"CAULINE" AND "COMMON" VASCULAR STRANDS 195 



in the leaf it would at best be only a pseudostele, secondary in origin, and 

 thus phylogenetically distinct from the stele of the axis. The primary 

 structure of the axis is monostelic: where isolated strands occur in the 

 axis, each with its sheath is a schizostele, a result of secondary segregation 

 of the component tissues of the stele. 



In this connection it is important to recall the old distinction between 

 "common" and "cauline" vascular bundles. In the former the lower 

 part of the course of the individual strand is in the axis, the upper extends 

 into the leaf: in the case of the tissues which may be styled cauline, the 

 course is within the stem throughout. From a theoretical point of view 

 the existence of cauline vascular tracts is important, for it accentuates 

 the axis as something more than a mere basis for insertion of leaves. 

 The further fact that the axial stele may be followed beyond the youngest 

 leaf-traces shows that the vascular system of the axis has an objective 

 existence independently of the leaf-traces, however closely it may be 

 connected with them in ordinary cases. These cauline extensions are 

 prevalent in early Pteridophytes, such as Lycopods, Psilotaceae, and Ferns ; 

 this fact must necessarily be of special interest in connection with any 

 theory of the origin of foliar developments in Vascular Plants. 



It is evident that the existence of a cauline stele bears directly 

 towards a strobiloid theory of the shoot. This suggests the question 

 whether any existing group of plants show a nascent condition of the 

 vascular system of the shoot such as a strobiloid theory would demand, 

 viz. a columnar conducting stele, with no appendages, or with appendages 

 anatomically accessory to rather than formative of the central stelar column. 

 In a paper on the conducting tissue-system in Bryophyta, Tansley has shown 

 that such a structure is found in the more complex Mosses. 1 In discussing 

 the points brought forward he very properly disavows at the outset any strict 

 homology with Vascular Plants, remarking that it is almost as certain as 

 any phylogenetic thesis is likely to be that the conducting tissues of Bryo- 

 phytes have nothing directly to do with the origin of the conducting 

 tissues of the higher plants. The main seat of the development of these 

 tissues in Bryophytes is the gametophyte generation, which is in any case 

 excluded from the comparison, since the vascular system in Pteridophytes 

 is confined to the sporophyte. And at the least it is extremely unlikely 

 that the Pteridophytes have been derived from a Bryophytic ancestor with 

 a sporophyte showing anything approaching the specialisation of the moss- 

 sporogonium, in which conducting tissues also occur. But it must not for 

 this reason be supposed that the Bryophytes are of no interest in consider- 

 ing the problem of the evolution of the vascular system in Pteridophytes. 



1 Ann. iif Bo/any, \v., 1901, p. 2. Kor Mr. Tansley's later views on this and kindred 

 subjects, especially as affecting the question of origin of the shoot in the Filicales, 

 reference should be made to his Lectures (New Phytologist, 1907). This chapter was 

 in type before Mr. Tansley's lectures were given. The opinions here expressed may have- 

 to be modified. 



