338 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



with seeds of the Trigonocarpus or related types. Further instances of seeds 

 attached to fronds of Sphenopteris were observed, but the most significant con- 

 tribution in recent years was made by Ilalle (1929), who studied no less than 

 five new cases of seed-bearing fronds, viz., one species of each of the genera 

 Sphenopteris, Pecopteris, Alethopteris, EmpJectopteris, and Nystroemia. Halle 

 got the impression that a terminal position of the seed was the rule among the 

 older pteridosperms, and that marginal and laminar attachments did not ap- 

 pear until later. It seemed likely to him, therefore, that the seed habit origi- 

 nated in plants which still had terminally placed sporangia. Much work was 

 also devoted to the elucidation of the internal structure of various paleozoic 

 seeds, found only detached, but believed to belong to pteridosperms (Scott, 

 1923). They differ in size and symmetry, in the structure of the testa, and in 

 the organization of the nucellus and the integument in the micropylar region. 



The male organs of Lyginopteris oldhmnia are still not known with certainty, 

 but Benson (1904), Crookall (1930), and others, have suggested that TeJangium 

 Scotti might be its microsporangia. It proved difficult in many cases to differen- 

 tiate pteridosperms from marattiaceous ferns (Kidston, 1923-1925). Some 

 plant remains were believed to represent male organs of the Medullosaceae, but 

 the situation was far from satisfactory, and it was not until after the end of 

 this period that better knowledge was gained. 



Research on the vegetative anatomy of the pteridosperms made further prog- 

 ress (Scott, 1923). This applies, inter alia, to the Calamopityaceae, which were 

 grouped with the pteridosperms entirely on the basis of their stem structure 

 and regarded as being, at least in part, most closely related to the Lyginopteri- 

 daceae. As regards the Medullosaceae, Sutdiffia indicated the probable deriva- 

 tion of the complex, polystelic, medullosean stem from a simple, solid, proto- 

 stelic type, such as existed in Heterangiurn. 



The old conception of the filicinean origin of the seed plants — which can be 

 traced back to Hofmeister's time — had had to be abandoned. The pteridosperms 

 and the contemporary ferns came to be regarded as distinct and in some re- 

 spects parallel series. 



Caytoniales 



In 1925 Thomas discovered in rocks of mesozoic age remains of a new group 

 of seed plants, which he named the Caytoniales. The remains consisted of fruit- 

 bearing stalks interpreted as pinnate megasporophylls (Caytonia), also branched 

 microsporophylls bearing quadrilocular sporangia in terminal clusters on each 

 subdivision, and palmately compound leaves {Sagenopteris). On each pinna, 

 the megasporophyll bears an almost closed, saclike body containing several 

 seeds; the testa is of three-layered complex structure. The pollen grains pos- 

 sess two wings placed opposite each other. Thomas believed that the Cay- 

 toniales occupied a position between the paleozoic pteridosperms and the recent 

 angiosperms, but their supposed affinities with the latter group were doubted 

 by the majority of paleobotanists. 



Cycadales 



The study of the cyeads was greatly in-omoted by Coulter (Coulter and 

 Chamberlain, 1917), and especially by Chamberlain (1935), not only in respect 



