. There is a clear space of some size at the anterior end of the cell, and two long cilia are attached imme- diately opposite. The reproduction takes place by the formation of 2 — 8 zoogonidia in a vesicle which bursts apart the two halves of the mother-cell-wall by a longitudinal split. P. lenticularis (Ehrenb.) Stein is not uncommon in stagnant water. Another genus of this sub-family is Pteromonas Seligo (1886) of which there are seven or eight species known from continental Europe. No doubt some of these occur in the lakes of the British Islands, but as yet there are no records of them. Sub-family III. VOLVOCE.E. The plants of this sub-family are composed of a motile coeno- bium of Chlamydomonadine cells, generally embedded in a considerable mucous envelope, and sometimes connected by proto- plasmic processes. Attached to each cell are two cilia which project through the mucous coat and give the coenobium a rotatory motion by their combined movements. The number of cells present in a single coenobium varies from four in one species of Gonium to 22,000 in some forms of Volvox. In the lower forms all the cells are equal and capable of reproducing the plant, but in the higher forms there is a differentiation amongst the cells of the coenobium, some of which are purely vegetative, whereas others are solely reproductive cells. Reproduction in the lower forms takes place by the formation of a daughter-coenobium from every cell of the mother-ccenobium. The daughter-coenobium is formed within the wall of the mother- cell, which swells up and ultimately sets the young colony free. In the higher forms only certain of the reproductive cells, often termed parthenogonidia, give rise to daughter-coenobia. Sexual reproduction occurs in the lower forms by the union of isogamous or heterogamous planogametes which arise by the Volvocacece 191 division of all the cells of the coenobium. In the higher forms there is a further differentiation of the sexual elements into antherozoids and oospheres, each of which arise from special repro- ductive cells termed respectively androgonidia and gynogonidia. Genus Gonium Muller, 1773. [Indus. Tetragonium West & G. S. West, 1896.] The coenobium is flat and plate-like, consisting Fig. 75. A, Gonium pectorals Mull., from Cambridge. B — F, G. lacustre nob., from Esher Common, Surrey. E and F show the formation of new colonies ( x 475). of four or sixteen ovoid or slightly polygonal cells arranged in one plane and enclosed in a common mucilaginous investment. The cells are connected by protoplasmic processes, and the cilia all arise from one surface of the plate-like colony. The chloroplast is bell-shaped, much hollowed, and contains a single pyrenoid. Two contractile vacuoles are present in each cell. Reproduction takes place by zoogonidia, which are formed four in each cell or by the dissociation of the colony into individual motile cells. Conjugation of isogamous gametes results in a zygospore with a smooth or rough cell-wall. Multiplication also occurs by the development of daughter-ccenobia from each of the cells of the mother-coenobium. G. pectorale Mull, consists of a flat, somewhat quadrate colony of sixteen cells, each of which is 7 — 11 p in diameter and of which twelve are peripheral and four central (fig. 75 A). It is a frequent and striking Alga in many stagnant ditches and ponds. G. lacustre nob. [ = Tetragonium lacustre W. 192 Chlorophycece & G. S. West1] possesses a colony of four ovoid cells, each of which is 1T5 — 19 p. in length and 7’5 — 15 '5 p. in breadth ; fig. 75 B — F. The anterior extremity of each cell is somewhat protracted and the cilia are only vibratile towards their extremities, the movements of the ccenobium being correspond- ingly sluggish. It is a much rarer plant than G. pectorale and prefers the open water of large ponds and lakes. It may possibly be identical with G. sociale (Duj.) Warming, but it appears to differ in the grouping of the cells and in the nature of the cilia. Genus Stephanosphsera Cohn, 1852. The coenobium consists normally of eight cells arranged in an equatorial zone within a tough, spherical or ellipsoidal investment. Occasionally young coenobia are observed which contain only one or two cells. The cells are ovoidal, and in the vegetative condition they possess several green or colourless protoplasmic processes. The zone of cells is situated rather towards the posterior pole of the coenobium anil the cilia, of which one pair is attached to the anterior pole of each cell, penetrate the mucous investment in the equatorial plane. The chloroplasts are rather irregular and contain from one to several pyrenoids. Multiplication occurs by the division of each cell of the ccenobium, after having assumed a more or less globular form, into four or eight daughter-cells, each group forming a new coenobium. Sexual reproduction occurs by the fusion of isogamous gametes, 8, 16, or 32 of which are formed from the division of a single cell. The gametes are fusiform in shape, conjugating laterally to form spherical ‘ zygozoospores ’ which soon become quiescent and of a yellowish-brown colour. St. pluvialis Cohn, which is the only species of the genus, is known to occur both in England and Ireland. It is one of the scarcest of the Volvo- cacese and is usually found in the rain-water which collects in the hollows of rocks. The cells are 6 — 12 p. in diameter and the colonies 26 — 60 p ; fig. 76 K. Genus Pandorina Bory, 1824. The coenobia are spherical or subspherical and usually contain 16 cells closely packed within the mucous investment. Sometimes coenobia of 8, or rarely of 32, cells are observed. The cells are pyramidal in shape and reach almost to the centre of the spherical colony, the pressure of contact often causing them to become quite angular. Two widely divergent cilia are attached to the broad end of each cell. Multi- plication takes place by the formation of 16-celled coenobia from each of the cells of the mother-coenobium. The daughter-coenobia 1 West & G. S. West in Journ. Roy. Micr. Soc. 1896, p. 160, t. iii, f. 1—13. Volvocacece 193 often remain within the old mucous investment of the mother- ccenobium for some time, and the 16-ciliated colonies present the appearance of a composite coenobium. Fig. 76. A — H, Pandorina morum (Mull.) Bory; A, adult colony; B, group of daughter-colonies within the swollen mother-cell-wall; from near Bradford, W. Yorks. (x475). C— H, formation of zygospore (z) and its development (after Pringsheim). K, Stephanosphcera pluvialis Cohn, ordinary vegetative colony (after Hieronymus, x 320). g, gamete. Asexual reproduction is by zoogonidia of a precisely similar nature to the cells of Chlcimydomonas1. The zoogonidia arise by the longitudinal division of the contents of the mother-cell, and each one secretes in addition to its own membrane a gelatinous investment which ultimately forms the common investment of the colony. Schroder2 has also described an asexual method of repro- duction in which the zoogonidia loose their cilia, vacuoles and pigment-spots, and form a new colony. The gametes arise by the division of the cells into 16 or 32 parts, and they exhibit con- siderable variability in size. Sexual reproduction occurs by the fusion of a pair of gametes, sometimes by the fusion of a smaller active (male) gamete with a larger and more sluggish (female) 1 Dangeard in Le Botaniste, vii, 1900. 2 B. Schroder in J. B. Schles. Gesellseh. Vaterl. Cult. 1898, Zool., bot. sect, w. A. 13 Chlorophycece gamete. The zygospores possess smooth cell-walls and the germination is indirect. P. morum (Mull.) Bory is abundant in ponds and ditches all over the country. The cells are 8 — 15 g. in diameter and the colonies 20 — 42 u ('fie 76 A— H). ^ h' Genus Eudorina Ehrenb., 1832. \Eudorvnella, Lemmermann, 1900.] The coenobium is globose or subglobose, rarely ellipsoid, and consists normally of 32 cells distantly arranged within the Fig. 77. Eudorina elegans Ehrenb. A, adult colony (x475); B, young colony formed by division of contents of mother-cell ( x 730), from Frizinghall, W. Yorks. C — E, development of antherozoid-clusters from mother-cell ; F, antherozoids (after Goebel). periphery of a copious mucous investment. The cells are globose, with a single bell-shaped chloroplast containing one or more pyrenoids. Small colonies of only eight or sixteen cells are some- times met with. The cilia (one pair) attached to each cell are parallel until they reach the outer surface of the investment, when they widely diverge. Multiplication takes place as in Pandorina by the division of all the cells of the coenobium to form daughter- , coenobia. Sexual reproduction occurs by the union of fusiform or pear-shaped antherozoids, produced 64 in a cell, with oospheres which are slightly larger than the vegetative cells. The oospores have smooth cell-walls and germination is direct. There seems no valid reason for the separation of Lemmer- Volvocacecr 195 mann’s genus Eudorinella1. Chodat2 also considers that the two described species of Pleodorina Shaw3 should be regarded merely as forms of Eudorina elegans , but the marked differentiation which exists between the vegetative and reproductive cells of Pleodorina does not support his view. Eudorina elegans Ehrenb. is widely distributed in ponds, ditches and lakes all over the British Islands. The cells are 10 — 25 g in diameter and the colonies 40 — 150 g ; fig. 77. The ellipsoid colonies of this plant are sometimes described as possessing several mucous mamillate projections at one pole, but I do not find this character in British examples, the colonies of which are invariably globose. Genus Volvox (L., 1758) Ehrenb., 1830. The ccenobium is globose, consisting of a large number of small cells (200 — 22,000) arranged in a single peripheral layer within the mucous investment. The coenobium is a hollow sphere and the cells are connected by protoplasmic threads of varying stoutness. There is a differentia- tion of the cells, the vast majority of them being purely vegetative (somatic), and the remainder either parthenogonidia, androgonidia, or gynogonidia. Each cell possesses a distinct chloroplast, two or more contractile vacuoles, and a pair of cilia. Asexual reproduction occurs by the development of new colonies from the partheno- gonidia, of which about 8 (1 to 16) are found in a single coenobium. These parthenogenetically formed individuals become detached in the hollow cavity of the mother-ccenobium and are ultimately set free on its death. Sexual repi’oduction takes place by the fusion of an antherozoid with an oosphere. The antherozoids arise by the division of few or many androgonidia, which are similar in appearance to the parthenogonidia. The divisions of the androgo- nidia produce either a plate-like or a spherical mass of antherozoids, each of which is a small fusiform body, much attenuated at the anterior end and furnished with two cilia. The gynogonidia are few in number, larger than the vegetative cells, and each one becomes an oosphere. The oospores are globular, either smooth or substellate, and their germination is direct, after a period of repose. Both asexual and sexual colonies occur, and the latter, which some- times possess parthenogonidia, may be either monoecious or 1 Lemmermann in Berichte Deutscli. Botan. Gesellsch. xviii, 1900, p. 307. 2 Chodat in Beitriige zur Kryptogamenflora der Schweiz, Bd i, Heft 3, Berlin, 1902, p. 152. ’ 3 Shaw in Botan. Gazette, 1894, xix, p. 279 ; Kofoid in Bull. Illinois State Lab. v, 1898, p. 273; also in Ann. Mag. Nat. Hist. ser. 7, vol. vi, July, 1900. 13—2 196 Chlorophycece dioecious. The most thorough investigations of the life-history of the genus have been made by Klein1. This genus is the culminating point in the evolution of motile coenobic forms, and although the individual cells are of the Fig. 78. A, C, and D, Volvox aureus Ehrenb. A, monoecious sexual colony ( x 210), from Bawcliffe Common, W. Yorks.; D, ripe oospore (x475); C, two anthero- zoids (after Klein). B, ripe oospore of V. ylobator (L.) Ehrenb., from Preston, Lancashire ( x 475). a, androgonidia ; an, antherozoid ; g, gynogonidia. primitive Chlamydomonadine type, the entire organism, by reason of the differentiation of its component cells, has attained along its own line a state of evolution quite comparable with the structural differentiation attained along other lines by the highest known forms of the green Algae. The Rotifer Notommata 'parasitica sometimes occurs as a parasite within the hollow coenobia of Volvox, swimming about within the central cavity and feeding on the green cells either of the adult or of the parthenogenetic daughter-coenobia. 1 Klein ‘Morphol. u. biol. Studien iiber die Gatt. Volvox,’ Pringsh. Jahrbiich. f. wissensch. Bot. 1889, Bd xx. 197 Endosphceracew Two well-known species of Volvox exist in the deeper ponds and ditches of the low-lying areas of the British Islands. They are often associated with Lenina , and prefer ponds which receive a plentiful supply of rain-water. The adult colonies of V. globator Ehrenb. are 680—800 /x in diameter, the cells are very numerous, the protoplasmic strands connecting them are of considerable thickness, and the ripe oospores are substellate (fig. 78 B). V. aureus Ehrenb. is usually smaller than the preceding species, the adult colonies being 200 — 500 g in diameter ; fig. 78 A, C, and I) ; the cells are less numerous, the proto- plasmic threads connecting them are extremely delicate, and the walls of the ripe oospores are smooth. Family 3. ENDOSPHCERACEW. This family includes a number of endophytic Algse in which the plant-body is either a simple rounded cell or a slightly branched ccenocyte, occurring wedged between the epidermal cells of aquatic or marsh-loving plants. The cells are somewhat variable in form and the cell-wall is sometimes thick and lamellose, often developing on one side a button-like excrescence of cellulose. Reproduction takes place by the formation of zoogonidia or planogametes, or both. In the best known genus, Ghloroch.ytrium Cohn, the contents of the endophytic cell become broken up by successive divisions, in a manner similar to that which occurs in Characium, into a large number of small zoogonidia or gametes. These are liberated by the gelatinization of the inner layer of the cell -wall, which becomes protruded as a large vesicle such as is found in the Pediastreas and many of the Protococcaceae. The biciliated gametes are isogamous and on their fusion the motile 4-ciliated zygospore comes to rest on the epidermis of the host- plant. Germination takes place without any period of rest, the new endophytic cell forcing a protuberance either between two cells of the epidermis or into a stoma. Some of these endophytic cells become akinetes and in this condition they pass the winter. There is a complete absence of ordinary vegetative division. Seven genera have been described, but some of these are doubtfully distinct. The researches of Klebs have shown that Cohn and other previous investigators were wrong in regarding certain of these plants as parasites, and that not merely can the endophyte live quite independently of its host-plant, but that the latter receives no injury beyond the little mechanical pressure exerted by the 198 CJdorophycece growing endophyte. The anomalous habit of these Algte is explained by the protection afforded by the intercellular spaces of the host. Freeman1 has suggested that the condition of affairs met with in Chlorochytrium lends itself to the development of parasitism and that the allied genus Pkyllobium is progressing in that direction. Genus Chlorochytrium Cohn, 1874. The cells are generally solitary, globose, ellipsoid, ovoid, or irregularly curved and lobed, Fig. 79. A, Vegetative cell of Chlorochytrium Lemncc Cohn, from Frizinghall, W. Yorks. (x475). B — D, Centrosphcera Facciolace Borzi ; B and C, from near Senens, Cornwall ( x 475) ; D, showing escape of zoogonidia (after Borzi, x 410). and they contain a large parietal chloroplast furnished with many pyrenoids. Both starch and oil are often found in the cells. Zoogonidia are formed by successive bipartitions of the cell- contents, at first by perpendicular, and afterwards by radially disposed walls. Gametes arise in a similar manner, and conju- gation takes place within the vesicle formed by the protrusion of the inner layer of the gametangium. Some of the species of this genus are endophytes in the leaves of Levina, Mentha, Rumex, Lychnis, Sphagnum, etc., and others are marine, occurring in the thalli of various marine Algse. Chi. Lemnce Cohn occurs frequently in the leaves of Lenina trisulca ; diam. cell. 40—143 p ; fig. 79 A. Chi. Knyanum Szymanski is found on the leaves of Lemna minor and L. gihba. The genus Stomatochytrium Cunningham (1888) differs from Chlorochytrium mainly in the absence of the vesicle which surrounds the gametes in the latter genus. This feature can scarcely be regarded as a generic difference. 