264 



STRUCTURE AND LIFE HISTORIES 



After the zygote is formed, the outer wall thickens and 

 assumes certain external characteristics, easily recog- 

 nized. It also becomes black as it ripens, and this fact 

 has given rise to the common name, "black mold." In 

 this condition the zygote rests, as a zygospore. 



FIG. 190. Sexual reaction between a hermaphroditic Mucor and (+) 

 and ( ) races of a dioecious species. Diagrammatic representation of a 

 Petri dish culture showing a heterogamic hermaphroditic mucor (+) in 

 the center separated by channels on either side from the (-{-) and ( ) 

 races, respectively, of a dioecious species. Sp., sporangia containing 

 spores by means of which the plant may be reproduced nonsexually. 1-6, 

 stages in development of a hermaphroditic zygospore from unequal male 

 and female gametes. A, sexual reaction between a ( ) filament and fe- 

 male gamete. B, sexual reaction between a ( +) filament and male gamete. 

 C, a male zygospore formed at stimulus of contact with a (+) filament. 

 (After Blakeslee.) 



265. Germination. At the close of the resting period, 

 and under favorable conditions of temperature and mois- 

 ture, the zygote germinates, sending out an erect hypha, 

 which at once develops a globular sporangium at its apex. 

 The asexual spores from this sporangium become the 

 starting point of another series of changes like those 

 just described. 



256. Sexuality of Rhizopus. It is well known that, 

 while conjugation often occurs freely between mycelia 

 from different spores, in other cases it fails entirely. The 

 explanation of this was not known until about eight years 



