TORREYA 



Vol. 45 March 1945 No. 1 



Alternation of Generations and Classification With Special Reference 

 to the Teaching of Elementary Botany* 



Herbert A. Wahl 



The value of a scientific theory is usually directly proportional to the 

 amount of thought and investigation which it stimulates. A theory loses its 

 value when it becomes so generally accepted that the implications of the theory 

 are considered as facts and stimulation of thought passes over to unquestioned 

 acceptance of ideas as facts. Since what seems to be a fact in the light of cer- 

 tain evidence may lose its factual nature in the light of supplementary evidence, 

 the periodic reexamination of established ideas is a constant scientific desidera- 

 tum. This has been aptly expressed by Parkin and quoted by Douglas (1944) 

 as follows : "It is well that from time to time there should be a stocktaking — a 

 full appraisment of our botanical generalizations." 



The various expressions of the phenomenon of Alternation of Generations 

 as it occurs in the plant kingdom, while mostly in the realm of fact rather than 

 of theory, embody various theoretical considerations some of which have be- 

 come so well established as to be accepted as fact. Yet, in the light of accumu- 

 lated evidence, they may now require additional appraisal. 



It has long been recognized that the life cycle of vascular plants and bryo- 

 phytes, as well as of many algae, consists of an alternation between two phases 

 or generations each of which by a characteristic reproductive process initiates 

 the other. These two growth phases or generations have constantly been re- 

 ferred to as the sporophytic phase or sporophyte and the gametophytic phase 

 or gametophyte, since the former is spore-producing and the latter is gamete- 

 producing. Further, it has been recognized since the time of Strasburger that 

 the number of chromosomes in sporophytic nuclei is characteristically double 

 that in gametophytic nuclei. The chromosome number is doubled by the fusion 

 of gametes, the process that initiates the sporophytic phase, and halved by 

 meiosis, the process which initiates the gametophytic phase. 



Theories of alternation of generations deal chiefly with the origin and de- 

 velopment of the phenomenon. One school of thought has maintained that the 

 alternating generations are antithetic in their origin, that is, that the sporo- 

 phytic generation arose by a gradual evolutionary development of the zygote 

 following sexual reproduction and is a new structure not homologous in origin 



* Contribution from the Department of Botany, The Pennsylvania State College, 

 No. 146. 



Torreya for March (Vol. 45, 1-32) was issued April 20, 1945. 



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