358 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



branches of which divide into ultimate jiranclilels, each hearing a (jiiadrisporan- 

 giate synanginm. 



Thomas (1933) discovered a new group of mesozoic pteridospermlike plants, 

 the Corystospermaceae, based on seed-bearing branches, male organs, and iso- 

 lated seeds. The female organs {Umkomasia, etc.) are made up of branches 

 partly borne in the axils of bracts and carrying terminal cupulate gymno- 

 spermous seeds characterized by curved bifid micropyles. The male organs 

 (Pteruchus) show groups of terminal sessile synangia produced on cuplike or 

 spatulate structures. The winged microspores resemble those of the Caytonia- 

 ceae. Dicroidium, Pachypteris, and similar types of leaves appear to belong to 

 this family. 



A third family of supposed pteridosperms of mesozoic age, the Peltasperm- 

 aceae, was instituted by Thomas (1933) and based on Harris' (1932-1937) and 

 his own studies of Lepidopteris, a genus of bipinnate fronds, and its reproduc- 

 tive organs. The female organ {PeUaspennum) first described by Harris, is a 

 peltate, cupulate "disc" with a circular series of seeds attached to its undersur- 

 face. The seed has a single integument and a curved micropylar beak. The 

 microsporophyll {Antevsia) is a dichotomously branched organ bearing ter- 

 minal groups of sporangia with wingless pollen grains. 



Generally (Thomas, 1938; Harris; Hirmer, 1937; Andrews, 1948; and others), 

 these families are placed tentatively in the pteridosperms and regarded as late 

 offshoots of the paleozoic stock. They may, however, represent independent 

 groups more or less related to one another. 



Cycadales 



Schuster (1932), Sehnarf (1933, 1937), Chamberlain (1935), and Gaussen 

 (1944-1952) have reviewed the cycad morphology and anatomy. Contrary to 

 Chauveaud's opinion, Messeri (1932) and Boureau (1950) found that the centri- 

 petal wood is not a late-formed addition to the foliar bundle. According to 

 Chrysler (1937), the pitted tracheids, together with parenchyma and rays, make 

 up the bulk of the stem in all living genera except Zamia and Stangeria, which 

 alone have secondary xylem consisting of scalariform tracheids. The stem wood 

 of tuberous Zamia species does not exceed the scalariform stage, and represents 

 therefore a persistent juvenile condition. Lam (1948, 1952) designated the 

 cycads as manifestl.y phyllosporous in both sexes, i.e., the ovules and microspo- 

 rangia are borne on many-telomed fronds or true sporophylls. 



Schuster (1931) also published a list of cycads of bygone ages. Florin (1933) 

 made this the starting-point for an investigation of the occurrence of cycads 

 in mesozoic deposits. It turned out that a certain detached megasporophyll of 

 early mesozoic age, PaJaeocycas, belonged to a cycad resembling the genus Cycas 

 itself. Studies of cycadlike leaves of the same age led to the recognition of a 

 taeniopteroid type (Bjuvia) as the leaf of Palaeocycas. The plant was referred 

 to the subfamily Cycadoideae of the Cycadaceae. Kiihle von Lilienstern (1928) 

 and Krausel (1949a) found pinnate, bi- or pluriovulate megasporophylls of tri- 

 assic age, Dioonitocarpidium, likewise referable to the Cycadoideae. Further, 

 Harris (1932-1937, 1941a) proved that the genus Beania, comprising female 

 cones of a type of Jurassic gymnosperms bearing Nilssonia leaves and male 

 cones of cycadean structure, belongs to this family and is most closely related 



