232 THE PLANT WORLD 



uess are the two great qualities required for such work ; but on the other hand, the 

 work itself will stimulate and invigorate these qualities. Experimental work on 

 heredity should now be done, in some line or other, by any one who takes interest in 

 the great questions which in botany and zoology are at the present so universally atul 

 so vividly discussed. 



SEXUAL REPRODUCTION IN THE MUCORACEAE. 



Mucor is such a well-known plant, and is so constantly used in ele- 

 mentary text books, but has withal proved so refractory in the matter of 

 producing zygospores on time in business-like fashion, that we welcome 

 the report of the studies of Blakeslee,* in which are embodied results that 

 will be of interest to every teacher of elementary botany. 



It appears that there are among the Mzicoraceae two physiological 

 types distinguishable by their behavior in the production of zygospores. 

 Certain forms, of which Sporodinia, very common on toadstools of various 

 kinds, stands as an easily obtainable example, produce zygospores if 

 grown from the non-sexual spores, or mycelium of a single individual 

 {,homothallic) , comprising the majority of the species. On the other hand 

 there are other forms, of which Rhizopus and Mucor are examples, which 

 refuse to do so under these conditions. It further appears, however, that 

 the individuals of the latter class fall into two races or strains denominated 

 by the author the (-h) strain and ( — ) strain ; and when the mycelia of 

 two individuals, one of which is of the ( + ) strain and one of the ( — ) 

 strain, come into contact, zygospores are produced Oieterothallic) ; while 

 if the mycelia of two plants of the ( + ) strain and of two plants of the ( — ) 

 strain come into contact, no zygospores are produced. These two strains 

 differ therefore physiologically, and this may further be evidenced in other 

 ways. Thus one strain called the ( — ) generally displays less vegetative 

 activity, as shown by the production of smaller non-sexual spores or 

 sporangia, or by less luxuriant, slower mycelial growth, or it may suffer 

 the rapid loss of the power to produce the sexual elements, which is re- 

 tained indefinitely by the other (called the +) strain. These differences 

 are regarded as indicating corresponding sexual differences, a view which 

 is borne out by the fact that, if two plants of different species or even 

 genera, and of different strains come into contact, attempts at zygosporic 

 reproduction are made, while if the plants are of the same strain, i. e. , both 

 ( + ) or both ( — ), no such attempt will be made. If this interpretation 

 prove correct, then the homothallic forms are bisexual or hermaphrodite 

 and the heterothallic forms are dioecious, a conclusion further justified 

 by the behavior of a homothallic type toward the ( + ) and ( — ) hetero- 

 thallic strains. Thus, if a homothallic mucor be grown between the ( + ) 



* Blakeslee, A. F.," Sexual Reproductiou iu the Mucoiaceae," Proc. Am. Acad. Arts and Sci., 

 40: No. 4, 1904. 



