STRUCTURE AND DEVELOPMENT OF MOSSES AND FEENS. 91 



modern conditions of existence, we come to the Vascular Crypto- 

 gams, with their three classes — FUkinece, EquisctitWiC, Lycopodiaetc. 



The FiiicinecB are arranged in two groups — Eusporanguttfp, and 

 Leptospomiu/iata;. Tlie former group comprises the homosporous 

 orders Ophior/lossacea; and Marattinccd, and also the heterosporous 

 Isoetacecc. The provisional transference hither of the latter order 

 from the Lijcopodinecc Mr. Campbell thinks to be justified by the 

 weight of our present evidence. The Juisporaiu/iatcB are regarded as 

 the remnants of widely divergent branches of a common stock, 

 which indicate the ancestry of the Lcptosporanglatm on the one hand, 

 and on the other possibly that of the Monocotyledons and Dicoty- 

 ledons along the line of the hoetacece (the Gymnosperms having 

 arisen probably along a different line, viz., that of the LycopmUnerB 

 via the Selnfiinellaceic). An interesting observation will be found on 

 p. 2i3, where it is shown that Botnjchuun alone among the Ferns 

 exhibits in its rhizome a true secondary thickening of its vascular 

 bundles. A thickening, but of a modified nature, also occurs in 

 the stem of Isoetes. The germination of the raacrospore and the 

 embryology of the sporophyte of this latter genus are amply 

 described and figured. The higher IJotnjckiecB and the jSLarattiaceoB 

 are both taken to show lines of approach to the leptosporangiate 

 Ferns, of which Osmnnda is the lowest step. 



The Leptosporangiatie are, like the Eusporangiata, arranged in 

 two groups — the homosporous FUices and the heterosporous Hydro- 

 pterides [MaruUaceir. and Salviniacea). As to the position of the 

 various orders, and the points of origin at which the SalviniacecB 

 and MarsiiiacecB arose, Mr. Campbell has a good deal to say ; and 

 his views will be found summed up in the diagram on p. 421. The 

 morphology and life-history of AzoUa are discussed at some length. 



Chap. XIII. is devoted to a consideration of the Eijuisetinea. 

 The nuclear changes that accompany the development of the 

 spermatozoids and of the spores are well figured and described: 

 the courtesy of the publishers enables us to give specimens of these 

 illustrations (see next page). Cor mack's recent observation of a 

 slight secondary thickening in the nodes of Equisetnm maximum is 

 interesting in connection with the thickening that occurred in the 

 fossil CalamitecB. Another fossil of this group — Calamostachys — is 

 of importance as being heterosporous, and confirming the opinion 

 that heterospory arose independently in several widely separated 

 groups of vascular cryptogams — an opinion of which Mr. Campbell 

 makes full use in his scheme of classification. 



In Chap. XIV. we reach the final class, the Lycopodinea, with 

 two homosporous orders, Lycopodiacea. and Psilotaceoi, and the 

 heterosporous order Sclaginellece. The author has closely investi- 

 gated the female prothallium of Selaginella Kraussiana, and shows 

 that the diaphragm in the macrospore is not a true cell- wall, but a 

 secondary thickening of the lower side of the prothallium which 

 occurs during a temporary cessation of the growth of the latter. 

 The Lycopodmeie on the one hand retain several primitive characters, 

 and on the other, through iSelayinella, show an affinity with the 

 Gymnosperms. 



