332 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Stur (1884) excluded these from the ferns and referred them to the cycads. 

 Williamson and Scott (1896) studied the stem anatomy of Heterangium and 

 Lyginodendron and concluded that they united the characters of ferns and 

 cycads. On the basis of these facts, H. Potonie (1897-1899) applied the name 

 Cycadofilices to this transitional group of vascular plants. Finally, Scott (1899) 

 examined the structure of a Medullosa, which represented the genus in its sim- 

 plest expression. Its stem was polystelic, each of the three pithless steles re- 

 sembling the single central cylinder of a Heterangium, and covered by the bases 

 of the petioles of Alethopteris fronds. The Poroxyleae, another group of extinct 

 gymnosperms apparently related to both the Cycadofilices and the cordaites, 

 were further investigated by Bertrand and Renault (1884-1887, 1889) and Re- 

 nault (1896), and found to be of significance in the discussion of the derivation 

 of the higher gymnosperms. 



The Mesozoic era had become known as the "Age of the Cycads." On closer 

 examination, however, the reproductive organs of the fossils mostly proved quite 

 different from those of present-day cycads. Our knowledge of the bennettites 

 was considerably enriched by Solms-Laubach (1890b), Scott (1900), Lignier 

 (1894), and Wieland (1899, 1901). The bennettites resembled the true cycads 

 in many ways, but their leaf-trace bundles had a much simpler course, the 

 woolly hairs were replaced by ramenta like those of ferns, the fertile shoots were 

 axillary, and the reproductive organs quite different. The apex of the fertile 

 shoot was modified into an ovuliferous receptacle enveloped by pinnate bracts 

 and carrying seed pedicels, each with a terminal orthotropous ovule, inter- 

 mingled with interseminal scales. A dicotyledonous embryo almost filled the 

 cavity of the seed. Wieland recognized the microsporophylls, which were pin- 

 nately compound, synangia-bearing organs, united at their bases to form a 

 sheath. The "flowers" were bisexual, and had a verticil of microsporophylls in- 

 serted below the base of the ovuliferous receptacle. The discovery of this group 

 is one of the most important advances in the history of paleobotany. 



The phylogeny of gymnosperms and the question of the monophyletic or poly- 

 phyletic origin of the group were discussed, and a variety of views put forth. 

 The scheme proposed by Engler (Engler and Prantl, 1897) marks a distinct 

 advance in the direction of a phylogenetic system of the gymnosperms. It in- 

 cluded no less than six classes, viz., the Cycadales, Bennettitales, Cordaitales, 

 Ginkgoales, Coniferae, and Gnetales, but the Cycadofilices were not yet taken 

 into consideration. The heterogenous nature of Engler's Taxaceae and Gneta- 

 ceae was not realized until much later, and the interrelationships of the Pinaceae 

 remained little understood. 



The Period 1900-1930 



This period was characterized by greatly increased activity in most fields, 

 and by several discoveries of fundamental importance. Speaking generally, the 

 most important of the latter was the sensational rediscovery in 1900 of Mendel's 

 till-then-neglected laws of heredity, formulated in 1865. This initiated a magni- 

 ficent progress of genetics. The concurrent advances in cytology and the asso- 

 ciation of this branch with genetics eventually led to studies of the evolution 

 of species in various plant groups. The gymnosperms played a subordinate part, 

 however, as objects of investigation in this connection. The subject of "macro- 



