344 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



the stele of typical scalariform and transitional types of bordered pitting. He 

 found that there is nothing to indicate whether opposite pitting is derived from 

 alternate or vice versa, or whether both types were independently originated. 



Gothan (1905) proved the value of the ray-cell structure to the classification 

 of coniferous woods. His abietinean pitting is characterized by strongly pitted 

 horizontal and vertical ray-cell walls and circular pits. He discerned several 

 types of tracheary pits visible through the crossing fields of the ray cells. The 

 normal occurrence of ray tracheids in the Pinaceae is according to him an ad- 

 vanced feature. Penhallow (1907) believed marginal ray tracheids to be de- 

 rived from parenchyma cells, while Thompson (1910) interpreted them as modi- 

 fied tracheids. In Penhallow 's opinion the rare occurrences of ray tracheids in 

 certain conifers constitute the first evidence of a tendancy in development which 

 was only fully realized at a later period, but Jeffrey (1917, 1925) and others, 

 read the series in the opposite direction, interpreting this feature as vestigial 

 or reversionary, and indicating pinaceous ancestry. Great importance was also 

 attached to the occurrence and distribution of resin cells and canals in the 

 conifers. Penhallow held that scattered resin cells indicate a primitive condi- 

 tion, and that their aggregation into groups containing resin canals, as in the 

 pines, is an advanced feature. At the end of the period under review, the general 

 opinion was still that the Pinaceae, with a well-developed system of resin canals, 

 were highest on the scale of conifers (Thomson and Sifton, 1925), but Jeffrey 

 considered their presence a primitive condition. The Taxodiaceae and Cupres- 

 saceae, as well as the Araucariaceae, are, according to him, mesozoic offshoots of 

 the Pinaceae, of which Pinus would be the most ancient and primitive genus, 

 directly derived from the cordaites. 



Jeffrey (1908) considered, moreover, leaf anatomy to be of phylogenetic sig- 

 nificance, and tried in that way to gain further support for his views. One argu- 

 ment was the occurrence in the Pinaceae of vestiges of double leaf-traces, but 

 in 1904 Chauveaud had shown that the doubling of the foliar bundle in Abies 

 and Pinus is a result of secondary modifications. Further arguments were: 

 (1) the presence of true centripetal wood in the genus Prepinus of cretaceous 

 age, believed to be a progenitor of Pinus; (2) the resemblance of the foliar bundle 

 of Prepinus to that of certain cordaites in the presence of centripetal wood and 

 a double sheath of transfusion tissue; and (3) the persistence of the double 

 transfusionary sheath in the true pines of the cretaceous. Prepinus had deciduous 

 short shoots of a generalized type bearing numerous spirally arranged leaves, in 

 contradistinction to a few whorled fascicular leaves of the highly specialized 

 living pines. The short shoot was a primitive attribute of the coniferous stock. 

 Thomson (1914), on the other hand, argued that in the pines the short shoot 

 is a specialized branch of finite growth with a determinate number of leaves, 

 whereas its progenitor apparently was an ordinary branch, and that therefore 

 the short shoot could not be considered an indicator of primitiveness. 



Burlingame (1915b) discerned three principal theories of the origin of the 

 conifers and their relationships. According to the "lycopod" theory, which still 

 had a few adherents in the 1920 's, the female conifer cone was a fiower, and 

 the cone scale a sporophyll differentiated into a spore-bearing and a foliar part. 

 The Pinaceae were consequently more specialized forms than most other coni- 

 fers. The "cordaitean" theory was adopted by the majority of students. The 



