234 BOTANY. 



cell-walls of the old filament surrounding it ; it then falls to 

 the bottom of the water and there remains until the proper 

 conditions for its growth appear. 



312. — Tlie conjugation described is the one best known ; 

 it prevails in a large part of the genus mentioned. There 

 are some curious modifications of the process. In what is 

 called genuflexous conjugation the opposing cells of parallel 

 filaments become strongly bent back so as to form an angle 

 at their central points ; then the angles approach each other 

 and fuse, allowing the cell-contents to pass over, as in the 

 other case. 



Lateral conjugation takes place between the cells of the 

 same filament. At the contiguous ends of two cells tubular 

 processes are pushed out, which, meeting, form a curved 

 channel from one cell to the other. Occasionally there ap- 

 pears to be only a slight enlargement of the contiguous ends 

 of the cells, and this is followed by the breaking away of a 

 portion of the separating wall. These cases of lateral con- 

 jugation show that the cells are, to a great extent, to be re- 

 garded as independent organisms, and that the conjugation 

 is primarily the union of two cells, instead of two filaments. 



313. — The germination of the zygospore is a simple pro- 

 cess. The inner mass enlarges and bursts the outer hard 

 coat ; it then extends into a columnar or club-shaped mass, 

 gradually enlarging upward from its point of beginning ; 

 after a while a transverse partition forms in it, and this 

 is followed by another and another, until an extended fila- 

 ment is formed. 



{a) The principal genera are Spir^^gyra, in which the chlorophyll 

 bands are spirally arranged in the cells, and Zygnema, in which the 

 chlorophyll is usually arranged in a stellate manner. Sixteen species 

 of 8piri gyra are recorded as occurring within the United States, and 

 of these Sp. longata and Sp. quini'ia are the most common. 01 Zyg- 

 mma but two species are recorded in the United States, and only one 

 of these, Z. i'signe, is common. 



(i) These plants may be found at any time in ditches and streams, 

 where they often form extensive masses of green felt ; but it is only 

 from the middle to near the end of spring: that they can be found in 

 conjugation. For the Northern States the time varies from April to 

 the first of June ; in the Seuth it is of course much earlier, being in 