1 Freeman, ‘Obs. on Chlorochytrium,’ Minnesota Botan. Studies, vol. ir, part in, p. 198. Characiece 199 Genus Centrosphaera Borzi, 1883. The cells are globose or shortly ellipsoid and generally occur aggregated in a more or less diffuse stratum beneath colonies of various members of the Oscillatoriaceae. The cell-walls are thick and lamellose, and possess a projecting lamellose button of cellulose. Reproduction takes place by biciliated zoogonidia, which usually arise in large numbers from the zoogonidangia. Gametes have not been observed. C. Facciolace Borzi1 is known from the south of England. Diam. of vege- tative cells 26 — 42 /i ; zoogonidangia up to 80 g ; zoogonidia 2 — 3 g in breadth ; fig. 79 B — D. There is scarcely any distinction between Centrosphaera Borzi, Endosphcera Klebs and Scotinosphcera Klebs2, the vegetative cells being almost alike in all three. In Endosphcera the reproduction is by gametes, zoogonidia not having been observed. In Scotinosphcera the only observed reproduction is by zoo- gonidia, which arise according to Klebs in a most extraordinary manner. Genus Phyllobium Klebs, 1881. The plants of this genus are endophytic coenocytes which occur in the leaves of certain marsh- loving plants. They send out branching tubes through the inter- cellular spaces of the host-plant after the manner of Phyllosiphon amongst the Siphonete. These branched tubes may or may not be septate, and projecting from the surface of the leaf are small, bright-green swellings. The latter contain a number of radiating chloroplasts, each with a pyrenoid. Biciliated macro- and micro- gametes occur. P. dimorphum Klebs, which occurs in the leaves of Ajuga , Lysimachia , etc., has not been observed from this country, but I have recently obtained what may prove to be another species from N. Uist, Outer Hebrides, thickly studding the leaves of damp Sphagnum, the whole tissue of the leaf being permeated by the branched tubes of the Phyllobium. Family 4. CHARACIE.®. The plants of this family are unicellular and they generally occur as epiphytes, either solitary or in clusters, on other larger Algte. The vegetative cells are of very variable form, but in most cases they are attenuated and slightly oblique. There is always a distinct differentiation into base and apex. The base is often drawn out into a stalk of variable length with a disc for purpose of attachment, and the apex is generally acuminate. There is a 1 Borzi, ‘ Studi Algologici I,’ Messina, 1883. 2 Klebs in Botan. Centralbl. 1881, xxxix, p. 16 — 21. 200 Chlorophycece single parietal chloroplast in each cell, generally of considerable extent and containing one pyrenoid. There is no vegetative division. Reproduction takes place by the formation of numerous zoogonidia in each cell. There is a repeated division of the cell- contents, several transverse divisions occurring before the first longitudinal division, and in a short time each portion loses its angular character, becomes rounded off, and forms a biciliated zoogonidium. The escape of the zoogonidia takes place either by a lateral or a terminal aperture. The pyrenoid disappears (in some species) during the formation of the zoogonidia, and reappears on their germination. Each zoogonidium on coming to rest develops into a new plant. Genus Characium A. Br., 1849. [ Hydrocytium A. Br., 1855 ; Hydrianum Rabenh., 1868.] The cells are fixed by a basal stalk, Pig. 80. A and B, Characium Pringsheimii A. Br.; A, from Mitcham Common, Surrey; B, attached to a frustule of Tabellaria flocculosa, from Gunwen Moor, Cornwall. C, Ch. subulatum A. Br., from Wimpole Park, Cambridgeshire. D, Ch. ensiforme Herm. , from Pilmoor, N. Yorks. (All x 520.) usually rather short, but sometimes much elongated ; in form they are spherical, ellipsoid, oblong, or fusiform, and they are generally asymmetrical. The cells as a rule give rise to 16 or 32 zoogonidia. Some eight or ten species of the genus are known to occur in Britain, of which Ch. Sieboldii A. Br. (length 40—70 g ; diam. 20—33 g), CL ambiguum A. Br. (length 15—30 g ; diam. 2'5 — 4 g), Ch. Pringsheimii A. Br. (length 18—35 g- diam. 5— 11 '5 g; fig. 80 A and B) and Ch. ornithoceplialum A. Br. (length 19—33 g ; diam. 8—12-5 g) are the most general. Ch. ensiforme Herm. (length 65—86 g; diam. 2'5— 3’8 g; fig. 80 D) is the most elongate species of the genus. Plmrococcacece 201 Family 5. PLEUROCOCCACE.®. The plants of this family are either unicellular or composed of very short, slightly branched filaments, which consist of few cells and are never attenuated to form hairs. The short filaments are generally creeping and are often compacted into pseudo- parenchymatous masses. Considerable polymorphism is exhibited by the different members of this family. The cells are variable in outward form and contain one or several parietal chlorop lasts, with or without pyrenoids. Multiplication takes place by division in two or three directions, and asexual reproduction occasionally occurs by the formation of biciliated zoogonidia. Gametes are rarely produced. In the two most frequent genera of the family, namely Pleuro- coccus and Trochiscia, the cells are more or less globose and occur aggregated in large masses ; the former prefers a subaerial habitat whereas the latter prefers an aquatic existence. In Hormotila there is a marked increase in thickness of the cell-wall at one side only, branched colonies of cells being formed by the more or less complete fusion of these remarkable lamellose excrescences. Protoderma is usually epiphytic, and its short cell-filaments commonly assume a pseudoparenchymatous character ; it is placed here on account of its close resemblance to certain of the more uncommon states of Pleurococcus. The cell- walls are usually very strong and firm, and the cells are associated to form indefinite colonies or irregular aggregations. This fact, and the complete absence of autospores, separate the Pleurococcacese from the Protococcacese. Chodat1 includes in this family the genera Microthamnion and Gongrosira, Algse which I have referred to a distinct family — the Microthamniacese — of the Chaetophorales. There is considerable resemblance between certain states of Pleurococcus and the young, germinating plants of Microthamnion, but the latter genus has reached a much higher stage of development than is ever attained by forms of Pleurococcus. It is very probable that the family Microthamniacete has had a direct origin from certain of the Pleurococcaceae. 1 Chodat in Beitrage zur Kryptogamenflora der Schweiz, Bd i, Heft 3, Berlin, 202 Chlorophycece Wille1 has placed the genus Trochiscia in the Volvocacese in close proximity to Chlamydomonas, but a careful consideration of the facts of the case shows this change to be unjustifiable. The ordinary vegetative condition of Trochiscia is a non-motile resting state, zoogonidia rarely being produced; whereas the normal vegetative condition of Chlamydomonas (and indeed, of all the Volvocaceae) is a motile one. Genus Pleurococeus Menegh., 1842. [Protococcus A g., 1824 (in part) ; Cystococcus Nag., 1849 ; Chlorococcum Fries, 1825 (in part); Chlorosplicera Klebs, 1883 (in part); Pseudopleurococcus Snow, 1899.] The cells are more or less globose, sometimes angular by compression, and they occur frequently in groups of two or four due to imperfect separa- tion after division. As division occurs in three directions a small cubic- al colony is occasionally produced, but this easily dissociates into its indi- vidual cells. The cell- walls are strong and firm. There is one parietal chloroplast in each cell, extremely vari- able in size and form, and with or without a pyrenoid. In moist places short filaments of cells are sometimes produced which exhibit a simple type of branching ; they frequently radiate from a few central cells of an angulai’, more or less parenchymatous form. This condition is readily produced in cultures and can be described as the ‘ Protoderma-stage.’ Vegetative multiplication takes place by division and sub- sequent separation of the cells. Reproduction is brought about 1 Wille, 1 Alg. Not. VII,’ Nyt Magazin f. Naturvidenskab. 1901, Kristiania, B. 39, H. 1, p. 9. Fig. 81. A, Pleurococeus vulgaris Menegh., from Cirencester, Gloucestershire. B, PI. ru- fescens (Kiitz.) Brdb. var. sanguineus W. & G. S. West, from near Arneliffe, W. Yorks. (All x 520.) chi, chloroplast; p, protoderma stage; pa, palmel- loid state ; pg, bright red pigment ; py, pyrenoid. Pleurococcacm 203 by the formation of one or many spores (aplanospores), by the rejuvenescence of the contents of a mother-cell, by biciliated zoogonidia, and by isogamous gametes. The polymorphism exhibited by plants of this genus under various conditions of humidity, temperature, etc., has caused much confusion with regard to the identity of the different forms. PL vulgaris Menegh. is one of the commonest of Algse, occurring in great profusion in all kinds of damp situations, on stones, walls, palings, tree-trunks, etc., and it usually forms a thin green incrustation on the windward side of the objects on which it grows. The cells are as described for the genus and the chloroplast is a massive lobed plate containing a prominent pyrenoid. Diam. cells 9 — 20 g ; fig. 81 A. In PI. rufescens (Kiitz.) Breb. the cell-contents are of a brick-red colour due to the presence of hsematochromin, which usually appears to be dissolved in oily material. This species has a preference for calcareous rocks; a variety of it (var. sanguineus West & G. S. West) has been observed in the limestone districts of West Yorkshire, forming large brilliant blood -red patches, covering those stones and rocks in the beds of streams which could not be displaced by the rapidity of the current and which are often left dry. Diam. cells 11—20 g; fig. 81 B. Genus Trochiscia Kiitz., 1845. [Accinthococcus Lagerh., 1883 ; Glochiococcus De Toni, 1888.] This genus is very closely related Fig. 82. A— F, Trochiscia aspera (Reinsch) Hansg., from Tremethick, Cornwall- A and B, vegetative cells ; C and D, formation of zoogonidia ; E empty cell from which zoogonidia ( zg ) have escaped ; F, palmelloid state. G and H I. hirta (Reinsch) Hansg., from Cambridge. I and J, T. paucispinosa West, from Ben Lawers, Perthshire. K, T . reticularis (Reinsch) Hansg., from Keston Common, Kent. (All x 520.) ° ’ to Pleurococcus, differing mainly in the external ornamentation of the cell-wall and in the aquatic habit. The cells are perfectly globose and usually occur in large aggregates in quiet water, or rarely on damp ground. The cell-wall is either areolated or thickly clothed with denticulations, spines, or other prominent 204 Chlorophycece processes. There are usually several parietal c.hloroplasts in each cell, some or all of which contain a single pyrenoid. Multiplication rarely occurs by cell-division as in Pleurococcus, but reproduction commonly takes place as in that genus by the formation of non-motile gonidia or spores. Zoogonidia are rarely developed and the plants sometimes pass into a palmelloid condition ( vide fig. 82 F). There are eight or nine British species of .this genus, distinguished by their external ornamentation. T.aspera (Reinsch) Hansg. (diam. veg. cells 13-5—29 /x ; fig. 82 A— F), T. aciculifera (Lagerh.) Hansg. and T. reticularis (Reinsch) Hansg. (diam. veg. cells 24—32 p ; fig. 82 K) are amongst the most frequent species met with in quiet waters. T. hirta (Reinsch) Hansg. (diam. veg. cells 17 — 27 fi; fig. 82 G and H) is often found on damp ground near the base of tree-trunks. Genus Radiococcus Schmidle, 19021. [Westellci De Wild.2, 1897 (in part).] The plants consist of microscopic families of few cells (generally four) arranged in a tetrahedral manner and enveloped in a mass of jelly. The cells are rounded or occasionally angular by mutual pressure, with firm cell- walls, and they contain a single parietal chloroplast with one pyrenoid. Reproduction takes place by the formation of four daughter-cells (spores) tetrahedrally disposed within the wall of the mother-cell, which ruptures and sets them free, the remains of the mother-cell-wall persisting after the manner of Tetracoccus and Schizochlamys. The genus is distinguished from Tetracoccus by the smaller families, which are enveloped in jelly, and by the tetrahedral disposition of the cells. The plants often occur attached to the under surfaces of water-lily leaves. There are two species of this genus3, of which R. nimbatus (De Wild.) Schmidle ( = Pleurococcus nimbatus De Wild. ; Tetracoccus nimbatus Schmidle ; Westella nimbatus De Wild.) is known from several parts of England. Diam. of cells 8 — 15 p.. In 1894 Schmidle4 erroneously placed this Alga under Tetracoccus and De Wildeman has further complicated matters by creating the useless name Westella. Genus Protoderma Kiitz., 1843 ; Borzi, 1895. This genus occupies a position in the Pleurococcacese by reason of the resem- blance between it and certain states of Pleurococcus vulgaris. The plants generally consist of a minute thallus of short cell -filaments radiating from a small central group of pseudoparenchymatous 1 Schmidle in Allgem. Botan. Zeitschr. 1902, p. 41. 2 De Wildeman in Bull, de l’Herb. Boissier, 1897, p. 503. 3 Schmidle in Hedwigia, 1902, Bd xli, Heft 4, p. 159. 4 Schmidle in Flora, 1894, Heft 1, p. 45. Pleurococcacece 205 cells. The cells are of a variable shape and the branches are sometimes a little at- tenuated. There is a large parietal chloro- plast in each cell con- taining a single pyre- noid. P. viride Kiitz. (fig. 83 A — C) is usually found epiphytic on larger Algae, such as Coleochcete orbicu- laris, or on the stems and leaves of aquatic Phanero- gams, such as Callitriche, Elodea, etc. I have pre- viously suggested 1 that certain plants described as Entocladia gracilis Hansgirg 2 ( = Endoderma gracile De Toni) are most probably referable to Pro- toderma viride Kiitz. Fig. 83. A — C, Protoderma viride Kiitz., from Baildon, W. Yorks. , epiphytic on Callitriche st ag- nails ; A and B, outlines of colonies, x 520 ; C, single cell, x 700. D, Honnotila mucigena Borzi, from Boston Spa, W. Yorks. ( x 520). Genus Hormotila Borzi, 1883. The vege- tative cells are spheri- cal, ovoid, or ellipsoid, rarely oblong, and from two to sixteen of them occur within a more or less ample, firm, gelatinous in- tegument, which is often concentrically lamellose. There is a large chloroplast in each cell, frequently very granulose and destitute of a pyrenoid. Multiplication takes place by cell-division, at first in three directions, but subsequently in two, and finally in one direction. In this way more or less moniliform series of cells are produced, all of which are connected by cylindrical lamellose integuments. Zoogonidangia arise from vegetative cells by an increase in size of the cell, and reproduction takes place by numerous minute zoogonidia, each with two cilia. Gametes have not been observed. The only known species is H. mucigena Borzi, an Alga which I have observed from Boston Spa in West Yorkshire. It forms an expanded, dull 1 G. S. West in Journ. Bot. Febr. 1899, p. 58. 2 Hansgirg, ‘ Ueber Entocladia Reinke und Pilinia Kiitz.,’ Flora, 1888, no. 33, t. xii, f. 6 — 15. 206 Chlorophycece green stratum on the damp surfaces of calcareous rocks. Diam. of veg. cells 4—12 g ; zoogonidangia up to 30 g in diameter ; zoogonidia 1— 2‘5 g in breadth and 3 5 g in length ; fig. 83 D. Chodat has observed ‘ Ilormotila stages’ of a species of Pleurococcus, and he also suggests that they may be in part Dacty- lothece Braunii Lagerh., but with this latter suggestion I cannot agree. Genus Urococcus Kiitz., 1849. The cells are much as in Hormotila Borzi, but are usually of a larger size and their contents are coloured with a red-brown pigment. The cell-wall is thick and lamellose, generally with a considerable increase in thickness on one side. There is frequently an exuviation of the outer layers of the cell-wall. The genus is usually attributed to Hassall, but this is due to a misconception. Hassall, in describing the characters of his “ First Subgenus” of Hcematococcus A g., stated1 that “the term Quracoccus might be applied to the species of this subgenus.” This suggestion was improved upon by Kutzing, who established the genus Urococcus of Hassall ; but, if Urococcus is to remain as a genus, it must be as “ Urococcus Kiitz.2” U. insignis (Hass.) Kiitz. [ = Hcematococcus insignis Hass.; Chroococcus macrococcus Rabenh.] is not uncommon in the bogs of moorland and upland districts. Diam. of cells without integument 25 — 51 g, with integument 41 — 78 g. It sometimes occurs in quantity amongst submerged Sphagnum, especially in peaty pools. Family 6. HYDRODICTYACE.®. In this family of the Protococcoidese the plant-body consists of a non-motile coenobium of coenocytes which floats freely in the water. The coenocytes are arranged either as a flat plate or after the manner of a net, and they are of very variable form. In Euastropsis there are only two cells (coenocytes?) in the coenobium, in Pediastrum there may be more than fifty coenocytes, and in Hydrodictyon there are often many hundreds, the coenobium reaching a length of several centimetres. Multiplication sometimes occurs by the formation of autocolonies. Reproduction takes place by the development of hypnospores (usually aplanospores), by the formation of new coenobia by the apposition of biciliated zoogonidia which have become quiescent, and in some by the fusion of isogamous gametes to form a zygo- spore. The germination of the zygospore is indirect. 1 Hassall, Brit. Fresliw. Alg. 1845, i, p. 322. - Kutzing, Spec. Algar. 1849, p. 206. Vide West & G. S. West in Journ. Bot. June, 1897, p. 239. 