350 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



considered the primary vascular system to be made up only of leaf traces form- 

 ing sympodia. Barthelmess studied the relations between phyllotaxis and stelar 

 structure, and supported the idea of the most equitable spatial arrangement being 

 at the growing apex. He believed that the leaf traces differentiate basipetally 

 towards preformed parenchymatous gaps in the primary meristem ring. Ster- 

 ling (1945) recapitulated the main views expressed in order to explain the regu- 

 larity of leaf arrangement. One of these is based on the concept of the genetic 

 spiral, and another considers the genetic spiral a secondary phenomenon re- 

 sulting from the influence of the contact parastichies. He suggested a third 

 interpretation, according to which the arrangement of foliar members at the 

 shoot apex is also to some extent determined by the vascular structure of the 

 shoot. The procambial strands were found to differentiate acropetally in the 

 shoot apex of Sequoia from older strands below. There are no cauline bundles; 

 the bundles are common to both stem and leaf. According to Gunckel and Wet- 

 more (1946) the two procambial strands of a Ginkgo leaf develop continuously 

 and acropetally from definite procambial strands already present in the lower 

 axis. Finally, Plant'efol (1948) presented a theory of the multiple foliar helices, 

 which he considered applicable to the phyllotactic patterns of all cormophytes. 



In respect to the nature of the various types of leaves in gymnosperms Florin 

 (1938-1945) arrived at the opinion that megaphylls and microphylls are not 

 fundamentally dift'erent. Both categories of leaves appear to originate from 

 radial dichotomized telome systems, which, however, differed in size and com- 

 plexity, and in their subsequent development. In the majority of conifers the 

 uninerved leaves appear to have been formed by direct reduction of little com- 

 plicated, cruciately dichotomized telome systems without either planation, fu- 

 sion or telones, or aggregation. According to Nemejc (1950) the cycadophytes 

 have megaphylous leaves, while the leaves of the coniferophytes are of the sphe- 

 nopsid type, i.e., they are transformed short lateral branches. 



The interest in the fundamental structure and evolution of shoot apices was 

 revived, and directed not least to the gymnosperms. In his review in 1939 of our 

 then rather meager knowledge of this subject, Foster recalled that neither the 

 apical cell theory nor the histogen theory had proved a satisfactory interpreta- 

 tion of the shoot apex in gymnosperms. The latter theory had been superseded 

 by the tunica-corpus concept, first stated by Schmidt (1924). In 1941 Foster re- 

 ported the main results of studies of this kind in the preceding five years. The 

 cells of the "primordial meristem" were segregated into more or less well-defined 

 tissue zones reflecting the type, direction, and distribution of growth. Later, 

 Johnson (1951; cf. Popham, 1951) summed up the subject and suggested that 

 the organization of the stem apex may be of value in tracing relationships. He 

 pointed out that the apices of all investigated gymnosperms agree in having a 

 superficial initiation zone, a group of subapical mother cells, and a flanking zone. 

 Comparative studies had revealed the existence of four types of terminal meri- 

 stem, viz., the cj^cadophyte type, the ginkgophyte type, the coniferophyte type, 

 and the tunica-corpus type, of which the latter appears to be the most advanced, 

 and has been attained in Araucaria (Griffith, 1952) and in chlamydosperms. In 

 this connection the initiation and differentiation of gymnosperm leaves attracted 

 attention from the point of view of the evolutionary history of the foliar types 

 in higher vascular plants, l)ut only a few complete accounts have yet been pre- 



