FLORIN: SYSTEMATICS Of THE GYMNOSPERMS 341 



male flowers differs only slightly from that of Renault (1879). Schoute (1925) 

 found, however, that the inflorescence is simple, and each fertile "bud" on the 

 main uniaxial. The flower axis carries scales and megasporophylls arranged in 

 a spiral. Mesoxylon (Scott, 1923; Maslen, 1930) is anatomically intermediate 

 between Poroxylon and Cordaites. Tlie wide pith is discoid; the wood is made 

 up of tracheids with multiseriate bordered pits, and has uniseriate rays. In 

 contrast to Cordaites, centripetal wood forms parts of the double leaf traces at 

 the margin of the pith. Other types of paleozoic stems allied to the cordaites 

 also became known. The genus Callixylon (Arnold, 1930), otherwise highly dif- 

 ferentiated, has mesarch primary wood. Cordaitean wood has a wide region 

 of transition between the spiral elements of the protoxylem and the first pitted 

 elements of the secondary wood (Penhallow, 1907; Bailey, 1925). The sequence 

 of structural changes exhibits recapitulation of successive evolutionary modifi- 

 cations of the derivation of multiseriate bordered pitting from scalariform pit- 

 ting. The roots and rootlets (Halket, 1930) were found to resemble those of 

 present-day gymnosperms. Leclercq (1928) and earlier authors showed that 

 cordaitalean leaves were rather variable in structure. Coulter and Chamberlain 

 (1917) and Scott (1923) believed that the Cordaitales and pteridosperms had 

 a common origin, while Chamberlain (1920), Sahni (1920), etc., regarded them 

 as belonging to two distinct evolutionary lines. 



GiNKGOALES 



Sprecher (1907) described in detail all the organs of Ginkgo. The female 

 flower was continuously debated (Haan, 1920). Sprecher, Coulter and Cham- 

 berlain (1917), Goebel (1923), Pilger (1926), and Sakisaka (1929) looked upon 

 the collar as a reduced megasporophyll bearing an ovule terminally, while Zim- 

 mermann (1930) characterized the flower in the terms of his telome theory as 

 a dichotomized truss with terminal megasporangia. Similarly the primitive male 

 organ of Ginkgo was, in Doyle's (1926) opinion, not a flattened leaf, but a 

 sporangiophoric structure carrying terminal microsporangia. According to Goe- 

 bel, Ginkgo differs from other gymnosperms by having an endothecium in the 

 microsporangium. Jeffrey and Torrey (1916) distinguished two types of micro- 

 sporangiate opening mechanism, an ectokinetic, characteristic of lower vascular 

 plants, and an endokinetic. The latter occurs in a flber layer derived from the 

 fibrovascular tissues, and is said to be present characteristically in living seed 

 plants (except the cycads). Jeffrey (1917) further pointed out that nearest 

 to the primary wood in the xylem of the peduncle the transition region shows 

 tracheids without rims of Sanio (crassulae), and the bordered pitting is largely 

 alternate, while opposite pitting and rims, characteristic of the mature sec- 

 ondary wood, are developed farther away. These conditions were believed to 

 indicate the derivation of the Ginkgoales from cordaitalean ancestry. In the 

 stem wood the transitional inner portion is narrower than in more primitive 

 gymnosperms (Bailey, 1925), and the circular type of bordered pitting tends to 

 work back into the protoxylem, so that typical scalariform and transitional types 

 are almost completely eliminated. Coulter and Chamberlain (1917) held that 

 the Ginkgoales were either derived from the Cordaitales, as Jeffrey believed, or 

 the two groups had become differentiated from some common stock of paleozoic 

 age, which was Scott's (1923) opinion. 



