THE NATURE OF ALTERNATION OF GEN- 

 ERATIONS IN ARCHEGONIATE PLANTS. 



AN HISTORICAL SKETCH. 



THERE are few scientific terms the meaning of which 

 does not become more or less modified with the 

 progress of investigation. Cases of the complete loss of the 

 original significance are, however, less frequent, and when 

 they occur usually involve a period of transition before the 

 the use of the term again becomes definitely fixed. The 

 phrase "alternation of generations " as applied to plants has 

 undergone such a change of meaning : this necessitates a 

 brief retrospect of its use in the past fifty years before 

 attempting to consider recent progress in our knowledge 

 and opinions regarding the phenomena now denoted by it. 



The term was introduced by Steenstrup, 1 in 1845, to 

 denote " the remarkable and till now inexplicable natural 

 phenomenon of an animal producing an offspring which at 

 no time resembles its parent, but which on the other hand 

 itself brings forth a progeny which returns in its form and 

 nature to the parent animal ". Plants are not referred to 

 until the last page of this monograph which deals with 

 various groups of invertebrata, and then the succession of 

 vegetative shoots culminating in the flower or reproductive 

 shoot is recognised as the corresponding phenomenon. 

 The diagrammatic representation of a similar view in the 

 frontispiece of Owen's classic work on Parthenogenesis may 

 be referred to in illustration. 2 That the importance of the 

 succession of sexual and spore-bearing forms in the life 

 history was not recognised can be readily understood, when 

 the fragmentary nature of the available data is remembered. 

 Observations pointing to the discontinuity between these 

 two stages in Bryophyta and Pteridophyta indeed existed, 

 but a comparative treatment was wanting. Even in 1851, 

 Braun 3 only uses the term alternation in the sense in which 



1 Steenstrup. 2 Owen. 3 Braun (1). 



