380 



SCIENCE 



[N. S. Vol. LI. No. 1320 



a fusion of nuclei in pairs in tlie zygospore 

 and a reduction division in the germ spor- 

 angium preceeding the formation of the 

 spores. From the diagram it will be observed 

 that the condition in the so-called hermaphro- 

 ditic lily is homologous with that in the so- 

 called diecious Phycomyces or Marchantia in 

 which I have found a similar differentiation 

 of sex in sporophytic sporangia. The forms 

 just mentioned are of the same type of sexual 

 differentiation and yet are termed hermaphro- 

 ditic or diecious according to whether the 

 sporophyte or the gametophyte is the more 

 conspicuous. To insure greater precision, I 

 have suggested the terms homo- and hetero- 

 thallic as applied to the gametophyte and 

 homo- and hetero-phytic as applied to the 

 sporophyte. If in further discussion of the 

 subject, I use the older terminology, it is 

 only to avoid terms unfamiliar to the major- 

 ity of my audience. The main point to be 

 brought out is that diecious mucors are not 

 to be homologized with diecious flowering 

 plants and higher animals. More nearly are 

 the sexual races of mucors to be compared 

 with the gametes themselves of such higher 

 plants and animals. 



ENVIRONMENTAL FACTORS 



Many investigators have succeeded in in- 

 hibiting the expression of one or both sexes 

 on the gametophyte of ferns by varying the 

 environmental factors to which they are ex- 

 posed. The question arises as to the in- 

 fluence of environmental factors on sexuality 

 in the mucors. Since gametes are formed 

 only after the stimulus of contact between 

 filaments with opposite sex tendencies and 

 not independently, the question is reduced to 

 the influence of external factors on zygospore 

 formation and upon the sexual activity of the 

 separate races. As a general rule, both for 

 hermaphroditic and diecious forms it may be 

 said that the limits within which zygospore 

 formation is possible are narrower than those 

 for non-sexual reproduction. Thus in Cun- 

 nmghamella echinulata, non-sexual reproduc- 

 tion takes place in abundance at low and high 

 temperatures while zygospores are formed 



only above 20° C. Certain other species will 

 not form zygospores, although able to produce 

 sporangia at a temperature as high as 26° C. 

 Further examples of other factors than tem- 

 perature might be given in support of the 

 greater environmental requirements necessary 

 for the sexual type of reproduction. So far as 

 has been investigated, external factors have 

 no influence in altering the inherent sex char- 

 acter in a given race, though they may change 

 ite power of showing a sexual response. 



The plus and minus races of Phycomyces 

 have been cultivated in separate test-tubes 

 since 1903 and have now reached the 242d 

 non-sexual generation of both plus and minus 

 paces. The plus and minus races of Mucor 

 Mucedo have been cultivated for the same 

 length of time. The minus race has grad- 

 ually become weaker and has this year finally 

 died out. There does not seem to have been 

 any actual loss or change of sex in the process 

 although the ability to respond sexually has 

 weakened with the weakness of vegetative 

 growth. 



MUTATIONS 



It is a question whether or not it will ever 

 be possible to induce genetic changes in the 

 mucors by changes in environmental factors. 

 Such changes do occur in some forms, how- 

 ever, apparently spontaneously. In 1912-13, 

 in an investigation as yet unpublished, I 

 found numerous variants of various degrees 

 of distinctness in the offspring of a single 

 plant obtained by sowing non-sexual spores. 

 Three forms from the hermaphroditic species 

 Mucor genevensis will suffice in illustration. 

 In the roll tube at the right is shown a num- 

 ber of mycelitun colonies of a dwarf mutant. 

 They are of about the same age as those in 

 the tube at the left. The dwarf has no 

 sporangia but is propagated by divisions of 

 the thallus. Perhaps the weakness of its 

 gTowth is responsible for the fact that it does 

 not form zygospores as other races of this 

 sjjecies do. 



At the right of the Petri dish culture (Fig. 

 3) are shown two colonies of the normal parent 

 race. The small dots scattered over the sur- 

 face are zygospores formed by the hermaphro- 



