COMPARATIVE ANATOMY OF GYMNOSPERMS 297 



strand embedded in the parenchyma of the fundamental tissue. 

 Part of a transverse section of such a stem is seen in Fig. 108, A. 

 In the center is the concentric fibrovascular bundle or stele, 

 which consists of a mass of xylem completely surrounded by 

 phloem. The stele or central cylinder is bounded in turn by 

 brown sclerenchymatous fundamental tissue. This type of 

 stem, since it is a very primitive one, may conveniently be 

 called " protostelic " (Jeffrey 19 ). 



Another common condition of the stem is seen in Fig. 108, 

 J9, which represents a cross-section of the rhizome of Adiantum 

 pedatum. In this case the central cylinder is not a solid fibro- 

 vascular strand as in the preceding example, but a hollow cyl- 

 inder filled with fundamental tissue like that external to the 

 stele. The plane of section is just above the point of origin of 

 a leaf-trace, which may be distinguished as the smaller of the 

 two concentric masses of fibrovascular tissue. At a higher level 

 the gap in the cauline central cylinder closes, and the stele be- 

 comes circular instead of crescentic in cross-section. Similar 

 gaps appear above all the outgoing leaf-traces, and as a conse- 

 quence the central cylinder is essentially a concentric fibrovas- 

 cular tube, with gaps in its walls corresponding to the leaf- 

 traces. The type of central cylinder which has just been de- 

 scribed may appropriately be termed " siphonostelic " (Jef- 

 frey 19 ). 



Fig. 108, C, is from a photograph of the adult stem of 

 Pteris aquilina, the common bracken fern. In this case there 

 are numerous concentric fibrovascular bundles present in the 

 fundamental tissue of the rhizome, and accordingly stems of 

 this type have been designated by Van Tieghem " polystelic." 

 It has been shown, however, that in such stems as are exempli- 

 fied by P. aquilina the primitive condition of the central cyl- 

 inder is a stelar tube with foliar lacunae (Jeffrey 19 ). Fig. 

 108, D, from the young stem of P. aquilina :, sufficiently demon- 

 strates the truth of this statement. The young stem gradually 

 passes into the condition represented in Fig. 108, (7, first by 

 the foliar gaps becoming so long as to overlap, and second by 

 the derivation of the large central strands from the inner wall 

 of the primitive stelar tube. Consequently the stem of P. aqui- 

 lina may be regarded on ontogenetic grounds as siphonostelic 

 and essentially similar to that of Adiantum pedatum. 