207 Hydrodictyacece The method of reproduction by the apposition of quiescent zoogonidia to form new coenobia distinguishes the Hydrodictyacese from all the other Protococcoidese, and the coenocytic nature of the coenobium also distinguishes them from the coenobic forms of the Protococcaceae (or Autosporaceae). It is most likely that Hydrodictyon and Pediastrum have no direct affinity, the resemblance being due to convergence of modification, but until more is known concerning the phylogenetic relationships of these genera the Hydrodictyacese are best divided into the two following sub-families : — Sub-family I. Hydrodictyece. New coenobium formed by apposition of zoogonidia within the mother-coenocyte. Plants macroscopic, cceno- cytes arranged in the form of a net. Sub-family II. Pediastrece. New coenobium formed by apposition of zoogonidia outside the mother-coenocyte. Plants microscopic, coeno- cytes arranged to form a flat plate. Sub-family I. HYDRODICTYEAI This sub-family includes only the one genus Hydrodictyon. The plants are macroscopic and consist of very large coenocytes which are disposed so as to form a more or less cylindrical net. The zoogonidia swarm and become quiescent within the wall of the mother-coenocyte, and there they become apposed to form the new coenobium. Genus Hydrodictyon Roth, 1800. The coenobium is a net- like sac, freely floating in the water, and reaches a length of 8 — 10 centimetres. The meshes of the net are of variable size and each one is bounded by either five or six coenocytes, the angles being formed by the junction of three coenocytes. The protoplasm of each coenocyte is confined to a lining layer containing many nuclei, the central portion of the segment being occupied by a large vacuole. There are no definite chloroplasts, the chlorophyll being more or less diffuse through the whole protoplasm, but numerous pyrenoids are present. The normal method of reproduction is by the formation of a very large number of zoogonidia within the mother-coenocyte, which swarm within the wall of the segment and then become quiescent, immediately forming a reticulated daughter-coenobium by the apposition of their extremities. The old cell-wall then ruptures and the young coenobium is set free. The zoogonidia are 208 Chlorophycece biciliated ; they possess one nucleus and contain a single pyrenoid, but as the young reticulum increases in size the pyrenoids of each segment multiply rapidly. Timberlake1 states that the swarming condition of the zoogonidia can he induced by the use of a reagent composed of 100 c.c. of 1 per cent, solution of iridium chloride and 3 c.c. of glacial acetic acid. He points out that the cell-contents first break up into large multinucleated masses, which in turn Eig. 84. Hydrodictyon reticulatum (L.) Lagerh., from the River Lea. A, nat. size ; B, small portion of a young colony ( x 110) ; C, part of a large coenocyte contain- ing a very young colony (xllO); D, quiescent zoogonidia (x480); E, zoogo- nidia which are becoming apposed to form a new colony ( x 480), p, pyrenoid ; P, slightly older coenocyte with four pyrenoids (p), x 480. break up into smaller masses, until each mass contains a single nucleus. Reproduction also occurs by the union of isogamous, 4-ciliated gametes, which escape from the mother-ccenocyte by a lateral pore. The gametes are smaller than the zoogonidia and their escape is preceded by a swelling of an inner layer of the cell-wall. This inner layer ruptures the outer layers and protrudes as a vesicle in which the gametes swarm. On becoming free they conjugate in pairs2. The zygospore is globose and after a 1 Timberlake in Botan. Gazette, xxxi, 1901, p. 203. 2 Klebs in Bot. Zeitung, xlix, 1891. 209 Ilj/d) 'odictyacece short rest forms two or four large biciliated zoospores, which on coming to rest assume a polyhedral form. The repeated division of the cell-contents of these polyhedral bodies results in the formation of numerous zoogonidia, which by apposition give rise to new net-like coenobia. The division of the protoplasm of the adult cells of this genus to form either zoogonidia or gametes is a splendid example of free- cell-formation. The only known species, H. reticulatum (L.) Lagerh., which has received the name of the “Water-net,” is a very rare plant in Britain. The average length of the adult coenocytes just before they become zoogonidangia is 4 or 5 mm., but they are known to attain a length of 1 cm. The length of the quiescent zoogonidia at the time of their apposition is 1 3*5 — 25 p. The swarming zoogonidia are 10 p in length by 8 p in breadth and the gametes are a little smaller. Fig. 84. Sub-family II. PEDIASTREiE. The plants of this sub-family are microscopic in size and consist of a number of small coenocytes united to form a flat, disc-like coenobium. The zoogonidia swarm in a vesicle which is protruded from the mother-ccenocyte and the new coenobium is thus formed outside the old coenocyte. Genus Pediastrum Meyen, 1829. The coenobium is always a free-floating, flat plate, disc-shaped or star-shaped, and consists of a single layer of small coenocytes which is rarely duplicated in the centre. The coenocytes are either parenchymatous and closely united, or there are perforations of variable size between them which give the coenobium a sieve-like aspect. The marginal coenocytes are of different form from those in the centre and they are generally furnished with a pair of diverging processes. There is a single chloroplast in each coenocyte, containing one pyrenoid. The number of coenocytes in a coenobium varies from 2 to 64, or even more. Coenobia of two coenocytes are very rarely observed, and possibly belong to the genus Euastropsis, but in one species (P. tetras ) four is the regular number. The coenocytes are often arranged in distinct rings round a central one, 8, 16, 32, or more being the number in the coenobium. Nageli1 pointed out that the coenobia were usually constructed as follows : — Colony of w. A. 1 Nageli, Gatt. einzell. Alg. Zurich, 1849. 14 - 1 0 Chlorophycece ^ = 1 "** ^ j colony of 16 = 1 -f 5 + 10 ; colony of 32 = 1 + 5 + 10 + 16; but this arrangement is not always observed. Multiplication occurs in some species bv the formation of autocolonies which arise by the division of the contents of a single coenocyte. Hypnospores are also frequently formed (fig. 85 E h). Fig. 85. A, Pediastrum integrum Nag., from Ben Lawers, Perthshire ( x 475). B, P. tricomutum Borge, from Glen Tummel, Perthshire (x475). C and D, P. tetras (Ehrenb.) Balfs, from Pilmoor, N. Yorks. (x475). E, P. duplex Meyen, from Lough Fea, Londonderry, Ireland ; hypnospores (h) ( x 475). F — H, P. Boryanum (Turp.) Menegh.; G, showing escape of zoogonidia; H, young colony formed by apposition of quiescent zoogonidia ; F, from Frizinghall, W. Yorks., x 475 ; G and H, x 220 (after Kerner). I, two marginal cells of P. glanduliferum Benn., from Bisley Common, Surrey. J — L, zoogo- nidia and gametes of P. Boryanum (after Askenasy); J, zoogonidia and K, gamete (x500); L, conjugation of gametes to form zygospores (z) (gametes x 730, zygospores x 220). Reproduction takes place by the successive division of the contents of a coenocyte to form a number of zoogonidia, which are suddenly liberated into an external vesicle through a slit in the wall of the mother-coenocyte. The zoogonidia swarm in this vesicle and at length become quiescent, arranging themselves in one plane as a new coenobium. Biciliated gametes have been observed by Askenasy1, which are much smaller than the zoogonidia 1 Askenasy, ‘Entwickl. von Pediastrum,’ Ber. Deutsch. Bot. Gesellsch. vi, 1888. 211 Hydrodictyacece, and conjugate in pairs, the zygospore being polyhedral in form. A new coenobium arises by the segmentation of the contents of this polyhedral body. The species of this genus exhibit great variation in the characters both of their central and marginal cells, the length of the marginal processes, etc. ; especially is this the case in cultures. Their natural habitat is in small ponds and ditches amongst other water-plants, and not uncommonly in quiet bog-pools. Sometimes they are numerous in the freshwater plankton. The two most abundant species are P. Boryanum (Turp.) Menegh. (fig. 85 F — H and J — -L) and P. tetras (Ehrenb.) Ralfs (fig. 85 C and D), the former sometimes reaching a relatively large size (200 p in diameter). The marginal processes of P. Boryanum are extremely variable and the cell-walls are often granulated. P. tetras generally occurs in coenobia of 4 (diam. 10 '5 — 18 p) or 8 (diam. 22 — 29 p) cells. P. duplex Meyen 1829 (fig. 85 E) ( = P- pertusum Kiitz. 1845) is also a widely distributed species. P. simplex Meyen ( = P. clathratum Lemm.) and P. integrum Nag. (fig. 85 A) are much rarer species. P. Boryanum and P. duplex are the most abundant species in the plankton. Genus Euastropsis Lagerh., 18941. In this genus the coenobium is free-floating and consists of two flattened cells. Fig. 86. Euastropsis Richteri (Schmidle) Lagerh. A and B, from near Senens Cornwall ( x 760). C — E, showing formation of young coenobia (after Lager- heim ; highly magnified). which are closely applied along their straight inner margins, the outer margins being widely notched. The entire coenobium has a certain resemblance to a minute species of the genus Euastrum, and was originally described as such2. There is a single parietal chloroplast in each cell containing one pyrenoid. Lagerheim describes the presence of one nucleus in each cell, but suggests that there may be more. Reproduction occurs by oval, biciliated 1 Lagerh. in Tromso Museums Aarshefter 17, 1894, pp. 12—21, t. i f. 8—27 2 Schmidle in Flora, 1894, p. 60, t. 7, f. 25. 14—2 212 Chlorophycece zoogonidia which swarm in a vesicle as in Pediastrum. On becoming quiescent they arrange themselves in pairs and assume the form of the adult cells, each pair forming one coenobium. E. Richteri (Schtnidle) Lagerh. is only known in the British Islands from Cornwall. Length of coenobium 10 — 40 g, breadth 6 — 25 p (fig. 86). Family 7. PROTOCOCCACE.® (or AUTOSPORACE.ffij. The Algae included in this family are free-floating, solitary or colonial, and are most commonly associated to form minute colonies of a definite construction. The cells are sometimes firmly united to form a definite coenobium, as in Goelastrum and Sorasirum, but as a rule the colonies easily dissociate into single cells or smaller groups (or families) of cells. With few exceptions there is very little mucus surrounding the colony. There is generally one chloroplast (sometimes many) in each cell, parietal or occupying the whole cell, and with or without a pyrenoid. There is one cell-nucleus. Multiplication takes place by the successive division of the cell-contents either to form spore-like bodies which assume the characters of the mother-cell before being liberated, or to form colonies which on liberation resemble the mother-colony in all except size. These are known respectively as autospores and autocolonies. Multiplication by autospores, although such a characteristic feature of the Protococcace®, is not confined to this family, as it occurs in Pediastrum, Radiococcus and certain other genera. Reproduction by zoogonidia or gametes is unknown except in the genus Dictyosphcerium. The Alg® of this family are well marked off from the Pleuro- coccace® and Palmellace® by their definite colonies and by their method of multiplication. They are little removed from the most primitive forms of green Alg® and a few of them are capable of profound modification by cultivation in different media. In their natural state, however, they exhibit a remarkable constancy of character and many of them are ubiquitous in all climates. The family is best subdivided into the seven following groups : — Sub-family I. Ccelastrece. Cells primarily globose or broadly lunate, forming a definite spherical or polyhedral coenobium. Sub-family II. Crucigeniece. Cells of variable form, arranged in a regular flat plate. P rotococcacece 213 Sub-family III. Selena&trece. Cells elongated, often greatly at- tenuated and frequently curved; solitary or associated in definite or loosely coherent colonies. One chloroplast in each cell. Sub-family IV. Oocystidece. Cells globose, ellipsoid, reniform or sometimes sublunate. Daughter-cells generally retained in the en- larged wall of the mother-cell. Several or many chloroplasts (rarely O one) in each cell. Sub-family V. Tetraedriece. Cells solitary ; flattened and angular, with a definite number of angles, or tetrahedral, octahedral or poly- hedral. Angles generally furnished with simple or furcate spines. Sub-family VI. Phythelieoe. Cells globose or ellipsoid, solitary or colonial, furnished with two or more long attenuated bristles. Sub-family VII. Dictyosphwi'iece. Cells globose, ellipsoid or ovoid, associated to form colonies, and joined more or less completely by the persistent old walls of the mother-cells, which sometimes become trans- formed into special connecting-threads. Sub-family I. CCELASTREiE. This is the only group of the Protococcaceae in which there is a definite and regular coenobium of spherical or polyhedral form. The cells are either globose or polygonal, with or without short projecting processes, by means of which they are united to form a hollow sphere ; or they are broadly lunate and united at the centre of the spherical colony by short stalks. Multiplication is by the formation of an autocolony in each cell of the coenobium, which is ultimately set free by the dissolution or splitting of the mother- cell-wall. Genus Ccelastrum Nag., 1849. [ Hariotina Dang., 1889.] The coenobium, which is spherical or polyhedral, is hollow and is com- Fig. 87. A, Ccelastrum cambricum Archer, from Lough Gartan, Donegal, Ireland, b D, C. sphcericum Nag. ; B and C, small ccenobia from near Penzance, Cornwall; D, large ccenobia giving rise to daughter- ccenobia (autocolonies)’ from Bowness, Westmoreland. (All x 475.) 214 Chlorophycece posed of a variable number of cells united by their lateral margins to form a single peripheral layer. In some species the cells are globose, in others more or less angular, and in others they are furnished with projecting processes by means of which they are joined together. The latter forms exhibit intercellular spaces of variable size and the cells are often furnished with a truncate projection (rarely two) on their free outer surfaces. The maximum number of cells observed in a single colony is 64, but in most species the number is 8, 16, or 32. Each cell contains a single chloroplast with one pyrenoid. The multiplication is by typical autocolonies, which are liberated by a split in the wall of the mother-cells or more rarely by the entire gelatinization of the mother-cell-walls. Single spores are sometimes developed from individual cells. The two most frequent species are C. sphcericum Nag. (diam. coenob. 18 — 92 fx\ diam. cells 4 — 23 p; fig. 87 B — D) and G. cavibricum Arch. 1868 ( = C. pulchrum Schmidle 1892); fig. 87 A. In the former species the cells are somewhat conical with a polygonal base, and in the latter species they are more or less distinctly lobed and furnished at the same time with a truncate surface projection. C. cubicum Nag., C. rnicroporum Nag., C. verrucosum Reinsch and C. proboscideum Bohlin are species of considerable rarity. Fig. 88. C 'oelastrum reticulatum (Dang.) Senn, a small irregular form from Churchill, Donegal, Ireland ( x 475). C. reticulatum (Dang.) Senn, which was made the type of the genus llariotina by Dangeard, differs from all other species in the nature and dis- position of the processes of the cells. The ccenobium consists of 4, 8, or 16 cells, and the young autocolonies are retained for a relatively long period within the walls of the mother-cells. The processes of attachment of the cells are narrow prolongations, often curved, and sometimes irregularly disposed. This plant has also been named G. subpulchrvm by Lagerheim and C. distans by Turner, but Dangeard’s name1 takes precedence. It is known from Donegal, Ireland2, and large colonies are not infrequent in the plankton of Lough Neagh. Diam. cells 6 — 24 /x ; fig. 88. 1 Dangeard, ‘Memoire sur les Algues,’ Le Botaniste, 1889 ; Chodat A Huber in Bull. Soc. Bot. France, tom. xli, 1894. 