ALGM. 



221 



together with Figs. 5, 6, and 15 (pp. 10 and 17), to give an idea of the process of 

 development of these plants. 



Among the Desmidieae ^ we may choose as an example for closer observation 

 Cosmarium Botrytis (after De Bary, /. c). The cells live isolated, and are symme- 

 trically bisected by a deep constriction (Fig. 157, X), and are also compressed at right 

 angles to the plane of constriction {I, a) ; in each half-cell are two grains of starch 

 and eight discs of chlorophyll, which curve and converge in pairs running from two 

 centres to the wall. The multiplication of the cells by division is brought about by 

 the narrowest part of the constriction elongating a little, when the thicker outer 

 layer of the cell- wall opens by a circular fissure ; the two halves of the cell hence appear 

 separated from one another and united by a short canal, the wall of which is a con- 

 tinuation of the inner layer of the walls of the half-cells. A septum soon appears in 

 the piece which unites them, by which the cell is divided into two daughter-cells, each 



Fig. in.— Cosmarium Botrytis (after De Bary, /. c). (I-III X390, Il^-A' X190). 



of which is a half of the mother-cell. The septum, at first simple, splits into two 

 lamellae, which immediately become convex towards one another (JX, h)\ each 

 daughter-cell now possesses a small rounded outgrowth which grows gradually and 

 assumes the form of a half-cell, so that each daughter-cell now again consists of two 

 symmetrical pieces [X). While the wall is undergoing this growth, the discs of chlo- 

 rophyll of the old halves grow into the newly-formed halves of the cell. The two 

 grains of starch of the old half-cells elongate, become constricted, and each divides 

 into two grains; of these four grains two pass over into the new half-cell, and all 

 four again arrange themselves in the original symmetrical manner. Conjugation takes 

 place between cells lying in pairs in a crossed position enclosed in thin jelly (Fig. is7, I)- 

 Each of the two cells emits from its centre a conjugating protuberance (/, c) which 

 meets the other; these protuberances are formed by a delicate membrane which is 

 a continuation of the inner layer of the cell, the firm outer layer of which is split 

 {I,c). Both protuberances swell up into a hemispherical bladder while in contact 

 with one another until the separating wall disappears, and the contents unite in the 

 broad canal thus formed; the protoplasm becomes everywhere loosened from the 

 cell- wall, and contracts into a spherical form. The united protoplasm appears as if 



[See also Ralfs, British Desmidiese, 1848.— Aixher in Pritchard's Infusoria.] 



