178 PHYLUM III. ZYGOPHYCEAE 



Class 5. CONJUGATAE 



251. In this class the most primitive forms are the fila- 

 mentous Pond Scums, well represented everywhere by 

 species of Spirogyra. In this genus the ribbon-shaped 

 chloroplasts are longer than the cells, and are therefore 

 more or less spirally coiled. In generation two cells unite 

 by pushing out short opposing tubes where 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 

 F^ 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. More commonly this sexual union 

 takes place between cells of different filaments, as de- 

 scribed, but in some species such a union takes place 

 between contiguous 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 