2 West & G. S. West in Journ. Bot. March, 1903. Protococcacece 215 Genus Sorastrum Kiitz., 1845. [Selenosphcerium Cohn, 1879.] The coenobium is globose, consisting commonly of 16 to 64 (rarely 4 or 8) stalked cells, the stalks uniting in the centre of the coenobium to form a small faceted sphere. In small colonies the stalks of the cells are very short and the central sphere is not always ap- parent. The cells are broadly sublimate, reni- form, or subtriangular in shape, and each extremity is furnished with two spines (rarely one) of moderate length. There is one chloroplast, containing a pyrenoid, in each cell. The multiplication is by the formation of autocolonies, but the details have not been worked out. The British species usually met with is S. spinulosum Nag. ; diam. of ccenobia 31 — 90 /x; length of cells without spines 11 — 26 /x; fig. 89. It is a scarce plant occurring in bog-pools or amongst aquatic Phanerogams in the quiet margins of lakes. S. Americanmn (Bohlin) Schmidle occurs in the freshwater plankton of the Outer Hebrides. Fig. 89. Sorastrum spinulosum Nag., A, small coenobium from Pilmoor, N. Yorks. ( x 475); B, large coenobium from Clifden, Galway, Ireland ( x 450). Sub-family II. CRUCIGENTExE. The coenobia consist of few or many cells regularly arranged in the form of a flat plate. The cells are very variable in form, generally somewhat rounded, and occasionally furnished with spines. They are disposed in groups of four, the latter being held in position by a tough mucilage. In some the chloroplasts possess a single pyrenoid, but in others pyrenoids are absent. The multiplication is by autocolonies, which in some cases are set free almost immediately, but in others remain for some time as part of the mother-colony. Genus Crucigenia Morren, 1830. [Staurogenia Kiitz., 1849 ; Lemmermannia Chodat, 1899; Willed Schmidle, 1900.] The crenobium consists of 4, 8, 16, or 32 cells arranged as a flat plate and held in position by a mucilaginous envelope, but under favourable conditions of environment as many as 128 cells have 216 Ohio i -ophycew been observed in one colony1. Even in the large colonies the cells are distinctly arranged in groups of four, the cells of each group being closely adherent except in the centre, where there is usually a small quadrate or rhomboidal space. The cell-walls are smooth, and each cell contains a single chloroplast with or without a pyrenoid. Multiplication occurs by autocolonies of four cells pro- duced by the cruciform division of the contents of a mother-cell. Schmidle* has given a good account of this genus under the name of ‘ Staurogenia,’ and has also described the formation of hypnospores. Chodat has separated the genus Lemmermannia from Crucigenia mainly owing to the absence of a pyrenoid from the chloroplast. For the present I prefer to regard the two as identical. There is no difference in the structure of the colonies and the mode of multiplication is pre- cisely the same in each, the daughter- cells persistently remaining as parts of the mother-colony in several species of Crucigenia as well as in Lemmermannia. In all the speci- mens I have observed of Crucigenia Tetrapedia, which was the plant upon which Chodat founded the genus Lemmermannia3, there was the same small gap in the centre of each group of four cells that is present in all other species of Crucigenia. C. rectangularis (Nag.) Gay is the most frequent species of the genus; length of cells 5 — 9 p ; breadth 4 — 6 /x ; fig. 90 A — C. C. quadrata Morren 1830 ( = ? C. triangularis Chod. 1900) is a much rarer species ; diam. cells 5 — 5 '5 jx ; fig. 90 D and E. C. Tetrapedia (Kirchn.) W. & G. S. West4 ( = Lemmermannia emarginata Chod.) is known from the plankton of Lough 1 West & G. S. West in Ann. Bot. xii, 1898, p. 36. 2 Schmidle in Berichte Deutsch. Bot. Gesellsch. 1900, Bd xviii, pp. 149 — 157. 3 Under the beading of Lemmermannia emarginata, Chodat (in Beitriige zur Krypt.-fl. Schweiz, Bd I, Heft 3, 1902, p. 222) makes some irrelevant remarks concerning Tetra'edron pentaedrica W. & G. S. West (which, for some reason, he seems to think was described as a Tetrapedia !) and Tetrapedia morsa W. & G. S. West, which clearly show that he is quite unacquainted with either the published descriptions or figures of the plants in question. 4 West & G. S. West in Trans. Boy. Irish Acad. vol. xxxii, sect. B, part i, 1902, p. 62. Fig. 90. A— C, Crucigenia rect- angular is (Nag.) Gay, from Lough Shannacloontippen, Galway, Ire- land; C, with formation of auto- spores. D and B, C. quadrata Morren, from Settle, W. Yorks. F, <7. Tetrapedia (Kirchn.) W. & G. S. West, from plankton of L. Neagh, Ireland. G and H, Tetrastrum Staurogeniceforme (Schrod.) Chod., from near Bievaulx Abbey, N. Yorks. (All x520.) ProtococcacecB 217 Neagh, Ireland ; diam. cells 4’8— 9’5 /x ; diam. coenob. of 4 cells 10-5— 15-5 g; fig. 90 F. All the species are regular constituents of the freshwater plankton. C. irregularis Wille1 is known from the plankton of several lochs in the Shetlands, and from Norway. It is a most interesting species, differing from C. rectangularis only in the somewhat irregular colonies and the absence of pyrenoids from all the cells of the colony. Some of the cells in a colony of C. rectangularis are often destitute of pyrenoids, and Wille was quite right in placing his Norwegian plant ( C . irregularis) as a species of Crucigenia. U nfortunately, however, the useless generic name ‘ Willea ’ has been put for- ward by Schmidle2 to include Crucigenia irregularis. This genus is simply founded upon the absence of pyrenoids from all the cells of the colony ; and Lemmerraann3 has since placed ‘ Willea 3 as a subgenus of Cohniella, also owing to the absence of pyrenoids ! Surely, in Willea, Lemmermannia, and Lemmermanu’s suggestion that Willea should be a subgenus of Cohniella, the climax of absurdity has been reached with regard to the presence or absence of pyrenoids as a generic distinction. No one who has been fortunate enough to observe colonies of Crucigenia irregularis could dispute their close affinity with forms of C. rectangularis. The former species has most likely had a direct origin from the latter. Genus Tetrastrum Chodat, 1895. [ Cohniella Schroder, 1897.] The coenobium consists of four cells arranged in one plane and surrounded by a thin mucous envelope. The cells are rounded or somewhat angular, and they possess from two to five spines of variable length on their external margins. The multiplication is by the formation of autocolonies of four cells in each mother-cell. Pyrenoids may be present or absent. The genus only differs from Crucigenia in the presence of the marginal spines and in the regular 4-celled condition of the coenobium. The only two known species of the genus are T. heteracanthum (Nordst.) Chod. and T. Staurogeniaeforme (Schrod.) Chod. The former is known from West Ireland and the latter (fig. 90 G and H ; diam. of cells without spines 3 — 6 g) occurs in North Yorkshire. Sub-family III. SELENASTREiE. This group of the Protococcacese is characterized by the elon- gation of the ceils, which are often very narrow with the extremities attenuated to fine points. They are frequently lunate or arcuate, and may be solitary or associated to form colonies of a more or less 1 Wille, 1 Algologische Notizen IV,’ Nyt Magazin for Naturvidenskb. Bd 38 Heft 1, 1900, p. 10, t. 1, f. 15. 2 Schmidle, 1. c. p. 157. 3 Lemmermann in Berichte Deutsch. Bot. Gesellseh. 1904, Bd xxii, p. 22. 218 Chlorophycece fragile character, the cells in some instances being held in position only by the enveloping mucus. There is a single chloroplast in each cell, which may contain one or several pyrenoids or may be destitute of them. In rare instances the chloroplast is fragmented. The cell-wall is firm but delicate. Multiplication is principally by autospores and autocolonies, and this often takes place by the oblique division of the contents of the mother-cell. The following genera of the Selenastrese are British : — A. Cells or colonies almost destitute of mucus. * Cells attenuated to acute apices ; multiplication by oblique division of contents of mother-cell, the daughter-cells often remaining loosely attached by their apices Dactylococcus. ** Cells ellipsoid or much attenuated, forming more or less definite colonies consisting of a row of cells in one plane Scenedesmus. *** Cells sublimate or ellipsoid, arranged in groups of four in one plane, the groups being attached to form an irregular colony Dimorphococcus. **** Cells elongate and acutely attenuated, often lunate, solitary or loosely grouped in irregular bundles. t Cells of moderate length, usually with not more than one pyrenoid Ankistrodesmus. ft Cells greatly elongated, pyrenoids numerous .. . Closteriopsis. ***** Cells ovoid, oblong, or club-shaped, often at- tached by their apices to form radiating colonies Actinastrum. ****** Cells attenuated, lunate, arranged back to back to form definite colonies Selenastrum. B. Colonies enveloped in a copious mucus. Cells lunate or much curved, disposed more or less irregularly in the enveloping jelly Kirchneriella. Genus Dactylococcus Nag., 1849. [Indus. Coccomyxa Schmidle, 1901.] The cells are ellipsoidal, fusiform or pyi’iform, often sublimate and generally with acute apices, which are some- times unequally prolonged into spine-like processes. They occur solitary or loosely connected by their acute extremities to form fragile colonies of few cells. The chloroplast is single and parietal, and sometimes contains a single pyrenoid ; occasionally it becomes fragmented into two, three, or four parietal pieces. Some species are truly aquatic, but most of them occur on wet rocks and moist surfaces, forming a dark green mucous stratum. Multiplication is P rotococcacem 219 by the oblique longitudinal division of the contents of the mother- cell. D. bicaudatus A. Br. is a lunate species with the apices greatly prolonged and its chloroplast is destitute of a pyrenoid ; length of cells 13—39 y ; diam. of cells 2-5— 5-8 y ; fig. 91 A. D. dispar W. & G. S. West is unequally Fig. 91. A, Dactylococcus bicaudatus A. Br., from near Bradford, W. Yorks. ° B and C, D. bicaudatus var. subramosus W. & G. S. West, from Widdale Fell, N. Yorks. D, D. dispar W. & G. S. West, from Dorking, Surrey. (All x 520.) developed at the extremities and the cells frequently become irregular; fig. 91 D. The Alga described as D. Debaryanus Reinsch, which often occurs in large numbers as an epiphyte on Cyclops and other small Crustacea, is a species of Characium. There is little doubt that D. infusionum Nag. is merely a state in the life- history of Scenedesmus obliquus (Turp.) Kiitz. It is usually aquatic and often occurs in immense quantity in the water of flower-pots and in aquaria. It exhibits great variability of form and its chloroplast commonly contains a pyrenoid ; length of cells 7'5 — 19 y, breadth 2-8 — 5'8 y. Genus Scenedesmus Meyen, 1829. In this genus there is a coenobium of more or less definite form, consisting of a variable number of cells arranged either in a single row or in two alterna- ting rows. The cells are generally disposed in one plane and in some species the terminal cells of the row differ considerably from the central cells. There is a wide range of variation in the form of the cells, which may be ellipsoid, oblong, or fusiform, and the extremities of all the cells of the coenobium, or of the terminal cells only, may be furnished with one or more spines. In some the cells are longitudinally ridged and in others spines may be attached to the middle region of the cells. There is a large parietal chloroplast, often completely filling the cell, and generally containing a single pyrenoid. Multiplication takes place by the formation of autospores and autocolonies, and rarely by globular resting-spores. In cultures these plants have been said to pass into a palmelloid or gloeocystiform condition. 220 Chlorophyceoe Grintzesco1 has shown that remarkable malformations of Scenedesmus obliquus can be produced by cultures in a nutritive medium of agar and glucose, and that this Alga possesses the physiological property of liquefying gelatine. He finds a Dacly- lococcus- stage equivalent to Nageli’s JJ. infusionum, and he attributes the extensive geographical distribution of this plant to the ease with which it adapts itself to different media and different temperatures. Fig. 92. A, Scenedesmus obliquus (Turp.) Kiitz., from Bradford, W. Yorks. B, the state of S. obliquus known as Dactylococcus infusionum Nag., from Bowness, Westmoreland. C, S. bijugatus (Turp.) Kiitz., from various localities. D — F, S. quadricauda (Turp.) Br6b. , from Bradford, W. Yorks. G, S. quadricauda var. horridus Kirchn., from S.E. Surrey. H, S. quadricauda var. maximus W. & G. S. West, from Pilmoor, N. Yorks. I— K, S. denticulatus Lagerh. var. linearis Hansg. ; I and J, from Westmoreland ; K, from Mayo, Ireland. L, S. spi- catus W. cfe G. S. West, from Saltburn, N. Yorks. (All x 520.) aut, autocolonies. There are about 10 British species, of which S. quadricauda (Turp.) Breb. (fig. 92 D — F), S. bijugatus (Turp.) Kiitz. ( = S', obtusus Meyen ; fig. 92 C) and S. obliquus (Turp.) Kiitz. ( = S. acutus Meyen ; fig. 92 A and B) are general and abundant. All three species vary much in size and also in general characters ; they are regular constituents of the freshwater plankton, but occur most abundantly in stagnant water, especially in association with Pediastrum Boryanum, Ccelastrum sphcericum, etc. The cells of the largest forms of S. quadricauda reach a length of 30 p and a breadth of 14 p. S. denticulatus 1 Grintzesco, ‘ Recherch. Experiment, sur la Morph, et la Phys. de Scenedesmus acutus,’ Bull, de l’Herb. Boissier, 2me. ser. 1902. Proto cocccicece 221 Lagcrh. var. linearis Hansg. (fig. 92 I-K) is widely distributed but rarely occurs in such abundance as the three preceding species. The cells of S. cos- tatus Schmidle and S. acutifonnis Schroder possess prominent longitudinal ridges. S. Systrix Lagerh. and S. granulatus W. & G. S. West are very uncommon species remarkable for their external ornamentation. Genus Dimorphococcus A. Br., 1849. The cells are arranged in composite colonies, each colony consisting of an irregular agglome- ration of definite groups of four cells. The cells of each group are disposed obliquely in one plane and are of two kinds ; the two central cells are ellipsoid or oblong and the two outer cells are sublimate. The groups of four are held in position by irregular portions of the old walls of the mother-cells. There is a large parietal chloroplast with one pyre- noid. Autocolonies of four cells are produced in each mother-cell ; these remain attached to the parent-colony until the latter becomes too large, when it breaks up into several smaller colonies. D. lunatus A. Br. is a rare plant in Britain1. It sometimes occurs in the plankton, but is met with more often in the small tarns of mountainous dis- tricts. The colonies are 57 — 86 p in diameter and the cells 11 — 25 p in length ; fig. 93. Genus Ankistrodesmus Corda, 1838; Ralfs, 1848; Archer, 1862. [Rhaphidium Ktitz., 1845; Schroderia Lemmermann, 1898.] The cells are fusiform with acute apices, rarely obtuse, and they are generally many times longer than their diameter. They are straight, lunate, arcuate, or sigmoid, and although frequently solitary, are more often variously grouped in loose aggregates. In some forms the apices are greatly produced and almost bristle- like. The cell-wall is very thin and there is a single parietal chloroplast, usually occupying the greater part of the cell-cavity. 1 Vide West in Journ. Boy. Micr. Soc. 1892, p. 735, t. ix, f. 39; West & G. S. West in Trans. Boy. Irish Acad, xxxii, sect. B, part i, 1902, p. 66. Fig. 93. Dimorphococcus luna- tus A. Br. ; A, from the plankton of Loch Mor Bharabhais, Lewis, Outer Hebrides; B, from Bowness, Westmoreland ( x 520). 222 Chlorophycece Pyrenoids are commonly absent, but one or two may occur in some forms. Multiplication takes place by means of autospores formed by the division of the contents of the mother-cell ; they generally arise by oblique divisions, but may be produced by repeated transverse or longitudinal divisions. There has never been any doubt concerning the identity of Ankistrodesmus Corda and Rhaphidium Kiitz., but this fact only seems to have been acted upon by Ralfs in his ‘British Desmids and Archer1 when he described his Ankistrodesmus acutissimus. I am indebted to Dr Nordstedt of Lund for furnishing me with full information concerning Ankistrodesmus from Corda’s scarce memoir2. The remarks made by Corda under the heading of “ Ankistrodesmus novum genus,” and the description and figures he gives of A. fusiformis, are quite sufficient to establish this genus, and also to show that Ktitzing’s later genus Rhaphidium is identical with it. Kuntze3 endeavoured to prove that these two genera should be placed as synonyms of ‘ Micrasterias Corda (1835)4,’ a genus which he tried to establish on the assumption that Micrasterias A g. (1827) should be relegated to Helierella Bory (1826). Nordstedt5 has clearly shown that the name ‘ Helierella ’ cannot be used as a generic name, and in consequence Micrasterias Ag. (1827) is a valid genus. It follows from this that Ankistrodesmus Corda is the earliest generic name given to the common plants which Ktitzing named Rhaphidium, and as Corda’s description and figures are also suffi- ciently characteristic, his name must' be accepted for the genus. The genus Scliroderia was established by Lemmermann6 in oi’der to include an Alga which had previously been found by Schroder7 and described by him as “ Reinschiella? setigera.” This Alga I have observed from North Yorkshire along with other plankton forms. The only distinction that can be drawn between Scliroderia and Ankistrodesmus is the greater attenuation of the 1 Archer in Quart. Journ. Micr. Sci., n. s. vol. 2, 1862, pp. 255 — 261, t. xii (in part). a Corda in Almanach de Carlsbad par J. de Carro, Prague, 1838, pp. 196 — 198. 3 Kuntze Kevis. Gen. Plant, n, 1891, pp. 904, 905. 4 Corda in Almanach de Carlsbad, 1835, p. 206. Corda’s only mention of Micrasterias falcata is in the description of the plates on p. 206 ; there is no text reference and the figures are on t. ii, f. 29. 5 Nordstedt in Hedwigia, 1893, Heft 3, pp. 149 — 151. 0 Lemmermann in Hedwigia, 1898, Bd xxxvii, p. 311. 7 Schroder, ‘Ueber das Plankton der Oder,’ Berichte Deutsch. Bot. Gesellsch. 1897, Bd xv. j Protococccicece 223 apices of the former, and I do not regard this as a sufficient generic difference. “ Rsinschicllci ? scticj era must be considered as an Ankistrodesmus having the same relationship to other species of the genus as Dactylococcus bicaudatus has to other species of Dactylococcus. Fig. 94. A, Ankistrodesmus falcatus (Corda) Ealfs, from Bowness, Westmoreland. B and C, A. falcatus var. acicularis (A. Br.) ; B, from Pilmoor, N. Yorks.; G, from the plankton of Loch M. Bharabhais, Lewis, Outer Hebrides. D, A. falcatus var. tumidus nob., from Puttenham Common, Surrey. E, A. falcatus var. mvrabilis nob., from Wimbledon Common, Surrey. F, A. setigerus (Schrod.) nob., from near Rievaulx Abbey, N. Yorks. G and H, A. Pfitzeri (Schrod.) nob., from the plankton of Loch M. Bharabhais, Lewis. (All x 520.) auts, autospores. As plants of this genus are often very abundant and the synonymy is much confused, the following synopsis will be found useful : — A. falcatus (Corda) Ralfs. [Micrasterias falcata Corda 1835 ; Ankistro- desmus fusiformis Corda 1 838 (in part) ; Rhaphidium fasciculatum Kiitz. 