PLANT PHYSIOLOGY PRESENT PROBLEMS 143 



Except in the case of Sporodinia grandis, and perhaps one or two 

 other species of Mucoracece, mycologists have experienced great 

 difficulty in securing the zygosporic stage of these fungi. The recent 

 paper by Blakeslee, announcing the conditions governing zygo- 

 sporic formation in this family, seems to open a field for investiga- 

 tion wholly novel and suggestive. The substance of his results is 

 that this family may be divided into two principal categories, desig- 

 nated, respectively, as homothallic and heterothallic, these terms 

 corresponding to monoecious and dioecious forms among higher 

 plants. Sporodinia grandis belongs to the homothallic type, both 

 gametes in every union developing from the same thallus. Rhizopus 

 nigricans belongs to the second and larger class, the heterothallic 

 type, in which the two gametes are invariably the product of two 

 mycelia, which mycelia are sometimes of diverse vigor. When, in 

 .culture, the two strains, as they are termed, grow together, zygo- 

 spores are abundantly produced along the lines of contact. These 

 are the striking results of this important paper; but other related 

 physiological facts have been observed, and only further investigation 

 can tell whether this is a special case of gametic union in the fungi, 

 or whether similar phenomena may be found to be characteristic 

 of other groups where there is gametic union. 



The discovery of Mendel's hybridization studies and the inde- 

 pendent confirmatory evidence furnished by De Vries, Correns, 

 and others, all indicate the necessity of differentiating unit charac- 

 ters and of following separately the inheritance of each unit char- 

 acter. The idea which it involves of the purity of the gametes with 

 respect to unit characters, the segregation of unit characters in 

 the formation of the gametes, is one of fundamental importance. 

 Such work has given a marvelous impetus to studies in inheritance. 

 Numerous investigators have followed up this work, but it will be 

 many years, perhaps, before a test of the Mendelian laws can be 

 carefully made with any great number of plants and animals. The 

 exceptional instances already reported of the appearance of mosaic 

 characters and the dissimilarity in the product of reciprocal crosses 

 themselves indicate further fields for experimental research. Only 

 a word need be said bearing upon the phylogenetic side of physi- 

 ological work, since phylogeny, as well as pathology or ecology, 

 constitutes a separate section of biological science. The admirable 

 work accomplished by De Vries, serving beyond all question to de- 

 monstrate experimentally the origin of species by leaps or mutations, 

 necessitates laying further stress upon discontinuous variation as 

 a factor in the origination of existing species of plants. It is to be 

 doubted, however, that most botanists will at present concur in 

 such an opinion as that the evidence advanced is sufficient to disregard 

 or disparage the part which is played by continuous variation in the 



