FLORIN: SYSTEMATICS OF THE GYMNOSPERMS 335 



and especially Strasburger, had laid the foundation of our knowledge of the 

 gametophytes, fertilization, and embryo formation, and the spermatozoids of 

 Ginkgo and cycads had been discovered, this branch of research attracted great 

 interest. 



Numerous progressive series were worked out, showing more or less gradu- 

 ated changes in various organs, and Zimmermann (1930) emphasized the im- 

 portance of the study of "phylogeny" of single characters. Great expectations 

 were attached to the new serodiagnostic method (Mez, 1926), the object of which 

 was experimentally to elucidate the relationships of the various plant groups, 

 but they were hardly fulfilled (Wettstein, 1925; Gilg and Schiirhoff (1927). 



Interest was centered about 1870 and 1880 on meristem research, but this 

 was for a long time displaced by research in other directions. In 1926, however, 

 Schiiepp summarized our knowledge of apical growth. Jeffrey (1917) treated 

 the anatomy of the woody plants with special reference to its historical and ex- 

 perimental aspects. Great stress was laid on three "canons" of comparative 

 anatomy which, however, were all more or less open to criticism : 



1. The doctrine of recapitulation of the history of the race in the development of the 

 individual. 



2. The doctrine of conservative organs (leaf, reproductive axis, root, the first annual 

 ring of the stem, and sporangium). 



3. The doctrine of reversion, an expression used for certain effects of wounding be- 

 lieved to be reminiscent of ancestral characters. 



As regards tissue systems, Sachs's old divisions were reaccepted by Jeffrey 

 as a consequence of the theory of the common origin of pith and cortex. In dis- 

 cussing the evolutionary tendencies of gymnosperms. Coulter and Chamberlain 

 (1917), following Jeffrey, emphasized the evolution from the protostelic condi- 

 tion to the siphonostelic. The universal tendency was to eliminate the centri- 

 petal xylem until the collateral mesarch bundles became collateral endarch, first 

 in the central cylinder, and finally also in the peripheral regions. The trans- 

 fusion tissue was especially studied by Bernard (1904); his conclusions came 

 very close to those of Worsdell (1897), but he went still further, and considered 

 this tissue to be actually centripetal xylem. Other investigators maintained 

 that it arises independently of the centripetal wood. Porsch (1905) and Reh- 

 fous (1917) investigated the stomatal apparatus of the gymnosperms from the 

 evolutionary point of view. Hill and De Fraine, (1913; cf. Dorety in Coulter 

 and Chamberlain, 1917) found that details of seedling anatomy are apparently 

 not of any great help in instances of questionable relationships between two 

 plants or plant groups. Finally, Fames and MacDaniels (1925) treated the cur- 

 rent status and opinion of general plant anatomy. 



Intense cytological activity gradually led to the revelation of, inter alia, the 

 constancy of the chromosome number in the organism and its importance as a 

 systematic criterion. The following seem to have been the only certainly known 

 haploid numbers at the end of the period : Ginkgo 12, Pinus 12, Larix 12, Tsuga 

 12, Picea 12, Podocarpus 12, Cephalotaxus 12, Juniperus 11, Taxus 12, and 

 Ephedra 7. The uniformity of conifer karyology appeared remarkable. Of other 

 events we need only mention that the first decade of this period was charac- 

 terized bv much work to elucidate the meiotic division. 