1845 ; R/i. polymorphism Fresen. var. falcatum Rabenh.] Cells lunate or arcuate, 16 — 24 times longer than the diameter (1’5 — 3 p), with the apices acute; associated in loose aggregates or close bundles, rarely solitary. Chloroplast usually devoid of a pyrenoid. Fig. 94 A. Var. acicularis — . [Rhaphidium aciculare A. Br. 1849 ; Rh. polymorplium var. aciculare Rabenh. ; Closterium subtile Breb. ; Ankistrodesmus acutissimus Arch. 1862.] Cells usually solitary, commonly straight or slightly curved, with acutely attenuated apices. Length 36 — 65 p ; breadth 2-5 — 3-5 p. Chloroplast sometimes furnished with one pyrenoid (or more). Fig. 94 B and C. It is most probable that Rh. pyrogenum Chod. belongs here. Var. duplex — . [Rhaphidium duplex Kiitz. 1845.] Cells elongate, sig- moid, associated end to end in pairs. Rh. nivale Chod. may possibly be a form of this variety. 224 Chlorophycece Var. tumidus — . [Rh. polymorphum var. tumidum W. & G. S. West 1897.] Cells solitary or in small aggregates, curved, in the middle inflated; apices most acute ; length 61 — 73 p ; breadth 4 '5 — 6 '5 p. Chloroplast with one or two pyrenoids, or destitute of them. Fig. 94 D. Var. mirabilis — . [Rh. polymorphum var. mirabile W. & G. S. West 1897. ] Cells solitary and considerably longer than in typical A. falcatus, variously curved, often sigmoid ; apices very acute. Chloroplast completely interrupted in the middle of the cell and often fragmented ; vacuoles in the cell- protoplasm often containing a single moving corpuscle. Length up to 117 p; breadth 2 — 35 p. Fig. 94 E. Var. spiralis — . [Rh. spirale Turn. 1893; Rh. fasciculatum var. spirale Chod. 1902.] Cells grouped in bundles of 4 or 8, twisted round each other in the central region of the cells, but free at their extremities. Var. spirilliformis — . [Rh. polymorphum var. spirale W. & G. S. West 1898. ] Cells always solitary, spirally twisted, making 1 — H turns; apices very acute. Often occurs in prodigious quantity in stagnant water. The above varieties of A. falcatus are widely distributed and some of them are abundant. They occur most abundantly in small ponds, but are also frequent in the freshwater plankton. A. convolutus — . [Rhaphidium convolutum Rabenh. 1868.] Cells solitary, short, only 3 — 6 times longer than the diameter, lunate or arcuate ; apices verj' acute; diam. 3 — 6 p. A. Pfitzeri — . [Rhaphidium Pfitzeri Schroder.] Cells straight, some- what asymmetrical, 5^ — 6 times longer than the diameter, much attenuated with rounded apices ; forming small colonies embedded in mucilage. Length 38 — 49 p ; breadth 7 — 8 p. In the Scottish plankton ; not uncommon. Fig. 94 G and H. Perhaps this species would be better placed as a form of A. biplex (Reinsch) nob. [ — Rh. biplex Reinsch 1867.] A. setigerus — . [Reinschiella ? setigera Schroder 1897; Schroderia setigera Lemm. 1898; Rhaphidium setigerum W. & G. S. West 1901.] Cells fusiform, with the apices much attenuated into fine hair-like prolongations. Chloroplast with one pyrenoid. Diam. 5 '7 — 6'5 p ; length 75 — 88 p. Mostly found in the plankton. Fig. 94 F. Genus Closteriopsis Lemm., 1898 h This genus only differs from Ankisti'odesmus in the great length of the cells and in the consequent increase in the number of pyrenoids. The cells are narrow, greatly elongated, and the extremities are much attenuated into bristle-like points. The chloroplast contains a dozen or more pyrenoids in an axial series. Cl. longissima Lemm. is only known from the plankton. In the British Islands it has been found in Lough Neagh, Ireland, from Finstown, Orkneys, and in Loch Asta, Shetlands. Length 330 p ; breadth 3’8 — 4-2 p. Genus Actinastrum Lagerh., 1882. The cells are ovoid, oblong or club-shaped, from three to six times longer than their 1 Lemmermann in Forschungsbericliten Biol. Stat. Plon. vii, 1898, p. 29. Protococcacece 225 diameter, and are generally attached by their apices to form radiating colonies of small size. Each cell possesses a parietal chloroplast furnished with a pyrenoid. Multiplication takes place by the longitudinal division of the cell-contents, with the occasional formation of another transverse wall. The products of division diverge outwards, but remain attached by their proximal ends, and the colonies sometimes reach a relatively large size owing to the repeated new divisions of the contents of the radiating cells. As a general rule four young cells (autospores) are produced in each mother-cell. A. Hantzschii Lagerh. is a very rare Alga in Britain and is confined principally to the plankton. Length of cells 10 — 24 g ; breadth 3 — 6 g. Genus Selenastrum Reinsch, 1867. In this genus the cells are arcuate or lunate, attenuated to fine points, and arranged back to back to form a 4- or 8-celled colony of small size. The cell-wall is thin and firm, and the chloroplast is without a pyre- noid. The multiplication is by auto- spores which arise as in Ankistrodesmus and Kirchneriella. The species of this genus are very un- common, usually occurring amongst other water-plants at the margins of ponds and lakes. S. Bibraianum Reinsch, S. gratile Reinsch (fig. 95 A — D) and S. acuminatum Lagerh. (fig. 95 E — G) all occur in Britain. Chodat considers the latter species to be a Scenedesmus, but that I cannot agree with. Genus Kirchneriella Schmidle, 1893. [Indus. Selenoderma Bohlm, 1897.] The cells are arcuate or crescent-shaped, attenuated or subcylindrical, often bent until the apices almost touch each other : they are loosely aggregated, without any definite disposition, within a large enveloping mass of jelly. The cell-wall is very thin and the chloroplast is parietal, being situated on the convex wall of the cell. There is one pyrenoid, but it is frequently absent. The multiplication is by autospores, four or eight of which are produced in a mother-cell by oblique or more or less transverse divisions. The genus differs from Selenastrum in the loosely aggregated colonies of cells, which 16 Fig. 95. A — D, Selenastrum gracile Reinsch; A — C, from near Settle, W. Yorks.; D, from Puttenham Common, Surrey. E— G, S. acumina- tum Lagerh., from Bowness, Westmoreland. (All x 520.) W. A. 226 Chlorophycece are irregularly disposed, and in the large enveloping mass of j<%- Fig. 96. Kirchneriella obesa (West) Schmidle. A, B, D and E, from Bowness, Westmoreland; C, from the plankton of Loch Mor Bharabhais, Lewis, Outer Hebrides ( x 485). K. obesa (West) Schmidle (breadth of cells 2 — 9‘5 p ; apices l-5 — 4 p apart ; greatest diameter of cell 6 — 16 p ; fig. 96 A — E) and K. lunaris (Kirchn.) Mob. are widely distributed in the British Islands, occurring sparingly in the small ponds and lakes, but in quantity in the plankton of the larger lakes. Sub-family IV. OOCYSTIDEB3. This sub-family is characterized by the globose or ellipsoid cells (curved or even sublimate in N ephrocytivm), which are frequently retained within the swollen wall of the old mother-cell. There may be one or many chloroplasts in each cell, which are parietal and usually contain one pyrenoid. In all except Palmellococcits the cell-walls are firm and somewhat thick. The multiplication is by autospores, which generally grow and attain their full size whilst still forming part of the mother-colony. The following are the British genera : — * Cells ellipsoid Oocystis. ** Cells curved, subcylindrical or sublunate Nephrocytium. *** Cells spherical. t Cells large, solitary and free-floating Eremosphcera. ft Cells minute, forming a thin stratum Palmellococcus. ttt Cells minute, sparsely scattered and symbiotic Chlorella. Protocoecacew 227 Genus Oocystis Nag., 1845. I he cells are ellipsoid, sub- cylindrical or panduriform, with a firm cell-wall which commonly possesses a nodular thickening at each pole. There are usually several parietal chloroplasts in each cell, destitute of pyrenoids. In some cases, however, there may be one pyrenoid in each chloroplast. Multiplication is by autospores, which are generally retained for some time within the greatly swollen wall of the mother-cell. Sometimes several generations are contained within one large mother-cell-wall. Fig. 97. A and B, Oocystis solitaria Wittr., from Ben Lawers, Perthshire. C and D, 0. crassa Wittr.; C, from Lanlivery Moor, Cornwall; D, plankton form from Lough Beg, Ireland. E and F, 0. panduriformis W. & G. S. West; E, from Pilmoor, N. Yorks.; F, from near Clifden, Ireland. G, 0. elliptica West, from Derryclare Lough, Galway, Ireland. (All x 485.) There are about ten British species of the genus, some of which are widely distributed in the quiet waters of ponds and lakes. 0. solitaria Wittr. (length of cells 15 — 48 p ; breadth 9-5 — 25 p ; fig. 97 A and B) is undoubtedly the commonest species, although 0. elliptica W'est (fig. 97 G) is abundant. 0. parva West & G. S. West (length 6 — 12 p ; breadth 4 — 7 p) is the smallest species and 0. gigas Arch, (length 41 — 505 p ; breadth 32'5 — 40 p) is the largest. 0. crassa Wittr. is not common except in the plankton ; fig. 97 C and D. 0. asymmetrica W. & G. S. West is another solitary species. Chodat states that 0. gigas Arch, and 0. panduriformis W. & G. S. West (fig. 97 E and F) are merely stages in the life-history of Eremosphcera, but this statement I do not accept. These plants often occur in localities from which Eremosphcera is absent and they reproduce themselves in the manner of true species of Oocystis. Moreover, I invariably find the life-history of Eremosphcera to be remarkably free from polymorphic forms (vide page 229). 15—2 228 Chlorophycece Genus Nephrocytium Nag., 1849. The cells are oblong, ellipsoid or subcylindrical, slightly curved or sublunate, sometimes almost reniform. There is at first a large expanded chloroplast in each cell, furnished with a single pyrenoid, but later the chloroplast fragments. The multiplication is by autospores similar to those of Oocystis, and formed within the mother-cell soon after the segmentation of the chloroplast. The young autospores are often spirally disposed round the inside of the mother-cell-wall. The genus is distinguished from Oocystis primarily by its curved cells without any trace of apical thickenings. Fig. 98. A, Nephrocytium obesum West, from Angle Tarn, Cumberland. B, N. ecdysiscepanum West & G. S. West, from near Goring, Oxfordshire. C F, N. lunatum West; C — E, from near Bowness, Westmoreland; F, from near Boundstone, Galway, Ireland. (All x 367.) JY. Agardhianum Nag. (inclus. N. Ndgelii Grun.) is a widely distributed species in the stagnant waters of small pools and lakes ; length of cells 12 22 p ; breadth 7—12 p. N. obesum West is the largest species, characterized by the short, stout cells and by the great thickness of the mother-cell-walls ; length of cells 34—42 p ; breadth 24—28 p ; fig. 98 A. Ar. lunatum West is a characteristic species (supposed by Cliodat to be a stage of N. Agardhianum) which is very local, but sometimes abundant among submerged Sphagnum ; length of cells 14—18 p ; breadth 4— 6'5 p ; fig. 98 C— F. N. ecdysiscepanum Protococcacece 229 W. & G. S. West is a curious species in which the mother-cell-walls throw oft a number of integuments, several generations being disposed m a fan-shaped manner and held in position by the partly exuviated layers of the mother-cell- walls; length of cells 24 — 26-5 p ; breadth 13 17 p; fig- 98 B. Genus Eremosphaera De Bary, 1858. [ Ghlorosphcera Hen- frey, 1859.] The cells are solitary, large, and spherical, with a thick, firm cell-wall distinctly differentiated into two layers. Each cell contains a large number of small parietal chloroplasts furnished with a conical projection directed towards the centre of the cell. There is one pyrenoid in each chloroplast. The nucleus is gene- rally contained in a small mass of protoplasm suspended in the Fig. 99. EremospTuera viridis De Bary, from near Clapham, W. Yorks. ( x 175). central region of the cell by a network of protoplasmic threads. Multiplication takes place by the division of the contents of a mother-cell into two or four smaller but similar daughter-cells (autospores), which are set free by the rupture of the mother-cell- wall. Chodat1 has described the occurrence of certain polymorphic forms of this Alga, but although I have examined large quantities of it from every part of the British Islands, and from elsewhere, I have never yet seen any trace of such forms. Specimens kept under cultivation for two years developed no forms other than globular daughter-cells. Moore2 also disputes the alleged poly- morphism of this Alga. E. viridis De Bary is widely distributed all over the British Islands, more especially in Sphagnum-bogs. It is a constant associate of certain Desmids. The cells vary from 55 — 200 p in diameter; fig. 99. Genus Palmellococcus Chodat, 1894. [? Protucoccus Ag. 1824 (in part).] The cells are more or less globular, with a firm cell- wall, and aggregated to form a thin mucous stratum. There is in 1 Chodat in Botan. Zeitung, liii, 1895, t. v. 2 Moore in Proc. Amer. Assoc. Adv. Sci, 1900, pp. 278, 279. 230 Chlorophycece each cell a parietal plate-like chloroplast devoid of a pyrenoid, which is often hidden by an orange- red oil. There are three methods of multiplication ; 1st, by division of the original mother-cell into two or four daughter-cells ; 2nd, by a rejuvene- scence of the cell-contents and an exuviation of the wall of the mother- cell ; 3rd, by the formation of 8, 16, 32, or 64 spores within the wall of the mother-cell, which then rup- tures and sets them free. Palmellococcus differs from Pleurococcus in the absence of pyrenoids from the chloroplasts, in the methods of reproduction, and therefore in the different aspect of the cell- aggregates. P. miniatus (Kiitz.) Chod.1 ( Pleurococcus miniatus Nag.) is not an in- frequent Alga on the outer surfaces of plant-pots and similar objects, forming a moist, brownish-green scum, which often turns to an orange-red colour. Diam. cells 2 — 12-5 g ; fig. 100. Certain plants belonging to this genus have in the past been referred to “ Protococcus Ag.” The latter genus is obsolete, having included Algae which are now referred to a number of other genera. Genus Chlorella Beyerinck, 18902. The cells are small, globular or ellipsoid, with firm cell-walls, and with a single parietal chloroplast containing a pyrenoid. They occur in abundance in symbiotic relationship with Hydra viridis, species of Amoeba, Paramoecium, Ophrydium, etc. Multiplication takes place by the quadripartition of the cell contents. Radais3 has confirmed Beyerinck’s observations that this Alga has the faculty of cell- increase and the formation of chlorophyll in the dark as in the light. Grintzesco4 also affirms that development takes place more rapidly in total darkness than in full daylight. 'Che latter author has cultivated Chlorella in various media and finds that glucose stimulates its development and that peptone is a better source of nitrogen than nitrates. Cultures of this Alga do not liquefy gelatine. Chi. vulgaris Beyr. is widely distributed and often abundant in cultures or in stagnant aquaria. The cells are 5 — 10 g in diameter. 1 Chodat in Bull. Herb. Boissier, tom. ii, 1894, pp. 429 and 599. 2 Beyerinck in Botan. Zeitung, xlviii, 1890. 3 Badais in Comptes Rendus, cxxx, 1900, p. 793. 