178 PHYLUM III. ZYGOPHYCEAE 
Cuiass 5. CONJUGATAE 
In this class the lowest type is that of the filamentous 
Pond Scums, well represented everywhere by species of 
Spirogyra. In this genus the ribbon-shaped chloro- 
plasts are longer than the cells, and are therefore more or 
less spirally coiled. In generation two cells unite by 
pushing out short opposing tubes until they come in 
contact; the contact walls then are 
absorbed leaving an open channel 
from cell to cell, and through this 
+ the protoplasm from one cell slowly 
Fic. 70.—Spirogyra. Passes to the other, the two proto- 
plasms uniting into one mass, which 
rounds up and covers itself with a thick wall, thus 
forming a resting spore. The resting spore thus formed 
is set free by the decay of the dead cell-walls of the old 
filament surrounding it; it then falls to the bottom of the 
water, and remains there until the proper conditions for 
its growth appear. . 
251. More commonly this sexual union takes place 
between cells of different filaments, as described, but in 
some species such a union takes place between contigu- 
ous cells in the same filament, the tubes forming at the 
contiguous ends. 
252. The germination of the resting spore is a simple 
process. The inner mass enlarges and bursts the outer 
hard coat; it then extends as a cylindrical cell, in which 
after a while a transverse partition forms, and this is 
followed by another and another, until an extended 
filament is produced. 
253. In the Desmids the filaments usually fragment 
easily into single cells, which then grow more or less after 
separation. However in the lower Desmids the cells are 
still in filaments (Family Desmidiaceae). In the second 
