THE OLDER SPOROPHYTE 



197 



first deeply depressed aperture or gap occurs at about the sixth or seventh leaf. 

 The leaf traces still continue to issue from the stele as single strands till a varying 

 number have been formed, but they begin to bifurcate while still within the cortex 

 of the stem. 



"As the stem increases the leaf traces become more numerous and crowded and 

 they take away a larger portion of the vascular tissue from the axile strand. The 

 result is that the leaf gaps become less rapidly repaired. The stele is already hollow 

 in this region, that is, it consists of a cylindrical vascular mass with perforations 

 corresponding to foliar gaps and inclosing a core of parenchyma. Sooner or later, 

 the gap above one leaf fails to be repaired until after the exit of the trace of the next 

 leaf and then the original vascular cylinder becomes broken up and assumes a 

 condition, in transverse section, conforming with that of polystely or dialystely." 



There is thus a gradual transition from the solid stele found in the earliest stages 

 to a hollow cylinder or siphonostele with a core of pith, and by the formation of larger 

 and larger leaf gaps there is a transition to. an open dictyostele like that found in the 

 other genera. Finally there appears the commissural strand which forms a thick 



A-D. Four transverse sections of a verv \oune, sporopln h- of Irigiopter, 

 the Fusion of tin- first three leaf traces. 



E. Centra) region of C. X200. 



F. Central region of D. X200. 



A. traverses stem apex. B, C, show 



sti'nd traversing the pith and undoubtedly of cauline origin. This central strand 

 becomes more and rnore important as the plant develops and from its central 

 position might be mistaken for the original axial strand of the young sporophyte. 

 The vascular system at this stage is described as follows: 



"The leaf traces become more complex and anastomoses take place at irregular 

 intervals with the strands which can still be recognized as the relics of the original 

 siphonostele, as well as with one another. Irregularities also commence to become 

 apparent as to the relative heights at which the two members ot the leal traces be- 

 come freed from the plexus of tissue, and a stage is thus reached at which the vascular 

 skeleton appears to consist of a stout axile strand, surrounded by upwardly diverging 

 zones of steles, which ultimately pass out above to the leaves." 



It is evident that at this stage there is a condition which is very much like that 

 figured by Kuhn for Kaulfussia, and it is pretty clear that the whole of the dictyostele, 

 exclusive of the axial or commissural strand, is made up of the "upwardly diverging 

 zones of steles which pass out above into the leaves," i. e., in other words, the 

 dictyostele is composed of a union of multiple leaf traces. 



