CHAPTER XXXII. 



ALTERNATION OF GENERATIONS, AND THE LAND-HABIT. 



THE expression " Alternation of Generations " was brought into 

 prominence by Steenstrup, who applied it to the succession of phases 

 in the life-history of Medusae, Trematodes, and other Animals. He 

 denned it as " the remarkable natural phenomenon of an Animal 

 producing an offspring which at no time resembles its parent ; but 

 which itself brings forth a progeny which returns in its form and nature 

 to the parent Animal." The publication of Steenstrup's essay pre- 

 ceded the demonstration by Hofmeister of the life-history of Mosses, 

 Ferns, and Conifers. These researches disclosed phenomena of 

 Alternation in Plants superficially so like those in Animals that it 

 was natural to use the same terms in describing them. But later 

 it has become clear that the resemblance is not an exact one, and that 

 the " generations " in Plants differ more essentially from one another 

 than those in Animals, to which the terms were originally applied. 



In many Plants the distinctness of the sporophyte and gameto- 

 phyte is marked by form and structure; for instance, that between 

 the prothallus and the Fern-plant. Nevertheless some contemplated 

 the possibility of the sporophyte having originated as a modification 

 of a gametophyte, and described the alternation as one of " homo- 

 logous" generations. Others, impressed with their distinctness not 

 only in form but also in their probable origin as indicated by com- 

 parison, held the two generations to be " antithetic," that is, distinct 

 in their origin and history from one another. The discussion of the 

 question seemed likely to pass into an inconclusive dialectic, when 

 a fresh point was given to it by the discovery that in Plants there 

 is a prevalent nuclear difference between the two generations. In 

 such Plants as maybe held to be normal, the sporophyte was found to 

 have diploid nuclei, and the gametophyte nuclei that are haploid. 

 This distinction is not matched by any corresponding known difference 



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