Februaet 27, 1914] 



SCIENCE 



317 



it is difficult to detect any clearly ancestral 

 type among them all. The structures con- 

 cerned with this process in the aecidium- 

 forming rusts certainly seem most readily 

 comparable with the reproductive organs 

 of other thallophytes. It seems probable 

 that the occurrence of fusion at the same 

 point in all forms is due to its being post- 

 poned in all forms, as long as it could be 

 without being pushed over into another 

 phase of the life cycle. 



It would be instructive to spend another 

 half hour, as we can not do here, in con- 

 sidering those peculiar short cuts in repro- 

 duction known as apogamy and apospory. 

 These phenomena are so patently second- 

 ary, and so relatively infrequent, that they 

 can not be looked to for evidence of funda- 

 mental importance concerning the history 

 or the significance of the essential sexual 

 process itself. Their study has, however, 

 served to correct certain false assumptions 

 concerning the relation between the differ- 

 ence in chromosome number, and the differ- 

 ence in structure, of the two generations. 

 For example, the apogamous production, by 

 Nephrodium molle, of a normal fern sporo- 

 phyte with the X number of chromosomes, 

 demonstrates, as no other kind of evidence 

 could, that de Vries was right in regarding 

 the normal sporophyte as really two beings 

 in one. Incidentally too, such phenomena 

 suggest how comparatively unimportant it 

 is, for the structure of the plant, in what 

 manner, and at what point in the life his- 

 tory, the association of the 2X number of 

 chromosomes is brought about. 



CONCLUSION 



In our I'apid glance at the progress made 

 in the study of this problem, during twenty 

 centuries, we have seen how for eighteen 

 centuries men attempted to solve the prob- 

 lem by recourse to philosophical reasoning, 

 without the aid of detailed observation or 



experiment. Then, in less than two cen- 

 turies, by the use of these means, Camerarius 

 proved that pollination is a necessary con- 

 dition of seed-formation ; Koelreuter demon- 

 strated that characters from both parents 

 appear in hybrid offspring; Amici, Schmitz 

 and Strasburger showed how the mingling 

 of parental qualities is made possible by the 

 approximation and fusion of parental 

 protoplasms and nuclei. 



The sexuality which was first suspected, 

 and first experimentally proven, in the seed- 

 plants, has now been demonstrated in all 

 groups of plants save the bacteria and their 

 allies. The primary feature of the process, 

 the union of two parental nuclei, is the 

 same in all. The method of bringing to- 

 gether the two nuclei varies widely, this 

 variation sometimes involving even the com- 

 plete disappearance of externally recogniz- 

 able sexual organs. During the evolution 

 of plants old methods of accomplishing the 

 approximation of the nuclei have been dis- 

 carded, and new methods have arisen. In 

 the latter case a fusion of nuclei of closer 

 kinship has often been substituted for the 

 primitive one of more distantly related 

 nuclei. This seems evidently the case, for 

 example in the apogamous aseomycetes, 

 perhaps also in the basidiomycetes, and 

 surely so in the cases of nuclear fusion in 

 the prothallia and in the sporangia of 

 apogamous ferns. 



In the process of fertilization, as we 

 understand it at present, there are brought 

 together two distinct sets of chromosomes, 

 which in the nuclear divisions of the sporo- 

 phyte, or 2X generation, are often found 

 associated in pairs. The exact manner in 

 which these chromosomes become paired, 

 and the possibility of their attaining any 

 more intimate association, either in the rest- 

 ing reticulum or in synapsis, are not yet defi- 

 nitely determined. If, as is indicated by 

 Mendelian phenomena, and as is demon- 



