38 MORPHOLOGY 



Individuals are multiplied by cell-division. The nucleus divides and 

 a wall is formed across the isthmus (fig. 101). The division thus results 

 in two new cells, each consisting of one half of the old cell and a portion 

 of the isthmus, which enlarges into a new half, when the two cells sepa- 

 rate. During this process the plastid in each half divides, so that the 

 new cell contains the usual two plastids (fig. 102). 



In sexual reproduction the cells pair, the walls are ruptured at the isth- 

 mus, the protoplasts escape and fuse, and a zygospore is formed (figs. 

 103-105). In some species a little tubular projection puts out from 



FIGS. 103-105. Closterium: 103, two cells pairing for conjugation; 104, the tvfj 

 protoplasts fusing; 105, the zygospore formed. 103, 104, after DE BARY; 105, after 

 WEST. 



each cell at the isthmus, and the two projections meet to form a short 

 tube in which the protoplasts meet and fuse. The sexual fusion in 

 desmids is true conjugation, in which two vegetative protoplasts fuse 

 without the organization of distinct gametes. In the germination of 

 the zygospore the heavy wall is ruptured, the protoplast escapes, and 

 four nuclei are formed, only two of which usually persist in the devel- 

 opment of new desmids. 



Mesocarpaceae. In this family the body is a filament of similar elongated 

 cells, in each of which there is an axial platelike chloroplast. Cell division is 

 accomplished by nuclear division and a centripetally growing wall. Sexual re- 

 production occurs between adjacent filaments, whose cells pair and come in con- 

 tact by kneelike bends or by short tubes, and the two abutting walls becoming per- 

 forated at their contact, permit the two similar protoplasts to come in contact 

 for fusion. The zygospore upon germination forms a new filament directly. 



Zygnemaceae. These are pond scums, and are among the most 

 common of the green algae. The characteristic genera are Zygnema 