4 J. Grintzesco in Bev. Gen. Bot. xv, 1903. Fig. 100. Palmellococcus mini- atus (Kiitz.) Chodat, from near Bradford, W. Yorks. ( x 720). Protococcacece 231 Sub-family V. TETRAEDRIE/E. The plants of this sub-family are always solitary unicells. Each cell is flattened and angular, usually with a definite number of angles, or it is tetrahedral, octahedral, or polyhedral. The angles may be rounded, emarginate, or furnished with spines. Genus Tetraedron Kiitz., 1845. [ Polyedrium Nag., 1849.] The cells of this genus, which occur as solitary individuals or rarely collected into temporary aggregates, are flattened and angular (triangular, quadrangular, or polygonal) or polyhedric. The angles are obtuse, acuminate, or furnished with one or more simple or furcate spines: There is a single large chloroplast, parietally disposed and containing one pyrenoid. Multiplication takes place by the formation of four or eight autospores, which are set free by the rupture of the wall of the mother-cell. Sometimes they are expelled into a delicate vesicle, which, however, soon disappears. Fig. 101. A, Tetraedron minimum (A. Br.) Hansg., from Keston Common, Kent. B, T. caudatum (Corda) Hansg., from Pilmoor, N. Yorks. C, T. regulare Kiitz., from near Bowness, Westmoreland. D, T. enorrne (Ralfs) Hansg., ‘ from Mickle 1 ell, N. 1 orks. E — (1, i\ horridum W . & G. S. West, from Putney Heath, Surrey. (All x 450.) auts, autospores. There are about 14 species of the genus known to occur in the British Islands. T. minimum (A. Br.) Hansg. is the most abundant of the flattened species; diam. of cells 6-5 — 16 p; thickness of cells 5 — 7p; fig. 101 A. T. regulare Kiitz. (= Polyedrium tetra'edmcum Nag.) is the commonest of the polyhedric species ; diam. of cells 13-5—40 p; fig. 101 C. T. enorrne (Ralfs) Hansg. Weis originally described as a Desmid; fig. 101 D. The genus Cerasterias Reinsch (1867) is sometimes separated from Tetraedron owing to the depth of the lobulation. The cells are solitary, tetrahedric in character, and the divisions into lobes 232 Clilorophycece are so deep that there is no central body. G. rhaphidioides Reinsch and C. longispina (Perty) W. & G. S. West are both plants of rare occurrence. Sub-family VI. PHYTHELIErii. This is one of the most interesting sub-families of the Pro- tococcacese, and the Algse contained in it have only recently been brought to light, largely by the plankton investigations of Lemmermann and Chodat. The plants are unicellular or some- times grouped so as to form a more or less definite coenobium, and in all cases they float freely in the water. As a rule the cells are almost devoid of a mucous envelope, and they are furnished with several stiff bristles considerably longer than their own diameter. Multiplication occurs typically by the formation of autospores, which usually attain all the characters of the adult before their liberation from the swollen wall of the mother-cell. Zoogonidia have been observed in Golenkinia. The name of the sub-family is derived from the genus Phythelios Frenzel (1891), an Alga which was originally described as a Heliozoan. All the genera are practically confined to the plankton of large lakes, although a few of them are occasionally observed in the surface waters of ponds. The long bristles of these Algae are protective characters developed as a result of a free-floating existence amidst numerous animals to which they would other- wise be an easy prey. The genera have been well worked out and monographed by Lemmermann1. Four of them are known from Britain. m A. Cells globular. * Cells solitary with evenly distributed bristles Golenkinia. ** Cells in colonies of 8, 16, or more, bristles attached to the outer faces only Richteriella. B. Cells ellipsoid or subcylindrical. * Bristles with a basal swelling Lagerheimia. ** Bristles without a basal swelling Chodatella. Genus Golenkinia Chodat, 1894. The cells are globular, usually solitary, with a firm cellulose wall, which is enveloped in a thin layer of mucilage. Each cell is furnished with a number of radiating bristles of considerable length, evenly disposed over the 1 Lemmermann in Hedwigia, Bd xxxvii, 1898. ProtocoecacecB 233 outer surface of the cell. There is a parietal chloroplast con- taining one pyrenoid. Multiplication normally takes place by autospores, but also by simple vegetative division, and reproduction by quadriciliate zoogonidia has been observed by Chodat. G. radiatci Chodat1 is known from Surrey ; diam. of cells 10 15 /x; length of bristles 26—45 p; fig. 102 U and E. G. paucispinosa West & G. S. West2 is known from the plankton of Lough Neagh, Ireland ; diam. of cells 15 16 /x , length of bristles 16 /x ; fig. 102 F. Fig. 102. A, Richteriella botryoides (Schm.) Lemm., after Lemmermann, x 520. B and C, R. botryoides forma quadriseta (Lemm.) Chod. ; B, from the plankton of Lough Beg, Ireland (x450); C, after Lemmermann ( x 520). D and E, Golenkinia radiata Chod., after Chodat ( x about 800). F, Golenkinia pauci- spinosa W. & G. S. West, from the plankton of Lough Neagh, Ireland ( x 450). Genus Richteriella Lemmermann, 1896. The cells are spherical and generally aggregated to form loose coenobia of 8, 16, 32, or 64 cells. The cell-wall is thin and firm, and is furnished with long, radiating bristles attached only to those surfaces of the cells which face outwards. There is a single parietal chloroplast with one pyrenoid. Multiplication has only been observed to take place by vegetative division. This genus only differs from Golenkinia in the aggregation of the cells and the fact that the bristles are more or less confined to the exposed surface of each cell. 1 Chodat in Morot, Journ. de Bot., Paris, 1894, p. 305, t. iii. 2 West & G. S. West in Trans. Roy. Irish Acad, xxxii, sect. B, part i, 1902, p. 68, t. i, f. 18. 234 Chlorophycem R. botry aides (Schmidle) Lemm., forma quadriseta (Lemm.) Chod. is known from the plankton of Lough Beg, Londonderry, Ireland ; diarn. of cells 3 — 9-6 p ; length of bristles 23 — 60 p ; fig. 102 B and C. Genus Lagerheimia Chodat, 1895. The cells are solitary, ellipsoid, or subcylindrical with rounded extremities, and with a firm cell-wall. There are four bristles arranged in diverging pairs at each pole, or disposed one at each pole and two equatorially. Each bristle has a wart-like thickening at its base. There is a single parietal chloroplast with one pyrenoid. The multiplication is by autospores. L. subglobosa Lemm. is known from Lough Gartan, Donegal, Ireland (diam. of cells 5 '5 — 9'4 p ; length of bristles 10'5 — 13 p ; fig. 103 D and E), and L. genevensis Chod. from the south of England (diam. of cells 3 p ; length of cells 9 — 10 p ; length of bristles up to 16 /lx ; fig. 103 A — C). Genus Chodatella Lemmermann, 1898. This genus only differs from the preceding one in the absence of the swellings or wart-like thicken- ings at the base of the bristles. The cells are solitary, ellipsoid, and furnished with four or many elon- gated bristles, which are sometimes radiating and sometimes curved. There may be one or several parietal chloroplasts, with or without pyre- Fig. 103. A — C, Lagerheimia genevensis Chod., after Chodat ( x about 850). D and E, L. subglo- bosa Lemm.', D, after Lemmermann (x520); E, from Lough Gartan, Donegal, Ireland ( x 450). F and G, Chodatella breviseta W. & G. S. West, from Lough Gartan, Ireland (x450). H and I, Ch. ciliata (Lagerh.) Lemm. var. amphitricha (Lagerh.) Chod. (x450); H, from Skipwith Common, E. Yorks.; I, from near Bowness, Westmoreland. auts, autospores. noids. The genus differs from Oocys- tis in the absence of the polar thickenings and the presence of the long spine-like bristles. Ch. ciliata (Lagerh.) Lemm. var. amphitricha (Lagerh.) Chod. [ = Ch. radians (West) Lemm.] occurs in several localities in the British Islands; length of cells 8 — 18 p ; breadth 4 — 13’5 p; length of bristles 12 — 20 p ; fig. 103 H and I. Ch. breviseta West & G. S. West is known from Lough Gartan, Donegal, Ireland; length of cells 12 — I2'b p; breadth 8 — 9-5 p-, length ot bristles 11'5— 17’5 p; fig. 103 F and G. Protococmcece 235 Sub-family VII. DICTY OSPHiEBIEiE. This sub-family contains a few genera the affinities of which are somewhat doubtful. The cells are globose, ovoid, or ellipsoid, and are associated to form more or less indefinite colonies. The colony is often of a fragile nature, the cells being held in position by the persistent old walls of the mother-cells, which sometimes become transformed into definite connecting-threads. A copious mucous investment is present in some, but in others it may be entirely absent. The multiplication is by simple vegetative divi- sion or by the formation of four daughter-cells (autospores) within the wall of the mother-cell, which gradually splits open and permits their escape. Reproduction by biciliated zoogonidia has been observed by Zopf and by Massee1 in Dictyosphcerium. The sub-family is most probably an artificial one and perhaps it should not have a place in the Protococcacese. The five following genera are British : — A. Cells indefinitely disposed. * With well-marked, subdichotomous connecting- threads; chloroplast parietal Dictyosphcerium. ** Cells in radiating series; connecting threads scarcely visible; chloroplast axile Dictyocystis. B. Cells grouped in fours in one plane ; colonies ir- regular Tetracoccus. C. Cells in botryoidal clusters. * Freely exposed in a thin gelatinous envelope Botryococcus. ** Clusters covered by a firm, irregular, tough membrane Inejjigiata. Genus Dictyosphaerium Nag., 1849. The cells are globose, ovoid, or subreniform in shape, with a firm cell-wall, and they are connected by dichotomously branched threads to form a roughly spherical or ellipsoidal colony. The entire colony is enveloped in mucus, and the cells are situated somewhat far apart towards its periphery, large colonies often becoming very irregu- lar. Each cell contains a more or less bell-shaped, parietal chloroplast, 1 G. Massee in Journ. Linn. Soc. Bot. xxvii, 1891. Fig. 104. Dictyosphcerium pul- chellum Wood. A, ‘from the plank- ton of Loch Shin, Sutherland ; B, from Cam Fell, W. Yorks, x 450. 236 C hlorophycece furnished with a single pyrenoid. The connecting-threads are often derived from the old mother-cell-walls, but in some cases it is doubtful if they do arise in this manner. Multiplication takes place normally by the formation of four daughter-cells within the mother-cell. Reproduction by biciliated zoogonidia occurs, but has been very rarely observed. D. Ehrenbergianum Nag. is a widely distributed British Alga, often occurring in quantity in the surface waters of ponds and in the plankton of lakes; diam. of cells 4 — 10 p. D. pulchellwm Wood (fig. 104), I), reniforme Buhl, and D. oviforme Lagerh. are more rarely found, but it is probable that the five so-called species of this genus are merely forms of D. Ehrenbergianum. Genus Dictyocystis Lagerh., 1890 l. The cells are ellipsoid or oblong, and are arranged in radiating series to form a small free-floating colony. The radiating series of cells frequently branch and the cells are held in place by delicate mucous threads. Each cell possesses a central chloroplast with one pyrenoid. D. Hitchcockii (Wolle) Lagerh. is a rare British Alga, occurring in the bogs of N. Ireland and N. W. Scotland, and also in the Scottish plankton. Diam. of cells 9 — 11 p ; the American specimens are larger. Genus Tetracoccus West, 1892'. [ Westella De Wild., 1897 (in part).] The cells are small, globose or subglobose, sometimes a little angular, and are closely arranged in groups of four. These groups are connected by the old mother-cell-walls into free-floating colonies of small size, consisting of a maximum number of about 80 cells. The four cells of each group are disposed in one plane, and the old walls of the mother- cells become transformed into deli- cate connecting-threads. There is one chloroplast which contains several large granules, but the presence of pyrenoids has not yet been definitely demonstrated. Multiplication takes place by the formation of four daughter-cells within the mother-cell, which arise by the division of the cell- contents in two directions in one plane. The colonies are almost entirely free from enveloping mucus. 1 Lagerheim in Nuovo Notarisia, 1890, p. 226. 2 West in Journ. Roy. Mier. Soc. 1892, p. 735, t. x, f. 43 — 48. Fig. 105. Tetracoccus botryoides West, from Bowness, Westmoreland. A and B, x450; B and C, two “te- trads,” x 715. P rotococcacece 237 Schmidle1 2 gave what he termed an ‘ amended description of this genus in 1894, but as the plant he included in it is not a species of Tetracoccus, his amended description is not a correct one-. T. botryoides West is widely distributed in the British Islands, generally occurring in the surface waters of ponds and in the plankton of large lakes. Diam. of cells 3'8— 5'7 y ; diam. of colonies 30—57 y, fig. 105. Genus Botryococcus Kiitz., 1849. The colony is free-floating and consists of an aggregate of botryoidal groups of cells. The cells are globose or ovoid in form and are closely aggregated to form clusters of 16 or 32 cells, the clusters being held together partly by old mother-cell-walls and partly by a gelatinous investment. There is a single cup-shaped chloroplast in each cell, but pyrenoids have not been observed. Chodat and Cretier3 have observed in the chloroplast a small body which can be regarded as a pyrenoid without an amylosphere. As a rule this Alga is of a bright green colour, but when occurring in large quantity, as it frequently does in the freshwater plankton, the cells become filled with a brick-red oil. B. Braunii Kiitz. is the best known representative of the genus, and is a frequent plant in bog-pools, large ponds, lakes, etc. Diam. of cells 5'5 — 9 y ; fig. 106. B. sudeticus Lemm. (which may only be a form of B. Braunii with globose cells) and B. calcareus West are rarer British species. Genus Ineffigiata West & G. S. West, 1897 ; em. 1903. This Alga consists of free-floating colonies of very irregular form and destitute of a gelatinous investment. The colony is composed of several families of cells agglutinated together, each family being small, more or less spherical, and consisting of a peripheral layer of cells surrounding a central cavity. The cells are ellipsoid or ovoid in form, and each one is furnished with a parietal chloro- plast, often containing what has been described as a single small 1 Schmidle in Flora, 1894, Heft 1, p. 45. 2 Vide West & G. S. West in Journ. Boy. Micr. Soc. 1896, p. 162. 3 Chodat & Cretier in Arch. Sci. Phys. et Nat. x, 1900. Fig. 106. Botryococcus Braunii Kiitz., from the New Forest, Hants. A, small colony ; B, two isolated cells ( x 450). 238 Chlorophycece pyrenoid, but which I am inclined to believe is a small granule of starch. In some cells starch appears to be quite absent. The outer surface of each family of cells is enveloped in a tough elastic membrane of irregular form, which contains a trace of cellulose, and is folded and produced into all manner of irregular wrinkles, lobes, pro- cesses and spines. Sometimes these irregular projections are wanting, but at other times they are exceedingly nume- rous. The membrane is a secretion of the underlying cells, and its presence renders observations on this Alga more difficult than on any other of the Protococcacese. Sometimes the smaller colo- nies are united by more or less rigid prolongations of the enveloping membranes into much larger colonies. The families multiply by division, forming larger and larger colonies, which ulti- mately become separated into smaller groups by the develop- ment of elongated processes of the enveloping membranes. The reproduction is unknown1. In situations in which Ineffigiata occurs in quantity, such as in the freshwater plankton, the cells develop the brick-red oily material which is found in the preceding genus. Fig. 107. Ineffigiata neglecta W. & G. S. West, from Harris, Outer Hebrides. A, outline of colony; B, smaller colony; C, part of single family in section; D, showing escape of cells from a small colony. I. neglecta W. & G. S. West is one of the most widely distributed of British Algae, occurring in all kinds of situations — in ditches, bogs, tanks, water- butts, etc.— and forming a regular and considerable constituent of the fresh- water plankton. Diam. of single families 21—56 p, of colonies 46 — 350 p ; length of cells 5-7 — 10'5 p, breadth 3-4 — 53 p ; fig. 107. 1 Vide Journ. Bot. March, 1903, t. 447, f. 1 — 6. Pal/mdlacece 239 Family 8. PALMELLACEAij. The Palmellacese is one of the most primitive families of green Alga?, primarily distinguished from the other groups of the Protococcoidese by the indefinite colonies of cells enveloped in a conspicuous mass of mucilage. In the ordinary vegetative con- dition these plants present the appearance of a group of more or less irregularly disposed cells embedded in a copious mass of jelly, which is either structureless or differentiated into concentric envelopes. The colonies are either microscopic or macroscopic, and sometimes reach a length of several centimetres. The cells are globose or ellipsoid, of small size, and are frequently arranged in pairs or in groups of four. Sometimes these groups of four or ‘ tetrads ’ are disposed in a tetrahedral manner, but at other times they are situated in one plane. Each cell contains a somewhat bell-shaped chloroplast which may or may not be furnished with a pyrenoid. The nucleus is situated in the hollow of the chloroplast. In some genera (e.g. Tetraspora and Apiocystis ) each cell is furnished with a pair of ‘ pseudocilia, ’ which consist of long motionless protoplasmic threads penetrating through the enveloping mucus to the exterior. These were first discovered by Thuret. Multiplication takes place by cell-division in two or three directions, followed sooner or later by a diffluence of a large part of the enveloping mucus and the consequent dismemberment of the colony into smaller portions, each of which increases as before either by simple cell-fission or by the formation of two or four daughter-cells within each mother-cell. Asexual reproduction takes place by biciliated zoogonidia. The latter are formed either by the transformation of a vegetative cell into a zoogonidangium in which several zoogonidia arise, or by the assumption by the ordinary vegetative cell of the motile Chlamydomonadine condition. The motile state greatly resembles the Chlamydomonad-type, and these plants have no doubt arisen by the intercalation of a simple though well-marked vegetative condition between two successive motile phases. Sexual reproduction has been observed in some of the Palmel- lacese. It consists of a fusion of isogamous planogametes, either similar in all respects to the zoogonidia and produced singly in a 240 Clilorophycece gametangium, or much smaller than the zoogonidia and produced in numbers from a gametangium. In some of these plants the colony has no definite form, but in others the cells, although irregularly grouped, are contained in a mucous mass which invariably assumes a definite shape. The family can be divided into three sub-families, in each of which the mucus may be indefinite or developed in accordance with some definite plan. Sub-family I. Palmellece. Cells irregularly grouped within a structureless mass of mucus. Sub-family II. Tetrasporece. Cells grouped in fours or sometimes irregularly disposed at the periphery of a structureless mass of mucus. Each cell with two pseudocilia. Sub-family III. Gloeocystideoi. Cells grouped in twos or fours within a lamellose mucous investment. Sub-family I. PALMELLECE. This sub-family is characterised by the large number of globose cells which are irregularly grouped within a structureless mass of mucus. The latter is usually of indefinite extent, but in Palmo- dcictylon it is more or less cylindrical and often much branched. The cell-walls are generally firm and thin, and in Schizochlamys the outer layers are periodically thrown off in one or several pieces. Genus Palmella Lyngb., 1819. The cells are spherical, with thin cell-walls, and they are surrounded by mucous coats which have fused to form an indefinite mass of jelly. The parietal chloroplast contains a pyrenoid. Multiplication takes place by repeated bipartitions of the cells in all directions of space, ac- companied by an extreme gelatinization of the mother-cell-walls. Reproduction occurs by macrozoogonidia, by microzoogonidia, and by small isogamous planogametes. One of the few true species of this genus is P. miniata Leibl., which occurs as a mucous expansion of a brick-red colour on damp ground, wet rocks, etc. ; diam. of cells 3 — 5 p. P. mucosa Kiitz. and P. hyalina Breb. are aquatic species of a green colour ; the former may possibly be a state of Tetraspora. Genus Palmodactylon Nag., 1849. This is a well-marked genus of the Palmellese, differing from Palmella mainly in the definite form of the enveloping jelly. The chloroplast is parietal and irregularly lobed, but is destitute of a pyrenoid. Multiplication of the cells takes place in all directions, but preponderates in Palmellacece 241 w Oo one direction, so that the cells are grouped irregularly within a cylindrical mass of mucus. This cylindrical mucous envelope frequently branches, each branch being similar to the primary mucous cylinder. P. varium Nag. (inclus. P. subramoswm Nag.) is a common British Alga in peaty ditches and in bog-pools. Diam. of cells 6-5— 9'5 /*; diam. of cylindr. mucous investment 11*5 — 33 /x; fig. 108. °8°S MS © °& OOo oo9 o0°g>° °6> oo oo oW A ecies. Fig. 152. A, Microcoleus delicatulus W. & G. S. West, from Glen Tummel, Perthshire ( x 350). B and C, Symploca muralis Kutz., from Frizinghall, W. Yorks. (B, nat. size ; C, x 350). 