Asexual

two equivalent gametes to form a zygote or zygospore. reproduction by means of swarm-spores is wanting in both classes, and since the gametes also are unprovided with cilia these forms are also classed as Acontae. The cells of the Conjugatae, which increase in number by cell-division, are uninuclear, and differ from those of the Diatoms in having a cell wall which is not silicified and in the presence of large green chloroplasts of complicated structure.

In

1. The Mesotaeniaceae, the simplest of the unicellular Conjugatae, include only a few genera. They are distinguished from the following order by the cell wall of the shortly cylindrical cells not being formed of two halves. The mode of conjugation presents some differences. Spirotaenia (Fig. 260) the protoplasts of the conjugating cells first divide and the daughter cells unite in pairs to form the zygotes; in Mesotaenium and Cylindrocystis the protoplasts conjugate without a preliminary cell-division. Similar differences are shown by the Diatomeae. The zygote on germination divides into four new individuals.

2. The Desmidiaceae are unicellular or their cells are united in rows; they are of great beauty and, like the Diatoms, exhibit a great variety of form. Their cells are composed of two symmetrical halves, separated, as a rule, from each other by a constriction. Each half contains a large, radiate chromatophore or a chromatophore composed of a number of plates. Within the chromatophores are disposed several pyrenoids, while the nucleus lies in the centre of the cell in the constriction. The cells themselves display a great diversity of form and external configuration, being sometimes rounded (e.g. Cosmarium, Fig. 261, A, B), sometimes stellate (Micrasterias, Fig. 261, D). The cell walls, which, as in the Diatoms, consist of separate halves, are frequently beset with wart- or horn-like protuberances and often provided with

FIG. 261.-A, Cosmarium coelatum, dividing. B, C, Cosmarium botrytis. C1, two cells at right angles preparing for conjugation--the lower cell shows the conjugation canal; C2, gametes fused into the young zygote; C3, mature zygote; D, Micrasterias crux melitensis. (After RALFS; C1, C2, after DE BARY.)

pores. In some genera there is no constriction between the two halves of the cell. This is the case, for instance, in the crescent-shaped Closterium moniliferum (Fig. 262, F), whose two chromatophores consist of six elongated plates, united in the long axis of the plant, while in each end of the cell there is a small vacuole containing minute crystals of gypsum in constant motion. Many Desmids are

characterised by heliotactic movements; they protrude fine mucilaginous threads through the cell walls at their ends; by means of these they can push themselves along, and take up a position in a line with the direction of the incident rays of light. Multiplication is effected by cell division. This is accomplished by the formation of a partition wall across the middle of the cell after the nuclear division is completed. Each daughter cell eventually attains the size and form of the mother cell, by the outgrowth of a new half on the side towards the new division wall (Fig. 261, 4).

In conjugation two cells approach each other, and surround themselves with a mucilaginous envelope. Their cell walls rupture at the constriction, and parting in half allow the protoplasts to escape; these then unite to form a zygospore. The zygospores frequently present a very characteristic appearance, as their walls are often beset with spines (Fig. 261, C). The four empty cell halves may be seen close to the spore. In some Desmidiaceae the conjugating cells undergo a preliminary division, the daughter cells uniting in pairs.

The two sexual nuclei in the zygote do not fuse until germination of the latter is about to commence. The resulting nucleus then undergoes division into four nuclei, two large and two small. Only two cells are formed from the zygote, each of which has thus two nuclei of different sizes; the smaller nuclei disappear. The production of two cells on germination thus to be derived from the diviappears sion into four cells seen in Mesotaenium,

and to stand to the latter as a reduced form.

3. Zygnemaceae. In this family, all of which are filamentous in character, the genus Spirogyra, with its numerous species, is the best known. It is commonly found in standing water, forming unattached masses of tangled green filaments. The filaments exhibit no distinction of base and apex, and are composed of simple rows of cells, which vary in length in different species.

Growth results from the division and elongation of the cells in one direction only. Each cell has a large nucleus and one or several spiral green band-like chromatophores (Fig. 263, C). The cells of Zygnema contain two star-shaped chromatophores.

