REVIEWS 



The Origin of a Land Flora: A Theory based upon the Pacts of Alterna- 

 tion. By F. O. Bower, Sc.D., F.R.S., Regius Professor of Botany in 

 the University of Glasgow. [Pp. 727, with 361 Figs, and Frontispiece.] 

 (Macmillan & Co., London, 1908.) 



A short statement of the theory forming the thread which binds this volume 

 together was published by the author in 1890. 1 Since that date his extensive series 

 of researches on pteridophytes has appeared under the title of Studies in the 

 Morphology of Spore-producing Members. When one bears in mind that these 

 solid contributions to our knowledge of the vascular cryptogams represent 

 preparatory investigations carried out with the object of supplying data for testing 

 or elaborating the original theory, one may be prepared to some extent for the 

 mine of information contained in the present work. The chief subjects coming 

 under consideration are the characters of the sporophyte generation in all the 

 groups of Pteridophyta, living and fossil, and in the Bryophyta ; the author's own 

 investigations and all other available sources being drawn upon to supply material 

 for theoretical sifting. 



The theory in question, called by the author " the biological theory of antithetic 

 alternation," may be outlined as follows : in the land flora the striking con- 

 stancy of alternation of generations is to be explained as the expression of an 

 amphibial existence ; the gametophyte is derived from aquatic ancestors (possibly 

 Algas), and remains aquatic as regards its motile gametes throughout the Arche- 

 goniatae, while the sporophyte is the characteristically subaerial generation, the 

 maturing and dispersal of its spores being normally connected with dryness. The 

 importance of the sporophyte is ascribed to the fact that in these terrestrial 

 organisms (Archegoniatas) sexual reproduction (which is aquatic) will be precarious, 

 hence increase in the number of individuals by another method suitable to drier 

 conditions will become of great value. Antithetic alternation of generations, as 

 distinguished by Celakovsky, is adopted as applying to the Bryophyta and Pterido- 

 phyta, i.e. the sporophyte generation is held to have been a phase intercalated 

 between successive gametophytes, and its origin is referred to certain cell-divisions 

 succeeding sexual fusion, such as occur in some of the lower plants, and are in 

 some cases connected with the reduction of the chromosomes. The sporophyte 

 generation at its initiation is thus pictured as a mass of spores resembling the 

 group of reproductive cells formed from the zygote in some Algae. From this 

 lowly beginning the sporophyte is held to have advanced to gradually increasing 

 complexity by means of various modifications, all of which are shown to be 

 intelligible as conferring a biological advantage, such as sterilisation of sporogenous 

 cells to supply protective tissue for enclosing the spores, and a vegetative system 

 for their nutrition ; the segregation of the sporogenous tissue into distinct pockets 

 or sporangia ; the origin of appendicular organs serving a variety of purposes. 



1 "On Antithetic as distinct from Homologous Alternation of Generations in Plants," 

 Annals of Botany, vol. iv. p. 347. 



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