24 T O R R E Y A 



statement. Barthelniess (1935), in his study of various Conifers, considers 

 the primary vascular tissue composed of leaf traces onl}\ variously united 

 into sympodia. However, his interpretation of procambium developing basi- 

 petally is not in agreement with Cross' findings (1942) in Ciinninghaniia 

 lanceolata. Crafts (1940) in Sequoia, and our as-yet unpublished findings in 

 Piniis Strobus and Ginkgo biloba. In general, Barthelniess points out the 

 similarity of the coniferous apical region to that described for the Angio- 

 spernis by Helm (1931). One observation of Barthelniess' should be referred 

 to. that of a shoot of Pseudotsitgo which in the course of its development failed 

 to produce the normal needle-like leaves and instead gave only membranous, 

 scale-like structures with no leaf traces. Yet this shoot when examined gives 

 a normal structural picture for a shoot of Pseitdofsuga except that the vas- 

 cular cylinder is unbroken by the usual interfascicular parenchyma or primary 

 rays. 



A summary survey of the literature and current research pertaining to 

 the organization of primary shoots of diverse groups of the vascular plants 

 certainly leaves the writer with no final dictum on the nature of the shoot. 

 There is cumulating evidence, however, that in Lycopodiiini and Sclaginclla 

 the vascular cylinder is mostly of cauline, to a lesser degree of foliar, origin. 

 Even here, though blocked out somewhat earlier in development, the differen- 

 tiation of metaxylem and metaphloem ordinarily does not seem to occur until 

 the leaf traces are themselves in a process of differentiation. 



The evidence for Eqitisefiiiii. Ferns, and (jymnosperms is still too incom- 

 plete to permit of generalizations. The rhizomes of Lycopodimn obsciiniiii. 

 the leafless stolons of Ncplirolepis, the unusual shoot of Psciidofsitga^ and 

 other leafless cauline axes considered by Troll (1937, p. 287-304), give 

 indication that stems in these groups may develop epidermis, cortex and vas- 

 cular tissues even though no leaves be present. Certainly the early appearance 

 of pith in the majority of species of Horsetails, Ferns, and Gymnosperms 

 would suggest for it a cauline origin, for this pith is often found higher in the 

 axis than the most apical, foliar, procambial connection. As Schoute ( Verdoorn. 

 1938 ) points out in his summary for the Ferns, there can be no question as to 

 the "influence" of megaphyllous leaves on the diflrerentiation of vascular tissues. 



In the Angiosperms, developmental studies generally give indication of 

 the importance of foliar structures on the entire developmental sequence of 

 events in the axis. There is little evidence to suggest a separate role for the 

 axis in the development of vascular tissues, except possibly in certain aquatics 

 such as Hippiiris, possibly in the interesting case of the cactus Trichoccrens, 

 and a few other instances. However, the fact that roots develop vascular and 

 cortical tissues without foliar appendages must not be forgotten. 



Is it possible to consider the shoot system as an entity within which a 



