No. 464.] STUDIES ON PLANT CELL.— VII. 567 



ciate the term fertilization with this phenomenon, whatever may 

 be the physiological significance of the nuclear fusions, because 

 we are not dealing with gametes and there cannot be involved 

 in the process anything of the long phylogenetic history of sex- 

 ual differentiation in the group. We considered these matters 

 in some detail in that portion of this section entitled " Fertiliza- 

 tion." 



With respect to the factors which determine apogamy it must 

 be confessed that we are still in the dark. Lang's ('98) studies 

 on fern prothalli, however, throw some light on the problem. In 

 some twenty forms of the Polypodiacese apogamy resulted when 

 the prothalli were kept from direct contact with the water {i. e., 

 were watered from below) and exposed to direct sunlight. 

 When watered from above these same forms developed normal 

 embryos from eggs. It is clear that the suppression of condi- 

 tions which make fertilization possible [i. e., water over the sur- 

 face of the prothallus), possibly aided by sunlight which may 

 cause irregularities of growth, induced the development of cylin- 

 drical processes from which the apogamous sporophytes arose 

 and which bore sporangia in two forms. It seems hard to draw 

 more precise conclusions from these experiments other than that 

 the normal life history is checked at a critical period (fertiliza- 

 tion) and the plant is forced into expressions of vegetative 

 activity. The conclusions of Farmer, Moore, and Digby (: 03) 

 offer an explanation of how the developments may take on 

 sporophytic characters through the fusion of nuclei in the tis- 

 sues and the establishment of a sporophyte number of chromo- 

 somes. 



Strasburger suggests that apogamy in Alchemilla may be the 

 result of a weakening of sexual power associated with excessive 

 mutative tendencies. This would seem to imply that excep- 

 tional vegetative activity, with the appearance of much variation 

 under favoring conditions, may be combined with apogamy. It 

 is of course a well known fact that a high degree of cultivation 

 tends to lessen the fertility of a form unless guarded by careful 

 selection. A weakened sexual fertility due to excessive vegeta- 

 tive activity is likely to be replaced by forms of vegetative 

 reproduction. When the process of sporogenesis becomes so 



