FLORIN: SYSTEMATICS OF THE GYMNOSPERMS 333 



evolution" is moreover hardly approachable by means of the experimental 

 methods of genetics. 



Bower (1908) further developed the antithetic theory of the alternation of 

 generations, according to which the neutral generation would be a new product 

 with a phylogenetic history of its own. The ultimate origin of all the vegetative 

 tissues of the sporophyte in the Cormophyta was the sterilization of potentially 

 fertile cells. Attention was subsequently especially directed to conditions in 

 the lower cryptogams, which led to the difference being stressed between the 

 alternation of nuclear phases on the one hand, and of morphological generations 

 on the other. Kidston and Lang (1921) stated that the morphology of the De- 

 vonian psilophytes did not entirely support either the homologous or the anti- 

 thetic theory. The simple sporophyte of Rhynia was part of an antithetic life 

 history, while its organization could be treated as homologous to the plant body 

 as realized in the sexual as well as in the spore-bearing stages of many algae. 

 The fundamental organization and evolutionary development of the sporo- 

 phyte was more eagerly discussed than ever before, and a good many theories 

 were propounded in explanation of the relationships of stem and leaf (Rudolph, 

 1921; Chauvead, 1921). Most of these may be passed over here. In continuing 

 his investigations on stelar structures, Jeffrey (1902, 1910; cf. AVorsdell, 1902a; 

 Schoute, 1903) divided vascular plants into two great stocks: the Lycopsida 

 — cladosiphonic and palingenetically microphyllous — and the Pteropsida — phyllo- 

 siphonic and palingenetically megaphyllous — the latter including ferns, gymno- 

 sperms, and angiosperms. The concentric type of siphonostele was regarded as 

 more ancient than the collateral and the primary bundle system of the vascular 

 plants to present a reduction series, of which the earlier and more complex 

 stages were found in ferns and lower gymnosperms and the more recent and 

 simplified stages in the higher gymnosperms and the dicotyledons. A stelar ter- 

 minology was elaborated (Brebner, 1902; Zimmermann, 1930), but no scheme 

 became generally accepted; Meyer (1917) definitely opposed the stelar theory. 

 Jeffrey's classification of the vascular plants was criticized by Tansley 

 (1908), who pointed out that many of the modern microphyllous forms appeared 

 to be reduced derivatives of megaphyllous ancestors and that the Lycopsida 

 could not all be considered palingenetically microphyllous. It was realized that 

 the formation of leaf gaps in the stele was not an absolute criterion of the mor- 

 phological nature of the leaf. The whole question appeared to Tansley to re- 

 solve itself into the actual size relation of leaf -trace to stele, as modified by the 

 ancestrally determined construction of the latter. Bower (1908) expressed simi- 

 lar views. At the beginning of the century all seed plants were considered to 

 have originated from vascular cryptogams, and the Cycadofilices to be actually 

 derived from ferns. The centripetal xylem was characteristic of the primary 

 bundles of both pteridophytes and seed plants -although it was gradually reduced 

 and finally disappeared in the stem of the latter (Scott, 1902). The views of the 

 relationships of the Cycadofilices subsequently changed, and it was admitted 

 that they differed from the ferns not only in their reproductive organs, but also 

 in essential features of their anatomical structure (Scott, 1923; Posthumus, 

 1924). The assumption of a close affinity between these two groups as expressed 

 in the phylum Pteropsida had to be given up ; Jeffrey's idea of a common mega- 

 phyllous origin of the gymnosperms was rejected. 



