LIFE CYCLES, SEXUALITY, AND SEXUAL MECHANISA1S 



45 



Seven basic types of life cycles can be clearly distinguished; 

 these are diagranimatically represented in Fig. 2 and are designated 

 by the letters A to G. Changes in nuclear phase are here considered 

 the cardinal events in the life cycle. These changes are indicated for 

 each cycle, progressing clockwise, by differences in shading. 



L/FE CYCLES IN FUNGI 



NUCLEAR 

 PHASE 



Fiu. 2. Schematic comparison of life cycles in fungi. 



In each cycle changes in nuclear phase are indicated, progressing clockwise, hy 

 changes in shading. The double vertical line at the top of the diagram represents 

 meiosis, and each of the two narrow sectors adjacent to the line represents a single 

 nuclear generation. 



Asexual reproduction by spores or other specialized organs oc- 

 curs at least in certain species belonging to each type and with few 

 exceptions propagates the phase of the cycle from which the special- 

 ized reproductive cells are derived; the few exceptions will be men- 

 tioned later. 



Asexual Cycle 



Species apparently lacking any sexual expression or alternation 

 of nuclear phase are fairly common throughout the fungi and con- 



