DESMIDIACE^E. 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 con jugate, f [See foot-note on p. 242.] 



II. CLASS COKJUGAT.E. 



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. 



30O. Order Desmidiacege. The Desmids are minute uni- 

 cellular algae ; 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. The 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 unsymmetrical ones ; these 

 grow by the rapid enlargement of the new 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 \. Areschoug, in " Observationes Phycologicae," 1874, records 

 having seen the conjugation in Cladophora and Ulva. 