334 Myxophycece Genus Symploca Kiitz., 1843. In this genus the filaments are densely interwoven to form a creeping stratum from which arise numerous erect tufts of variable height. A false branching is exhibited by the filaments of some species, the branches arising singly- The sheaths are thin, hyaline, and generally firm; they partially coalesce m the erect tufts. Only one trichome is present within a sheath and its apex is straight. S. murahs Kiitz. is known from W. Yorkshire. It occurs on damp earth, walls, or trunks of trees; thickness of trichomes 34 — 4 g. ; fig. 152 B and C. Other species occur among mosses and dead leaves, and others in hot springs, the genus having a world-wide distribution. Genus Lyngbya C.Ag., 1824. [ Leibleinia Endlicher, 1836 ; Leptotlirix Kiitz., 1843 (in part) ; Spirocoleus Mobius, 1889 (in part).] The filaments are un- branched, free-floating or densely intricate, forming a floccose mass or an expanded stratum. The sheaths are firm, of vari- able thickness, and some- times lamellose. They are generally hyaline, but in a few species they become of a yellow-brown colour. The trichomes are either obtuse or slightly attenuated at the apices, and sometimes there are evident constrictions between the cells. Species of this genus are widely distributed and occur in very varied habitats. Those belonging to the subgenus Leibleinia are entirely marine and live as epiphytes on other larger Algse. There are about 10 British freshwater species, of which L. Martensiana Menegh. (thickness of trichomes 6 — 10 fx) and L. cerugineo-ccerulea (Kiitz.) Gom. (thickness of trichomes 4 — 6 /x; fig. 153 B and C) are the most abundant. L. ochracea (Kiitz.) Thur. occurs in water containing much iron, the oxide of iron ultimately forming a thick deposit round the sheaths of the filaments. L. cestuarii (Mert.) Liebman is the largest British species which occurs in fresh water (thickness of tri- chomes up to 24 fx), although it is principally brackish or marine in habit. Fig. 153. A, Lyngbya major Menegh., from Wimpole Park, Cambridge. B and C, L. atru- gineo-ccerulea (Kiitz.) Gom., from Bradford, W. Yorks. D, Phormidium molle (Kiitz.) Gom. (a narrow form), from Bradford, W. Yorks. E and F, Ph. tenue (Menegh.) Gom., from Wicken Fen, Cambridge. (All x 460.) Oscillator iacew 335 Genus Phormidium Kiitz., 1843. [ Hyphceothrix Kiitz., 1843 (in part); Leptothrix Kiitz., 1843 (in part).] This genus is inter- mediate in character between Lyngbya and Oscillatoria, and undoubtedly serves a useful purpose for the reception of species which cannot be strictly referred to either of those genera. The filaments are simple and invested by delicate hyaline sheaths, which frequently become confluent or altogether diffluent. The filaments are often agglutinated to form an expanded stratum, either on damp earth, wet rocks, or entirely submerged. Some- times this stratum becomes hard and leathery, but it is more often soft and slimy. The trichomes are cylindrical, and there is frequently a constriction between the cells, a feature which is so prominent in some species as to give the trichome a moniliform or torulose appearance. The apices of the trichomes may be straight or curved, and the apical cells are attenuated, capitate, or even thickened at the extremity. Plants of this genus are amongst the commonest of the blue-green Algse, occurring in all kinds of damp and wet situations, and often giving a decided tint to large areas of damp ground, vertical limestone rocks, or those rocks and stones subject to the spray of waterfalls. There are some 13 British species occurring in damp situations or in fresh water. Ph. autumnale (Ag.) Gom. (diam. trich. 4 — 7 /x) is abundant on damp earth, and Ph. tenue (Menegh.) Gom. (diam. trich. 1 — 2/x; fig. 153 E and F) is frequent among other Algae in ponds, ditches, and rivers. Ph. purpurascens (Kiitz.) Gom. forms reddish-purple patches on the vertical faces of wet limestone rocks (diam. trich. 1'5 — 2 /x). [Note: — Clonothrix gracillima W. & G. S. West is one of the Schizomy- cetes of the genus Cladotlirix. ] Genus Oscillatoria Vaucher, 1803. [ Oscillaria Bose, 1800 '.] The trichomes are frge and cylindrical, without a sheath, or with one so thin as to be almost imperceptible. Faint constrictions are sometimes evident between the cells, but more often the edges of the filaments present a continuous and unbroken line. The cells vary much in relative length, but in the larger species they are always much shorter than their diameter. The extremity of the trichome may be straight or curved, and it is often attenuated. The apical cell is sometimes much attenuated, and may be capitate, being frequently furnished with a terminal thickening of the cell- wall known as a calyptra. The genus is found in great profusion 1 For reasons for adopting the generic name “ Oscillatoria ” consult Gomont in Journ. Botan. Morot, v, 1891, p. 273. 336 Myxophycece in all kinds of wet situations, and sometimes on damp ground, or in hot springs. Flocculent masses of Osdllatoria are often found float- ing on the surfaces of ponds and ditches after sunshine, and they commonly give off a more or less distinctive odour. The movements exhibited by the trichomes of this genus are discussed on page 315. There are about 20 British species Of the genus, one of which (0. decolorata G. S. West) is destitute of pigment and lives a saprophytic existence in stagnant ditches. 0. princeps Vauch. is the largest species (thickness of trichomes 25 — 50 p) and 0. angustissima W. & G. S. West is the narrowest (thickness of trichomes (EG p). 0. limosa Vauch. (thickness of trichomes 12 — 17 p ; fig. 154 A) and 0. tenuis Ag. (thickness of trichomes 4 — 10 p; fig. 154 C) are the most abundant species. 0. irrigua Kiitz. (fig. 154 B) is common in quickly running water. Fig. 154. A, Osdllatoria limosa Ag., from Wimbledon Common, Surrey. B, O. irrigua Kiitz., from Shipley Glen, W. Yorks. C, 0. tenuis Ag., from Sheep’s Green, Cambridge. D, 0. splendida Grev. var. attenuata W. & G. S. West, from Baildon Moor, W. Yorks. E, 0. acuminata Gom., from Sheep’s Green, Cambridge. (All x 460.) Genus Arthrospira Stizenberger, 1852. The trichomes are cylindrical, commonly devoid of a sheath, and twisted into a regular spiral. The latter character is the only distinction from Oscillatoria. The cells are generally shorter than their diameter. A. Jenneri (Hass.) Stizenb. is a rare Alga, which occurs in stagnant water, or forms a dark-green mucous stratum in trickling water ; diam. of trichomes 5 — 8 p; diam. of spiral whorls 9 — 15 p. Genus Spirulina Turpin, 1827. The trichomes are very narrow and are twisted into a regular spiral. They consist of a single elongated cell, sometimes of great length, and without transverse septa. This feature at once distinguishes the genus from Arthrospira. S. major Kiitz. is frequent in stagnant water (diam. trich. 1-5 — 2-5 p ; fig. 155 B), and S. tenu- issima Kiitz. ( = S. subsalsa (Ersted) is often abundant in salt and brackish water, rarely occurring in fresh water (diam. trich. 1 — 2 p). Fig. 155. A, Spirulina turfosa Cram., from Lough Neagh, Ireland. B, Sp. major Kiitz., from Wicken Fen, Cambridge ( x 480). Rivulariacece 337 Sub-order 2. TRICHOPHOREiE. This is a small group containing some of the most character- istic of the blue-green Alga;. The trichomes are strongly attenu- ated either towards one end, or from the middle towards both ends, and in all cases they are sheathed. The sheaths are generally strong, rarely hyaline and delicate, often lamellose, and fimbriate or ocreate along their margins. In certain genera, such as in Dichothrix and Ammatoidea, a false branching is present. Hetero- cysts are present in some genera, but absent from others. Asexual reproduction takes place by the formation of hormogones, but in Gloeotrichia spores are developed from the basal cells next the heterocysts. There are two sharply differentiated families : — Family 1. Rivulariacece. Trichomes attenuated from the base to the apex, which is piliferous ; heterocysts basal (rarely absent). Family 2. Camptotrichacece. Trichomes attenuated from the middle towards each extremity ; heterocysts absent. Family 1. RIVULARIACECE. Algae of this family are fairly abundant in mountainous dis- tricts, and they also occur sparingly in the less elevated parts of the country. They are found principally on the dripping rocks of waterfalls, cataracts and streams, or at the margins of rocky lakes. They form soft felt-like expanses, or hard hemispherical masses, generally of a brown colour, but a few of them occur as blue-green nodules attached to the stems and leaves of submerged plants. The trichomes are* all attenuated to a hair-like point, and at the broad base, in all except a few species, one or two heterocysts are located. Occasionally intercalary heterocysts are present in addition to the basal ones. The sheath is tubular, gelatinoiis or membranous, and is frequently strongly lamellose. In some species the lamellae of the sheath become dilated upwards, thus giving the exterior of the filament a fimbriate appearance. It frequently happens that the sheaths of adjacent filaments become fused to such an extent that their individuality is lost. The usual colour of the sheaths is yellow or yellowish-brown, and in some cases they are indurated with carbonate of lime. The filaments exhibit a false branching due to the growth of 22 W. A. 338 Myxophycece new trichomes from some of the inferior cells of the original trichome, the new growth often occurring immediately above an intercalary heterocyst. It is in Dicliothrix that the false branching and fusion of the sheaths are most marked. Asexual reproduction takes place by the formation of hormo- gones, the hair-like apical portion of the trichome being lost. Asexual spores commonly arise in Glceotrichia (and also in Caiothrix ) in close apposition to the basal heterocysts. There are five British freshwater genera, which can be distinguished as follows : — A. Without heterocysts. Filaments free, very narrow, forming a thin stratum Amphithrix. B. With heterocysts. * Filaments free, simple or forming a dichotomous thallus. t Filaments simple or falsely branched; branches distinct and free Caiothrix. ft Filaments branched ; branches several (2 — 6) within a common sheath Dichothrix. ** Filaments forming a hemispherical or globular thallus, closely united by mucus. t Filaments radiately disposed in a tough, globose or hemispherical, attached thallus. Spores unknown Rivularia. tt Filaments radiately disposed in a soft, globose, free-floating thalhis. Spores regularly pro- duced Glceotrichia. Genus Amphithrix Kiitz., 1843 ; em. Born. & Flah., 1886. The filaments form a thin expanded stratum of a purple or violet colour, which consists of two layers. The inferior layer is com- posed of densely intricate filaments, or of minute radiately disposed series of cells ; the superior layer consists of simple, erect filaments, closely packed together and attenuated to fine points. The sheaths are thin, close and continuous, and there are no heterocysts. A. janthina (Mont.) Born. & Flah. occurs on wet rocks and is known from W. Yorkshire; thickness of filaments 15 — 2-25 p. Genus Caiothrix Ag., 1824. [ Mastigonema Schwabe, 1837 ; Mastigothrix Kiitz., 1843; Schizosiphon Kiitz., 1843 (in part); Sympliyosiphon Kiitz., 1843 (in part).] lhe filaments are simple or slightly branched, forming penicillate tufts or soft velvety expansions, which are generally attached to submerged rocks and stones. The heterocysts are basal or intercalary, and in a few species they are absent. The plants often exhibit a slight Rivvlariacece 339 branching, caused by the close application ol the base of one filament to the side of another, the sheaths being continuous. The upper and more attenuated parts of the filaments are, how- ever, always free. Borzi1 has observed the formation ol basal, seriate spores in one species. There .are about six British freshwater species, C. parictina (Nag.) Thur. being the most frequent (thickness of trichomes 6—12 p ; fig. 156 A and B). C. fusca (Kiitz.) Born. & Flah. and G. epiphytica W. & G. S. West are epi- phytes on other larger Alga:, such as Vaucheria or Batrachospermum. Fig. 156. A and B, Calothrix parietina (Nag.) Thur., from Arncliffe, W. Yorks. C, Dichothrix interrupta W. & G. S. West, from Slieve Donard, Ireland. D, D. Orsiniana (Kiitz.) Born. & Flah., from Langdale, Westmoreland. (All x 420.) Genus Dichothrix Zanardini, 1858. In this genus the filaments are more or less dichotomously branched, several trichomes with their sheaths being enclosed within an outer common sheath. The heterocysts are basal or intercalary, and in one species they are absent. The plants generally occur in penicillate tufts on dripping rocks. 1 Borzi in Nuovo Giornale Botau. Ital. 1882, xiv, p. 374. 22—2 340 Myxophycece There are five British species, all of which are rare. They occur princi- pally on dripping rocks in mountainous regions. D. Nordstedtii Born. & Blah, and D. Orsiniana (Klitz.) Born. & Flah. (thickness of filaments 10—12 fi, of trichomes 6-7'5 M ; fig. 156 D) are widely distributed, and D interrupta W. & G. S. West is known from the Mourne Mts, Down, Ireland (fig. 156 C). Genus Rivularia (Roth) A g, 1824 (in part); em. Thuret, 1885. [ Zonatrichia J. A g., 1842 ; Limnactis Kiitz., 1843 ; Schizosiphon Kiitz., 1843 (in part).] The plants consist of a globose or hemispherical thallus, of a tough character, composed of radiating filaments which are repeatedly ‘ branched.’ The thallus is attached to submerged plants (such as Chara, Myriophyllum, or the bases of the stems of Phragmites)ox to the stones of streams and cataracts, • and is sometimes indurated with lime. The heterocysts are basal and the extremi- ties of the filaments are piliferous. Spores have not been observed. There are four freshwater species occurring in Britain, of which It. hcematites (D. C.) Ag. [ = Zonatrichia calcarea (Eng. Bot.) Endlicher] is the most frequent, sometimes occurring in quantity attached to the stones of mountain streams in limestone districts. It. dura Both and It. minutula (Kiitz.) Born. & Flah. (thickness of trichomes 2 — 12-5 y ; fig. 157 D and E) are occasionally met with attached to submerged plants, forming small globose masses of a blue-green or greenish-black colour. Genus Glceotrichia J. Ag., 1842. The thallus is globose and free-floating, solid when young, but inflated and hollow when Fig. 157. A — C, Rivularia Biasolettiana Menegh., from Arncliffe, W. Yorks. ; A, nat. size, on surface of stone ; B and C, x 480. D and E, R. minutula (Kiitz.) Born. & Flah., from Chippenham Fen, Cambridge; D, nat. size, on stem of P hr ag mites ; E, x 480. Camptotrichacecv 341 old. The filaments radiate from the centre outwards, and, as in R midaria, exhibit a false ‘ branching.’ The sheaths are only conspicuous near the base of the trichomes, being gelatinous and confluent near the periphery of the thallus. The trichomes are strongly attenuated from the base, and are more or less torulose. From the cell immediately above the basal heterocyst elongated cylindrical spores are developed, which remain for some time within the basal part of the sheath. There are two British freshwater species, G. Pimm (Ag.) Thur. and G. natans (Hedw.) Babenh. The former possesses a thallus which does not reach a greater diameter than 2 mm., but the latter is a larger species with a thallus up to 10 cms. diameter. G. Pisum is one of the Algae frequently concerned in the phenomenon of the “breaking of the meres.” Family 2. CAMPTOTRICHACE^l. This is a small family sharply marked off from the rest of the Trichophorese by the attenua- tion of the filaments from the middle toiuards each extremity. The plants are epiphytic and there are no heterocysts. The sheath may be very thin and delicate, or thick and lamellose. Only two genera are known, one of which ( Camptothrix ) is a genus of small tropical epiphytes. Genus Ammatoidea West & G. S. West, 1897. The filaments are epiphytic and exhibit a false ramification comparable with that met with in Rivularia. They are suddenly bent in their median or widest portion, the two more or less parallel extremities being gradually attenuated to piliferous apices. The sheaths are firm and lamellose, in young filaments colourless, but in older ones of a yellow-brown colour. The tri- chomes exhibit a slight constric- W. & G. S. West, from Dartmoor, Devonshire ( x 420). 342 Myxophycece tion between their median cells, and the apical cells are about six times longer than their diameter. A. Normanii W. & G. S. West is known from Devonshire as an epiphyte on Batrachospermum momliforme. Thickness of filaments 5-5 — 12 '5 //, of trichomes 3-5 — 5-5 p, ; fig. 158. Order II. COCCOGONE^. In this order the plants are unicellular or colonial, commonly occurring as colonies of unicells enveloped in a copious gelatinous investment. The colonies vary much in size and shape, being spherical, ellipsoidal, or expanded gelatinous masses. The cells are of varied form and are disposed in a variety of ways within the mucous envelope. The latter is sometimes conspicuously lamellose, but more often it is hyaline and structureless. The Coccogonese constitute the lowest group of the Myxophyceae, and indeed they are the most primitive of all the Algae. They occur free-floating, or more rarely as epiphytes, and some of them form gelatinous masses on wet rocks. Not a few are regular and abundant constituents of the freshwater plankton. The normal method of multiplication is by simple cell-fission, the larger colonies ultimately breaking up to form smaller ones. Sometimes young colonies arise at the sides of the older ones by a process of budding. Rounded asexual spores or gonidia have been observed in some species, being formed within the wall of a mother-cell. Zoogonidia are unknown. There are two well-marked families. Family 1. Chcimcesiphoniacece. Cells epiphytic, with a distinct base and apex ; reproduction only by the formation of gonidia. Family 2. Chroococcacece. Cells or colonies free-floating or form- ing a gelatinous stratum, very rarely epiphytic, not differentiated into base and apex ; multiplication by simple cell-division (very rarely by gonidia). Family 1. CHAM^ESIPHONIACEjE. The plants of this family are epiphytes, generally occurring in clusters around the filaments of larger Algae. They are in all cases differentiated so as to present a distinction between base and apex, and reproduction occurs by the formation of a number Cliroococcacece 343 of non-motile spores or gonidia from the contents of a mother-cell. Most Algm of this family are marine, the only British freshwater genus being Chamcesiphon. Genus Chamaesiphon A. Br. & Grun., 1864. [Indus. Sphcero- gonium Rostaf., 1883.] The cells are small, ovoidal, pyriform, or cylindrical, with very thin cell-walls. The cell-contents are homo- geneous and of a blue-green, violet, or yellow colour. The cells are attached by their base and generally widen upwards to their free apex. The gonidia are successively cut off from the upper part of the cell which has become a gonidangium, gradually escaping from the open apex. Fig. 159. Chamcesiphon incrustans Grun., on a filament of Rhizoclonium, from Heaton, W. Yorks. ( x 416). Ch. confervicola A. Br. is found as an epiphyte on Chcetomorpha, Rhizoclo- nium, Vaucheria , etc., and Ch. incrustans Grun. (diam. of cells 3-5 — 4-8 p ; fig. 159) often thickly covers filaments of Rhizoclonium and OEdogonium. Family 2. CHROOCOCCACE^E. This is the largest family of the Coccogoneae, and includes a great variety of unicellular and colonial blue-green Algae. They are abundant in all kinds of damp and wet situations, frequently forming a gelatinous stratum on the damp surfaces of dripping rocks. The cells vary much in shape in the different genera, and the colonies assume different forms according to the direction of division of the cells. In some the cells divide in every direction of space within a gelatinous envelope, producing an irregular colony, often of large size. In others the cells divide only in two dfrections in the same plane, giving rise to a tabular colony ; and in others cell-division takes place in one direction only. In one genus ( Tetrapedia ) the cells are flattened and they possess a striking symmetry of form. Synechococcus and Tetrapedia are the only genera which ai’e commonly destitute of a mucous envelope. In all others the cells are invariably embedded in a mass of mucus, 344 Myxophycew which varies much in its consistency. It may be firm and lamellose, or very hyaline and diffluent. The cells often contain red, orange, or violet pigments. Reproduction has been observed in a few species to take place by the formation of non-motile spores or gonidia within a goni- dangium. There are two sub-families : — Sub-family I. Chroocystece. Epiphytes on larger Algae, with a well-defined dorsiventrality. Sub-family II. Chroococcece. Free-floating or forming a gelatinous stratum, with no dorsiventrality. Sub-family I. CHROOCYSTECE. This sub-family includes only a single genus, the plants of which differ from the rest of the Chroococcaceae in being epiphytic, and in the possession of distinct upper and lower surfaces. They are likewise characterized by the possession of prominent bristles. Genus Gloeochsete Lagerh., 1883. [ Schrammia Dangeard, 1889.] The cells are globose or sub-globose, enveloped in a wide Fig. 160. Glceochcete Wittroclciana Lagerh. A, from Cray Moss, W. Yorks.; B, from Pilmoor, N. Yorks. ( x 416). mucous coat, and they occur singly, or in twos or fours, attached to larger filamentous Algae. More rarely they are attached to the Cliroococcacece 345 leaves and stems of submerged Mosses or Phanerogams. Each cell is furnished at its upper pole with one or two long, thin bristles, which sometimes give off small branches or spurs near their apices. The primitive chromatophore is bell-shaped, of a brilliant blue-green colour, and there is a clear space in the centre of the cell. Lagerheim in his original description of the genus1 says “ nucleus singulus.” The mother-cells give rise to two or four daughter-cells on division. Many authors have placed this Alga in the Chlorophycese, or have recorded a chlorophyceous Alga under the name of ‘ Gloeochcete.’ The genus as I have often found it, and as here figured, most certainly belongs to the Chroococcacese. Gl. Wittrockicma Lagerh. is known from several parts of the British Islands, occurring as an epiphyte on Vaucheria, Cladophora , or the leaves of Sphagnum. Diam. of cells 6 — 21 p ; length of bristles 96 — 260 p. ; fig. 160. Gl. bicornis Kirchn. has a pair of bristles attached to each cell. Sub-family II. CHROOCOCCEA:. The sub-family Chroococcese embraces almost all the unicellular and colonial Myxophycese. They are unquestionably of a lower type than any other of the Algae, and some of them bear resemblance to certain of the Bacteria. The cells display considerable variety of form, and with the exception of the genera Synechococcus, Tetrapedia, and to a certain extent Merismopedia, they are enveloped in a copious covering of mucus. The mucous coat is sometimes firm and lamellose, as in some species of Ghroococcus and Gloeocapsa, but more often it is an ample, homogeneous covering, very hyaline in character. It often happens that the coverings of numerous cells have -fused together, thus forming gelatinous colonies of various sizes. Some of these colonies are large macro- scopic masses, containing many thousands of cells, and possessing a tough exterior. In some genera, such as Merismopedia and Goelosphcerium, the colonies are of limited size and of definite shape. Multiplication takes place by the repeated division of the cells and the final fragmentation of the original colony, each fragment growing in size and repeating the processes. In the spherical colonies of Goelosphcerium a kind of budding takes place by means 1 Lagerheim in Nuova Notarisia, 1890, p. 231. 34G Myxophycece of which a new colony is developed from the side of the old one, ultimately becoming separated from it. Cell-division may be in one direction only, in two directions in one plane, or in every direction of space. Reproduction occurs in Gomphosphceria by non-inotile spores or gonidia1. There are thirteen British freshwater genera, which can be arranged as follows : — A. Cell-division in one direction only. * Cell-division transverse. + Cells enveloped in a wide mucous coat. J Cells elongated, each with a mucous coat JJ Cells little longer than broad, enveloped in a common mucous investment ... ft Cells destitute of mucus ** Cell-division oblique ; cells enveloped in mucus Glceothece. Aphanothece. Syneckococcus. Dactylococcopsis. B. Cell-division in two directions in one plane. * Cells globular or rounded-quadrate ; colonies often large Merismopedia. ** Cells of a definite symmetrical form, solitary, or forming small colonies Tetrapedia. C. Cell-division in all directions of space ; cells enveloped in mucus. * Cells forming large colonies. t Cells arranged at or towards the periphery of spherical colonies. J Cells closely and regularly arranged . . . Ccelosphcerium. H Cells geminate and sparsely scattered, markedly pyriform in shape Gomphosphceria. ft Cells densely aggregated in globose, elon- gated, or clathrate colonies Microcystis. tft Cells aggregated to form irregular gelatinous colonies. | Individual mucous coats clearly evident round each cell Glceocapsa. Cells enveloped in a common mucous covering Aphanocapsa. + + + Cells arranged in a compact gelatinous stratum Porphyridium. ** Cells more or less solitary, or forming very small colonies Chroococcus. Genus Gloeothece Nag., 1849. The cells are cylindrical and elongated, each one being surrounded by a thick mucous coat, which sometimes shows indications of lamellation. Cell-division 1 Schmidle in Berichte Deutsch. Botan. Gesellsch. 1901, xix. Chroococcacece 347 is only in one direction, and a number of cells are usually associ- ated to form a small colony. Species of this genus occur on wet rocks, among wet mosses, and in bog- pools. 67. linearis Nag. (length of cells 10-5 — 18 /x ; breadth 1'3 — 23 g ; fig. 161 A) and Gl. confluens Niig. (length of cells 57 — 7'5 p ; breadth 2 '6 — 3 p ; fig. 161 B) are the most frequent British species. Rhabdoderma linear e Schmidle1 seems very closely allied to Glceothece linearis Nag., if not actually identical with it. Genus Aphanothece Niig., 1849. This genus only differs from Gloeothece in the ag- Jr f Oq % °- <220%^ O % QL o o I gregation of large numbers of cells within a common mass of mucus. The cells are cylindrical and longer than their diameter. A. microscopica Nag. (length of cells 5 — 8 p , breadth 3 '5 — 4 p ; fig. 161 C) and A. saxicola Nag. are the most frequent species. They are found in bog-pools, at the margins of lakes, and on wet rocks. Genus Synechococcus Nag., 1849. The cells, which are cylindrical with hemispherical apices, are larger than in the preced- ing genera, and are desti- tute of the outer mucous coat. They occur free- floating in ponds, ditches, and bog-pools, often in considerable quantity. The cell-contents are usually of a brilliant blue-green, rarely of a rose-purple colour, and contain numerous large granules. S. ceruginosus Nag. and S. major Schroeter (length of cells 26 — 29 p ; breadth 155 — 17 '5 p ; fig. 161 D and E) are the most abundant British species, the latter often occurring in quantity in bogs. Genus Dactylococcopsis Hansg., 1888. The cells are generally associated to form small colonies, rarely solitary, and in many 1 Schmidle in Berichte Deutsch. Botan. Gesellsch. 1900, xviii, p. 149, t. vi, f. 8—11. Fig. 161. A, Gloeothece linearis Nag., from Old Cote Moor, W. Yorks. B, Gl. confluens Nag., from near Settle, W. Yorks. C, Aphano- thece microscopica Niig., from Withiel, Corn- wall. D and E, Synechococcus major Sehroet., from Adel Bog, W. Yorks. (All x 450.) 348 Myxophycece cases the colonies are embedded in a copious mucus. The cells are elongated, fusiform or lanceolate, with attenuated extremities, or sometimes sigmoidal in form. The chromatophore is almost homogeneous, of a pale blue-green colour, and somewhat lateral in position. Division of the cells occurs much as in Dactylococcus, by oblique septation. D. montana W. & G. S. West (length of cells 8‘6 — 11 '5 p, breadth 3'5 — 4 p ; fig. 162 A) occurs in bog-pools amongst Sphagnum. D. rhaphidioides Hansg. is known from the plankton of Lough Neagh. Genus Merismopedia Meyen, 1839. This is one of the most striking genera of the Chroococcacese, consist- ing of a flat rectangular colony, the cells of which are arranged in rectilinear series. Cell-division takes place in two directions and the cells appear to be usually arranged in groups of four. The cells are globose, ellipsoidal, or oblong, sometimes slightly angular by compression, and the cell-contents are homogeneous. M. glauca (Ehrenb.) Nag. is the commonest British species, occurring in ponds, ditches, bogs and lakes; diam. of cells 3 3 — 3'8 p ; fig. 162 B. M. punctata Meyen and M. ceruginea Breb. are not infrequent in stagnant water. M. elegans A. Br. is the largest species of the genus and is known from few localities in the British Islands; diam. of cells 6‘5 — 9-5p; fig. 162 C. The colonies of M. glauca and M. elegans often reach a large size (diam. up to 220 p) and may contain as many as 1856 cells. Genus Tetrapedia Reinsch, 1867. In this genus the cells are of some definite and symmetrical shape, often constricted into two equal half-cells. The cell-wall is firm and the cell-contents are homogeneous. In some species the cells are almost invariably solitary, but in others they are grouped in flat colonies similar to those of Merismopedia. V * St#8838 838388S8 B Fig. 162. A, Dactylococcopsis montana W. & G. S. West, from Widdale Fell, W. Yorks. B, Merismopedia glauca (Ehrenb.) Nag., from Thursley Common, Surrey. C, M. elegans A. Br., part of a large colony from Wicken Fen, Cambridge. D, Tetrapedia Reinschiana Arch., from near Goring, Oxford. (All x 450.) Chroococcacece 349 All the species are rare, but T. Reinschictna Arch. (diam. of cells 5-5 — 6 g ; fig. 162 D) and T. glciucescens (Wittr.) Boldt occur in lakes and bog-pools. T. setigera Arch, is a beautiful little species known from N. W. Scotland and W. Ireland, and which has been erroneously referred by some authors to the chlorophyceous genus Tetra'edron. The cell-contents of Tet.rapedia setigera are homogeneous and of a pale blue-green colour. Genus Coelosphaerium Nag., 1849. The cells are more or less closely grouped to form a hollow, spherical colony. They are em- bedded in a mass of mucus and are arranged just within the periphery of the spherical mass. In form they are globose, ellip- soidal, or ovoidal, and the cell-contents are granular, with so-called gas- vacuoles. The genus is a common constituent of the freshwater plankton, and is also found frequently in large ponds. C. KUtzingianum Nag. (diam. of colony 48 — 90 p ; diam. of cells 3 — 3‘8 yu. ; fig. 163 A) is the most abundant species, although C. Nagelianum Unger and C. minutissimum Lemm. both occur in quantity in the British freshwater plankton. Fig. 163. A, Ccdospluerium Kutzjngianum Nag., from the plankton of Lough Neagh, Ireland. B, Gomphosphceria aponina Kiitz., from Kestou Common Kent ( x 350). Genus Gomphosphaeria Kiitz., 1836. In this genus the colonies contain fewer and more scattered cells, which are disposed chiefly towards the periphery of a globular or ellipsoid mass of mucus. The cells are grouped in pairs and are distinctly pyriform in shape. Schmidle has observed the formation of ‘ microgonidia.’ The entire colony is solid and the cells divide alternately in three directions. G. aponina Kiitz. (diam. of colonies 64 — 78 p; length of cells 8‘5— 11 -5 g- fig. 163 B) is not uncommon in ponds, lakes, and stagnant ditches. Genus Microcystis Kiitz., 1833. [. Polycystis Kiitz., 1845 ; Glathrocystis Henfrey, 1856.] The cells are small, mostly globose, 350 Myxophycece and are densely aggregated to form solid colonies of variable shape. In some species the colonies are globose, ellipsoidal, or oblong ; in others they are much elongated; anrl in others they become cla- thrate and almost anasto- mosing. The cell-contents are blue-green, olive-green, or rose-purple in colour, and often contain gas- vacuoles. Fig. 164. A, Microcystis stagnalis Lemm., M. marginata Menegh from the plankton of Lough Neagh, Ireland. /j- „ » 0 „ 7 B, M. marginata Menegh, from Old Cote .• ■ ■ ■