November 5, 1908] 



NA TURE 



here, and the case of Dictyota (as well as the more 

 complex case of certain Florideffi) shows that the 

 cytological distinction may exist between generations 

 which are clearly homolofjous with one another. The 

 author explains the similarity of the two generations 

 in such cases by the similarity of the conditions to 

 which thoy are exposed (p. 8i). We can well under- 

 stand (though this is not the author's view) how, 

 when the conditions became different, as in the 

 Archegoniatse, generations likewise homologous may 

 have come to be sharplv difl'erentiated. The author, 

 in chapter v., gives an admirable account of the 

 facts, but perhaps hardly realises how unfavourable 

 they are to the theory of intercalation. 



There appears to be no satisfactorj' case among 

 the Thallophyta of the origin of a diploid asexual 

 phase by intercalation, unless it be among certain 

 fungi, too remote from the archegoniate series to 

 afford any serviceable analogies. 



The fruit-body of the green alga Coleochaete, 

 formerly regarded as comparable to a simple bryo- 

 phytic sporogonium, has been shown by Allen 

 to have haploid structure, reduction taking place on 

 the first nuclear division in the germinating zygote 

 (p. 73). Hence this time-honoured comparison will 

 no longer hold good, though some biological analogy 

 may still be traced. 



So far as the evidence from the Thallophyta is con- 

 cerned, it seems that recent work favours the origin 

 of the alternating generations by the modification of 

 homologous individuals rather than by the inter- 

 calation of an entirety new phase in the life-cycle. 



Those morphologists who maintain the intercala- 

 tion hypothesis differ among themselves as to the 

 relation between the leafy sporophyte of the higher 

 plants and the sporogonium from which they believe 

 it to have been evolved. To some, the leaf is the 

 primary structure, derived directly from the sporo- 

 gonial head, and the axis is entirely subsidiary (Cela- 

 kovsky and VVorsdell), while on Prof. Bower's view 

 the axis is primary, the leaves (sporophylls) arise 

 from it Ae novo, by " enation," and the roots are 

 likewise accessory. The author lays great stress on 

 the predominance of the axis, as the foundation of 

 his " strobiloid " theory, according to which the whole 

 plant represents an elaborated strobilus, which in its 

 turn was derived from a simple sporogonium-like 

 fructification. He supports his view by a wealth of 

 argument, based on anatomy, embryology, and com- 

 parative morphology (see especially chapter xi., the 

 theory of the strobilus). All this, however, is sub- 

 sidiary to the main question. The predominance of 

 the axis is no necessary part of the " antithetic 

 theory," nor is it in any way opposed to the homo- 

 logy of the sporophyte with the vegetative body of 

 the lower plants. We see quite clearly among the 

 Bryophyta how, starting from a thalloid structure, 

 the axis may become predominant, and analogies are 

 not wanting among the Thallophyta also. 



We will not, however, pursue these controversial 

 matters further. Prof. Bower deals in the fullest 

 manner with a great problem, and nothing could be 

 better than the way in which he states his case. He 

 is not, perhaps, quite so happy in his treatment of 

 NO. 2036, VOL. 79] 



alternative hypotheses, which he sometimes dismisses 

 rather curtly, though to many botanists they will 

 appear worthy of more serious consideration. The 

 question, as the author points out, scarcely admits of 

 any final solution. The gaps in the evidence are such 

 that no theory (least of all the author's) can dispense 

 with the postulation of " hypothetical organisms," nor 

 have we much reason to hope that the fossil record 

 will ever supply a more substantial ancestry. 



The second and longest division of the book, the 

 detailed statement of the facts, will probably prove 

 of most value to the student, for it gives a full 

 account of the morphology, anatomy, and embryology 

 of the sporophyte of the Archegoniatae (including 

 extinct groups), with incidental references to the other 

 generation. Here also the strobiloid theory permeates 

 the whole, and great importance is attributed to the 

 Lycopodiiim Selago type, as the best living repre- 

 sentative of the hypothetical " strobiloid condition " in 

 which all the leaves were sporophylls. A figure of this 

 species forms the frontispiece to the book. 



The Sphenophyllales (including Psilotaceae) and 

 Equisetales are appropriately grouped together under 

 the head of " Sporangiophoric Pteridophyta," charac- 

 terised by the sporangia being borne on definite out- 

 growths from the axis or leaf, the peltate scales of an 

 Equisetum affording the most familiar example. The 

 author maintains at length the view that the spor- 

 angiophore is an organ sui generis, not homologous 

 with a leaf or leaf-lobe, a position which is tenable 

 and simple, but not wholly convincing to those whose 

 point of view is different from that of the strobiloid 

 theory. 



The Ophioglossacese are treated in much detail ; 

 the author upholds his well-known opinion that this 

 family forms, as a whole, an ascending series, prob- 

 ably derived from some sporangiophoric type com- 

 parable to that of the Psilotacese or other Spheno- 

 phyllales. He thus makes the series a parallel 

 development to the ferns, without actual affinity with 

 them. The alternative, and, in the reviewer's opinion, 

 more probable view, that the Ophioglossaceae are 

 derived from a somewhat primitive group of ferns, 

 not very remote from the Botryopteridese, is not dis- 

 cussed. The author argues vigorously against sapro- 

 phvtic reduction as a factor of any importance in the 

 evolution of this family, though in his description of 

 Ophioglosstim simplex he has himself supplied the 

 most convincing proof that such reduction has 

 occurred in an extreme degree. The extraordinary 

 embryology of the genus Ophioglossum (the embryo 

 in some species consisting of a root and nothing 

 else) appears to indicate that we are here dealing 

 with very highly modified plants, and by no means 

 with types of primitive simplicity. 



The account of the ferns is extremely full and in- 

 teresting, and less influenced by theoretical consider- 

 ations than the rest of the book. The author's classi- 

 fication of the homosporous ferns according to the 

 arrangement and succession of development of their 

 sporangia was first published in iSgg, and has been 

 recognised as a convenient and natural grouping. The 

 three series are characterised as follov/s (p. 497) : — 



The Simplices, in which the sporangia of a sorus 



