WETMORE: LEAF-STEM RELATIONSHIPS 23 



Lycopodmm, I should like to mention that all the underground rhizomes of 

 L. ohscunnn so far examined have only membranous scale-like leaves with no 

 traces. It is significant that this rhizome develops vascular tissues and cortex, 

 however, not dissimilar to those normally present in a leaf-bearing rhizome. 



In substance, these living Lycopsida seem to have a shoot system composed 

 of a cauline part with foliar primordia borne initially in either erect or lateral 

 position. Here there seems to be a peripheral set of bundles, which form a 

 primary network and which are connected with the leaves, bundles originating 

 from procambial strands which are truly acropetal and continuous in their 

 origin. In addition, ordinarily differentiating only after leaf connections are 

 established, though blocked out earlier, is the whole central portion of the 

 central cylinder which is cauline in nature and which is never directly connected 

 with the leaves. 



What of the Horsetails, Ferns, and Gymnosperms? In Equisetiim, the 

 story is far from complete. A study of native species of this genus now in 

 progress in our laboratories gives evidence of a continuous acropetally develop- 

 ing procambium to the leaves and branches. It is not yet clear, however, from 

 this work, nor that of Barratt (1920), Oueva (1907) or Vidal (1912), just 

 how the nodal ring is developed. Certainly it would be difficult to think of 

 this ring as entirely foliar in nature. 



In the Ferns, the study is fragmentary. The work of Gillette (1937) 

 on Psaronius and of Schoute (1926) on living Marattiaceae suggest the 

 complicated stele to be of foliar origin. In the three native species of Os- 

 mundaceae, as yet unreported studies from our laboratories show no sign 

 of cauline bundles, though Kaplan reports such. In his summar}^ on the Ferns 

 in Verdoorn's Manual of Pteridology, Schoute (1938) states (p. 84) : "In 

 the Ferns the original Pteridophyte stele with its external sheaths, its phloem 

 and its central solid xylem has been reduced into a mere topographical tissue 

 column, acting as a recipient for leaf-traces, but without any tissue dif- 

 ferentiation of its own." 



Before leaving the Ferns, I must refer to certain scale-bearing stolons of 

 species of Ncphrolepis, studied by Lachmann (1885, 1889) and others. 

 Originally described as roots, the}' proved to be stems with a cortex and a 

 protostelic central cylinder resembling much more that of sporeling Ferns 

 or mature axes of Glcichnia, Lygodiinn or H ymenophyllum which remain 

 permanently protostelic. 



In the Gymnosperms, the classical account of Koch (1891), followed by 

 those of Barthelmess (1935), Cross (1939, 1940, 1941, 1942), Foster (1938, 

 1939b, 1940. 1941a, 1941b), Gift'ord (1943), Korody (1937), and Louis 

 (1935), indicates diverse patterns of apical meristem in the different gymno- 

 spermous assemblages. The Avork is too incomplete to give any summar}- 



