54 INTRODUCTION. 



There is a possibility that this doctrine may not be applicable to 

 cases of apogamy, apospory, and normal parthenogenesis among 

 plants. It has been suggested by Strasburger ('94, p. 300) that the 

 number of chromosomes may become doubled under the influence of 

 correlative pi'ocesses in an apogamously developed fern which arises 

 as a bud from the pro thallium, the nuclei of whose cells contain the 

 reduced number, and for the same reason the reverse may take place 

 in cases of apospory, /. e., the aposporous development of prothallia 

 may be attended with a correlative reduction in the number of chromo- 

 somes. Until the facts are determined by actual observation, all 

 discussion of this subject must remain a matter of pure speculation. 



The researches of Juel (1900) upon the normal parthenogenesis of 

 Antennaria alpina are of the highest interest in this connection, as 

 they throw light upon this question so far, at least, as the seed-bearing 

 plants are concerned. In Antennaria alpina, in which the egg 

 develops parthenogenetically under normal conditions, Juel finds that 

 no reduction in the number of chromosomes takes place in the develop- 

 ment of the embryo-sac, and, consequently, the nucleus of the egg- 

 cell which gives rise to the parthenogenetic embryo contains the same 

 number of chromosomes as the vegetative cells. Contrary to Anten- 

 naria dioica, in which fecundation regularly occurs, the mother-cell 

 of the embryo-sac of A. alpina develops immediately into the embryo- 

 sac, the heterotypic and homotypic nuclear divisions which follow the 

 appearance of the reduced number of chromosomes being omitted. 



In cases of normal parthenogenesis among the angiosperms, the 

 facts, so far as they are known, are certainly not at variance with the 

 doctrine of the reduction of the chromosomes as applied to the alter- 

 nation of generations. 



As has been intimated in preceding paragraphs, the sexual genera- 

 tion has been spoken of as the more primitive condition, and, as will 

 be seen from the following, the reduction in the number of chromo- 

 somes in the spore mother-cell is regarded by Strasburger as the 

 return of highly organized plants to the original unicellular condition : 



The morphological cause of the reduction in the number of chromosomes 

 and of their equality in number in the sexual cells is, in my opinion, phylo- 

 genetic. I look upon these facts as indicating a return to the original generation 

 from which, after it had attained sexual differentiation, offspring was developed 

 having a double number of chromosomes. Thus the reduction by one-half of 

 the number of the chromosomes in the sexual cells is not the outcome of a 

 gradually evolved process of reduction, but rather it is the reappearance of 

 the primitive number of chromosomes as it existed in the nuclei of the genera- 

 tion in which sexual differentiation first took place (1. c., p. 288). 



The phenomenon under consideration is essentially that of the return of the 