CONJUGATION, in the case of Spirogyra, is preceded by the development of converging lateral processes from the cells of adjacent filaments. When two processes from opposite cells meet (Fig. 263, 4), their walls become absorbed at the point of contact, and the whole protoplasmic contents of one cell, after contracting from the cell wall, passes through the canal which is thus formed into the opposite cell. The protoplasm and nuclei of the conjugating protoplasts then fuse together while the chloroplasts do not unite, but those of the entering protoplast disorganise. The resulting cell forms the zygospore invested with a

FIG. 263.-4, Conjugation of Spirogyra quinina (x 240). B, Spirogyra longata (x 150); z, zygospore. C, Cell of Spirogyra jugalis; k, nucleus; ch, chromatophores; P, pyrenoid. (× 256.)

thick wall, and filled with fatty substances and reddish-brown mucilage-spheres. This form of conjugation, which is the one exhibited by most species, is described as scalariform (Fig. 263, 4), as distinct from the lateral conjugation of some species, in which two adjacent cells of the same filament conjugate by the development of coalescing processes, which are formed near their transverse wall (Fig. 263, B). In some genera the zygote is formed midway in the conjugation tube. The conjugation nucleus of the young zygospore gives rise, according to CHMIELEWSKY, to four by two successive divisions. According to KARSTEN this is to be regarded as a reduction division (20). On germination the zygote grows into a tubular cell, from which by cell division the filament is derived.

CLASS VII

Diatomeae (Diatoms) (8, 21)

The Diatomeae (Bacillariaceae) constitute a very large class of unicellular Algae. They occur, usually associated together in large numbers, in both fresh and salt water, and also on damp soil.

The cells are either solitary or form colonies; they are free-floating, or are attached by means of gelatinous stalks, excreted by the cells themselves (Fig. 264). Sometimes the cells remain connected and form bands or zigzag chains, or, on the other hand, they are at

FIG. 264.-Licmophora flabellata. Colony of Diatoms with branched gelatinous stalks. (After SMITH, from, GOEBEL'S Organographie.)

FIG. 265.-Formation of auxospores in Navicula viridula. A, Cell seen from the valve side. B, Two cells lying alongside one another; their contents have divided into two daughter cells, each of which possesses two nuclei. C, D, Conjugation in pairs of the daughter cells to form the auxospores, which at first contain four nuclei. E, The two full-grown auxospores; the two larger nuclei in each have fused into one, the two smaller ones have disintegrated. (After KARSTEN, X 500.)

tached and enclosed in gelatinous tubes, while in the case of the marine genus Schizonema they lie embedded in large numbers in a

gelatinous branching thallus, often over 1 dem. in size. The cells also display a great diversity of shape; while generally bilaterally symmetrical, they may be circular or elliptical, rod- or wedge-shaped, curved or straight. The structure of their cell walls is especially characteristic ; it is composed of two halves or VALVES, one of which overlaps the other like the lid of a box (Fig. 3, B). The cells thus present two altogether different views, according to the position in which they are observed, whether from the GIRDLE (Fig. 3, B) or VALVE SIDE (Fig. 3, 4). The two valves are so strongly impregnated with silica, that, even when subjected to intense heat, they remain as a siliceous skeleton, retaining the original form and markings of the cell walls. The walls of the cells, particularly on the valve side, are often ornamented with numerous fine, transverse markings or ribs, and also with small protuberances and pits. They are often perforated by open pores which serve to give exit to the gelatinous

secretion.

The cell has always a central nucleus and one (Fig. 3) or two to four (Fig. 265) large, or numerous smaller chromatophores (Fig. 267) embedded in its parietal protoplasm. These chromatophores are flat, frequently lobed, and of a brownish-yellow colour. Pyrenoids are often present. Globules of a fatty oil are also included in the cell contents, and take the place of starch as an assimilation product.

The Diatomeae multiply vegetatively by longitudinal division which always takes place in one direction. In this process the two valves are first pushed apart from one another by the increasing protoplasmic contents of the mother cell, which then divides longitudinally in such a direction that each of the two new cells retains one valve of the mother cell. After the division of the protoplasm of the mother cell is accomplished, each daughter cell forms, on its naked side, a new valve fitting into the old one. The two valves of a cell are therefore of different ages. In consequence of this peculiar manner of division, as the walls of the cells are silicified and incapable of distension, the daughter cells become successively smaller and smaller, until finally, after becoming reduced to a definite minimum size, they undergo transformation into AUXOSPORES. The auxospores

are usually two or three times larger than the cells from which they arise, and by their further development they re-establish the original size of the cells.

According to the structure of the cell walls and the mode of reproduction the Diatomeae are subdivided into the two orders of the Pennatae and the Centricae. Order 1. Diatomeae Pennatae. In shape these are elongated, elliptical, or boat-shaped, but may be wedge-shaped and zygomorphic; the valves have their sculpturing pinnate (Figs. 264, 265). In many of the Pennatae (Fig. 3) a longitudinal line corresponding to an opening in the cell walls, and exhibiting swollen nodules at both extremities and in the middle, is distinguishable in the surface of the valves. Forms provided with such a median suture or RAPHE are