i66 COOK 



terizing the sporophyte is apparently brought about by the 

 migration and fusion of prothallial nuclei.^ In JVe^hr odium 

 ■pseiido-mas var. ci'istata apospora^ out of 80 prothalli examined, 

 only two showed possible cases of nuclear migration, and these 

 were open to doubt as regards their interpretations. 



The reason for the absence of fusion in other cases is obvious, 

 for the prothalli of JVephrodium j[)seudo-7nas var. n'istata apos- 

 pora, as we have seen, already possess the full complement of 

 somatic chromosomes. Hence there is no need for the fusion 

 of two nuclei which, by their union, double the number of 

 chromosomes." ^ 



Two varieties of the same fern show two very different sys- 

 tems of apospory, thus illustrating the range of choice of 

 aposporous methods of reproduction which lay open to the 

 ancestor of the angiosperms. In the one variety {cristata 

 apospord) apospory is accomplished without a reducing-division 

 and the resulting prothallus develops a new plant without fer- 

 tilization. In the other variety {polydactyla) the development of 

 the aposporous prothallus is initiated by the chromatin-fusion of 

 a vegetative cell. Two vegetative cells of the prothallus after- 

 ward unite, and thus restore the double number of chromosomes, 

 so that the new plant can be developed. 



A less violent departure from normal methods of reproduction 

 would place a primitive plant on the road toward evolution into 

 an angiosperm. The type of apospory which the angiosperms 

 appear to represent is somewhat intermediate between these two, 

 and somewhat less abnormal than either. Apospor}'-, as illus- 

 trated in angiosperms as a group, does not involve that mitapsis 

 be omitted altogether, as in one of the fern varieties ; it is merely 

 postponed to a later generation of cells. Neither is there an 

 abandonment of fertilization by the male gamete as in the other 

 fern variety. If the aposporous prothalli were to retain more of 

 the functions of true prothalli, they would be able to produce 



' While tlie aj'ticle was in the hands of the printer this supposition has been 

 confirmed. See Farmer, J. Bretland, and Digby, L., IQ07. Studies in Apos- 

 pory and Apogamy in Ferns, in Annals of Botany 21 : 161-199, Pis. 16-20, Apr. 

 1907. 



^ Digby, L., 1905. On the Cytology of Apogamy and Apospory, Proc. Royal 

 Soc. London, 76: 463. 



