222 ^H^- LLOPHYTES. 



surrounded by a delicate gelatinous wall (i/,/) by the side of which lie the empty cell- 

 walls (//, e, b). The zygospore now becomes rounded into a ball ; its wall forms, as it 

 matures, three layers, an outer and an inner colourless layer of cellulose, and a middle 

 firmer brown layer. This stratified cell-wall grows out at several points into spiny 

 protuberances which are at first hollow and afterwards solid, each of them producing at 

 the end a few small teeth (///). The starch-grains of the conjugating cells become 

 transformed into fat in the zygospores. Germination commences by the protrusion of 

 the colourless inner layer through a wide split in the outer layer {IF); the thin-walled 

 sphere thus set free considerably exceeds the zygospore itself in size. In the contents 

 of this sphere {F) may be recognised two masses of chlorophyll surrounded by fatty 

 protoplasm which might have been distinguished even before their escape from the 

 external layer of the zygospore. The contents now contract and become surrounded 

 by a new wall (F) from which the older wall detaches itself as a thin vesicle. After 

 some time the protoplasm becomes constricted by a circular furrow, and splits into two 

 half-balls, each of which contains one of the two chlorophyll-grains {FI). Each half-ball 

 remains for a time naked and again constricts itself; but this time the constriction does 

 not advance to the centre ; the body changes its form in other respects also, and each half 

 of the germinating cell now appears as a symmetrically divided Cosmarium-cell {FII), 

 which surrounds itself with a wall of its own. The planes of the constrictions of the two 

 cells derived from the zygospore cut the dividing plane of the zygospore itself at right 

 angles; they themselves also stand at right angles to one another, and therefore lie 

 crossed in the mother-cell. In each of these the contents now arrange themselves in 

 the manner above described ; the mother-cell-wall is absorbed and the new cells separate 

 from one another. All these processes of germination are completed in one or two 

 days. The new cells, whose outer wall is smooth, now divide in the usual manner, 

 but the newly grown halves are larger and rough on the outside {Fill, IX, X) ; \\\e 

 four daughter- cells of the two cells produced from the zygospore are therefore of two 

 different forms ; two have the halves equal and two unequal ; the latter constantly 

 produce by division one with equal and one with unequal halves ; the former only cells 

 with equal halves. 



The Diatomacese^ (Bacillarieae) extremely rich in species follow naturally after the 

 Desmidieae ; in particular they are allied to the Conjugatae, their processes of develop- 

 ment coinciding with the conjugation of the latter, or at least bearing a certain 

 resemblance to it^. They also resemble the Desmidiege in the configuration of their 

 cells, in the manner of division, and in the mode of completion of the daughter-cells. 

 Like the Desmidieae, the similar cells of the Diatoms may be united into threads, 

 or may live entirely isolated. The tendency of the Diatoms to secrete a thin jelly 

 in which they live socially is found also in the Desmidieae, although less strongly dis- 

 played. In the same manner the movements of Diatoms are not altogether dissimilar 

 to those of the Desmidieae, and even the silicification of the cell-wall, which is very 

 strong in the former, is found, though to a smaller extent, in Closterium and other 

 Desmidieae; and the fine sculpturing of the silicious shell also finds an analogue, 

 although in a coarser form, in the cell-wall of some Desmidieae. The Diatoms are 

 the only Algae, except the Conjugatae, in which the chlorophyll occurs in the form 

 of discs and bands, but in some forms it is also found in grains, and the green 

 colouring matter is concealed, like the chlorophyll-grains in Fucaceae, by a buff- 



^ Luders, Ueber Organisation, Theilung und Copulation der Diatomeen, Bot. Zeitg. 1862, 

 no. 7 et seq. — Millardet and Kraus discuss their colouring-matter in Compt. Rend. vol. LXVI. 

 p. 505; and Askenasy in Bot. Zeitg. 1869, p. 799. — Pfitzer in Heft II, of the Botanische Abhand- 

 lungen edited by Hanstein. Bonn 1871. [Quart. Journ. Micr. Sc, 1872, 1873.] 



^ [Thwaites first discovered the conjugation of the Diatomacese, Ann. Nat. Hist. 1847, 

 vol. XX; see also Carter: Ann. and Mag. Nat. Hist. 1856. — Schmitz, Quart. Journ. Micr. Sc., 

 1873, p. 145. — Smith, Synopsis of British Diatomacese. — Ed.] 



