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 
FTGS. 103-105. Closlerium: 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 
