52 



HEREDITY AND EVOLUTION IN PLANTS 



the latter case the nuclear divisions, some time preceding 

 spore-production, result in separating out the female (+) 

 and male (— ) strains, so that the spores in a given sporan- 

 gium are unlike as to sex — some being female (+), some 

 male (— ), (Fig. 37). This will be discussed more fully 

 in the next chapter. Such changes result merely in dis- 

 tributing the heritable units (genes) of the mother-cell 



Fig. 37. — 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 (J) 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 

 a female gamete. B, sexual reaction between a (-|-) filament and a male 

 gamete. C, a male zygospore formed by stimulus of contact with a (-|-) 

 filament. (After Blakeslee.) 



unequally to the daughter-cells, but introducing nothing 

 new; they may, however, result in the complete loss of 

 one or more heritable units, or in the formation of a new 

 one, not existent in the parent. In the latter two cases 

 we recognize a mutation. No hard and fast line can be 

 drawn between the various kinds of asexual reproduction; 

 there are various degrees of transition between reproduction 



