246 PROCEEDINGS OF THE AMERICAN ACADEMY. 



necessarily different in character. Every species whicli possesses this 

 type of zygosporic reproduction is capable of being separated into 

 two oi^posite strains, which may be cultivated separately to an in- 

 definite number of generations without the formation of zygospores, 

 but which will, when allowed to grow in contact on proper nutrients, 

 produce them by means of gametes derived from their respective my- 

 celia. If these strains are sowed side by side, a distinct line of zygo- 

 spores may be produced when the respective mycelia come in contact. 

 On account of its enormous jet black zygospores thickly beset with 

 forked spines, and the luxurance with which they are produced in a 

 broad line many layers thick, when the two opposite strains of the 

 species are properly contrasted, Phycomyces offers the most striking 

 example of zygospore formation in a heterothallic species. 



While in some forms, in addition to the inherent difference between 

 the strains which becomes apparent by the formation of zygospores when 

 they are allowed to come in contact, there is a more or less marked dif- 

 ferentiation in vegetative luxuriance, in others the differentiation is less 

 marked. To distinguish the two strains thus differentiated in the hetero- 

 thallic group, it has been found convenient to use the signs (+) and (— ). 

 As has already been mentioned in the Introduction, hybridization will 

 occur only between those strains which have unlike signs, and these 

 designations, it may be mentioned, were first used in distinguishing 

 strains where a vegetative differentiation was apparent. For this rea- 

 son it has become evident, notwithstanding certain theories to the con- 

 trary, that the process of conjugation is sexual in character, and that the 

 (+) and (— ) strains represent the two sexes repectively. In several 

 species certain strains have been found which, as far as they have been 

 tested, fail to respond to (+) and (— ) strains, the character of which had 

 already been determined in these species, and for this reason they have 

 been called " neutral." 



Rhizopds nigricans. 



This heterothallic species is the most widely distributed form among 

 the Mucorineae, and shares with Peuicillium the doubtful distinction of 

 being the most common fungus weed in laboratory cultures. As has 

 been already mentioned (p. 233), the " Harvard strain," supposed to be 

 a race in which the faculty of zygospore formation was specially de- 

 veloped, has been in use in the principal botanical laboratories in this 

 country, and that the peculiarity of its zygosporic superiority to ordi- 

 nary llhizopus has been the subject of some investigation goes without 



