356 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Of interest is also the decisive difference in respect to phenolic compounds estab- 

 lished between the subgenera Ilaploxylon and Diploxylon of the pines (Lind- 

 stedt, 1951). Regarding the "Konigsberg genealogical tree," built up by Mez 

 (1926) and his students on the basis of serological investigations, Molisch (1933) 

 and Chester (1937) defended serosystematics. The latter thus believed in its 

 having a sound basis, provided that sufficient care was taken to exclude non- 

 specific reactions, but admitted that the whole subject was still in an imperfect 

 stage and that further development of its techniques was needed. 



Pteridospermae 



Our knowledge of this group has increased considerably. Halket (1932) 

 pointed out that even such minor characters as the structure of the root apex 

 and the vertical orientation of the diarch xylem plate in lateral rootlets agree 

 with modern gymnosperms rather than with ferns. Three new genera of the 

 Lyginopteridaceae, viz., Tetrastichia (Gordon, 1938), Schopfiastrum (Andrews, 

 1945), and Microspermopteris (Baxter, 1949), were discovered. The last-named 

 type is striking because of its small stem and leafless condition, and combines 

 characters of both Lyginopteris and Heterangium. The evolutionary trends of 

 stelar structure in the MeduUosaceae were discussed by Schopf (1939), Baxter, 

 Stewart (1951), and Stewart and Delevoryas (1952). A main evolutionary line 

 and a divergent lateral branch were recognized. The former, which starts with 

 Sutcliffia and continues through the permian species, has abundant stelar branch- 

 ing, ontogenetic and phyletic fusion, and foliar steles with conspicuous secondary 

 tissue. The latter has transitional forms, in which little or no secondary tissue 

 is associated with the foliar steles, and advanced species in which stelar branch- 

 ing is strongly reduced except for foliar steles. Contributions to the stem anata- 

 omy of the Calamopityaceae were made especially by Read (1936-1937), who 

 described a new genus, Diichna, and proposed to divide the members of this 

 family in two major groups, viz., a manoxylic, protostelic group and a pycnoxy- 

 lic, medullated group. Its ancestral members were probably simple protostelic 

 forms with but little difference between stem and leaf. The stelar histology of 

 the pteridosperms in general was studied by Andrews (1940), who pointed out 

 that this group can no longer be regarded as intermediate between the ferns 

 and the cycads, and that its origin must be sought among the psilophytes, a view 

 previously expressed by Halle (1937), Bertrand and Corsin (1938), and others. 

 The tracheids of the secondary xylem do not appear to have ever had scalari- 

 form pitting. The characters of the primary wood are less stable than those of 

 the secondary xylem. 



Importance advances concern the polleniferous organs of the MeduUosaceae. 

 These appear to Halle (1933, 1937) to be in the main of two different types, 

 the Potonieineae and Whittleseyineae. Potoniea, belonging to paripinnate Neu- 

 ropteris fronds, is composed of stalked, cuplike, basisporangiate structures with 

 elongate sporangia filling the whole cup. This may have been formed, phylo- 

 genetically, by collateral fusion of sterile telomes. The Whittleseyineae — com- 

 prising Whittleseya, Aulacotheca, and other forms — appear to be a natural 

 group characterized by gigantic synangia borne on Alcthopteris and imparipin- 

 nate Neuropteris fronds, long tubular microsporangia, and large spores of bi- 



