ZYGOSPORE FORMATION IN SPIROGYRA 79 



adjacent cells of the same filament. If the conjugation is 

 between different filaments, are the zygospores all formed 

 on one side or are some formed in the cells of one filament 

 and some in the other ? 

 B. Find and draw a number of stages under h.p. illustrating 

 the history of the cell union or conjugation. Note : 



1. That the union takes place through processes put out 

 from adjacent cells. These unite to form a connecting tube. 



2. That the protoplast from one cell passes into the other 

 and fuses with its protoplast. 



3. That the product of this cell union is a fusion protoplast, 

 which forms a heavy wall about itself, thus becoming a 

 well-protected reproductive cell or spore. Note the changed 

 appearance of the contents of the spore, and the presence 

 of food material. Test for starch. 



Cell unions of this character are sexual processes. The cells 

 which unite are called gametes and their product is a sexually 

 formed fusion cell. The fusion cell in Spirogyra is called a sygo- 

 spore, or zygote, because the gametes are similar. For this reason, 

 also, this type of sexual reproduction is called isogamy (meaning 

 similar gametes). 



Kbfeeencb (on the plant cell). Principles, Chap. XVIII. 



Questions.* Describe the cell structure of Spirogyra. What 

 part of it is living substance and what part of it is non- 

 living ? Why are the cross walls in the filament flat planes ? 

 What would you expect to be the form of the wall at the 

 free end of a filament? How are new filaments of Spi- 

 rogyra formed ? How do the filaments grow and is the 

 growth confined to any special region ? What are the essen- 

 tial features in the formation of zygospores which define 

 it as a sexual process ? What part does the zygospore play 



* To THE Insteuctob : The sets of questions presented in connection with the 

 type studies of Part II are intended to bring before the student fundamental 

 principles in connection with his laboratory and field work, and his reading. 

 Written or oral exercises may be planned on them if desired. 



