THE ALG^E 



109 



in each half of the cell (Fig. 79, D). These are often composed of several 

 radiating plates, united at the axis of the cell. Pyrenoids are present, and in 

 the elongated forms, like Closterium (Fig. 79, D) and Docidiuin, there may be 

 seen at each end of the cell a small vacuole, containing minute crystals of sul- 

 phate of lime, which show an active dancing movement in the fluid within the 

 vacuole. Very generally there is also a more or less evident constriction on the 

 equator of the cell. 



Movements. Movements not unlike those found in the Diatoms may often 

 be detected in the Desmids, and are probably due to protrusions of protoplasm 

 through minute openings in the cell-membrane. 



Cell-division. Where the cell is constricted, as it is in most forms, the cell- 

 division is somewhat peculiar. While the nucleus is dividing, a short, cylindrical 

 membrane is formed, just inside the place where the edges of the two valves 

 meet, and the valves begin to push apart. Next a ring of cellulose is formed, 

 running round the inner face of the 



A j%i^ B 



cylindrical membrane, and this ring 

 grows rapidly and cuts the protoplast 

 in two. One of the original valves 

 and half of the cylindrical membrane 

 go with each of the new cells. The 

 result is at first two very unsymmet- 

 rical cells (Fig. 80, B), but the new 

 valves, developed from the cylindrical 

 membrane, rapidly grow, and in a few 

 hours reach the full size, and assume 

 the characteristic sculpturing found 

 upon the old valves. The chloroplast 

 in each semi-cell also divides, and 

 one of the new chloroplasts passes 

 into the new semi-cell, and the two 

 Desmids are complete. 



In some genera of Desmids (e.g. 

 Desmidium, Gymnozyga, Fig. 80, A) 

 the cells remain together and form 

 long chains. 



Sexual Reproduction. In Meso- 

 tsenhun, the lowest of the Desmids, 

 the zygote arises by the complete 

 fusion of two individuals, very much 

 as in the lowest Volvocacese, the prin- 

 cipal difference being that in Meso- 

 tsenium the gametes are not ciliated. 

 The result is a resting-spore, from 

 which, after a period of rest, four to 



eight cells are formed by internal division, much as in the formation of swarm- 

 spores from the germinating resting-spores of the Confervacese. 



In the higher types (e.g. Cosmarium) only the protoplasts of the conjugating 

 cells unite (Fig. 80, C, D). Two cells approach each other, and may become 

 invested with a mucilaginous envelope. More or less evident conjugating tubes 

 grow out from between the valves, and when these come together they fuse and 

 form a short channel, into which pass the contents of both conjugating cells, 

 leaving the old valves empty. The zygote, thus formed, develops a heavy 

 wall often having spines projecting from it (Fig. 80, D). It has been found that 



- sp 



of Gymnozyga Brebis- 

 sonii, in division (X500). B, cell of 

 Cosmarium botrytis, dividing. C, D, 

 formation of the zygospore in Cosmarium 

 sp. (X450). 



