FILICALES 



349 



the introduction of new incidents, others by the excision of certain phases. 

 For instance, buds may be produced either on the Fern-Plant or on the pro- 

 thaHus, which repeat respectively the one or the other (Fig. 277, p. 336). These 

 are merely amplifications of the soma, without any change of constitution of 



the tissues, or of the nuclei. But others are of the nature of short-cuts. For 

 instance, a prothallus may arise from the Fern-Plant without the intervention 

 of spores (apospory), as in certain forms of Atkyrium or Polystichum (Fig. 292, 

 A, B). Or a Fern-Plant may spring directly from a prothallus without the 

 sexual process (apogamy), as in Pteris cretica (Fig. 293).- Such examples show 

 that the events of the life-cycle are not immutable. But they raise difficulties 

 of interpretation in terms of chromosomes. 



