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- 

 thallus, which repeat respectively the one or the other (Fig. 2 77, p. 336). These 

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





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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 interi-ention 

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

 A, B). Or a Fem-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. 



