DESMIDIAOE^. 225 



reproduce new filaments. Smaller bodies — swarm-spores — 

 are also produced, and these are said to conjugate.* 



298. — In Ulva the plant-body is flat, and composed of a 

 single layer of polyhedral cells, in which are found zoospores, 

 which are asexual (Fig. 152, c), and smaller swarm-spores, 

 which are said to conjugate, f 



§ II. Class Conjugate. 



299. — In this class the sexual process is a distinct conju- 

 gation, and it always takes place in the mature plant. 

 Swarm-spores are wanting. The orders of this class are well 

 marked. 



300.— Order Desmidiacese. The Desmids are minute uni- 

 cellular algse ; the cells are of very various forms, mostly 

 more or less constricted in the middle, and divided into two 

 symmetrical half-cells ; they are free, or united into loose 

 families, sometimes involved in a jelly. Tiie cell-wall is 

 more or less firm, but not silicious. 



301. — The reproduction of Desmids takes place asexually 

 and sexually. In the first the neck uniting the two halves 

 of the cell elongates and becomes divided by a transverse 

 partition, so that instead of the original symmetrical cell 

 there are now two exceedingly unsymnietrical ones ; these 

 grow by the rapid enlargement of the now and small halves ; 

 eventually the two cells become symmetrical, by which time 

 they have separated. This process, which is essentially fis- 

 sion, may be repeated again and again. 



The sexual process takes place in this way : each of 

 two cells which are near one another sends out from its 

 centre a conjugating tube, which meets the corresponding 

 one from the other {d, Fig. 153). At the point of meeting 

 the two tubes swell up hemispherically, and finally, by the 

 disappearance of the separating wall, the contents unite and 

 form a rounded zygospore {e, Fig. 153), which soon becomes 



* and -f. Areschoug, in " Observationes Phycologicse," 1874, records 

 having aeen the conjugation in Claicyphora and Uliia. 



