686 CONCLUSION 



in the case of Lycopodiales (p. 337), Equisetales (p. 391), Sphenophyllales 

 (p. 418), Ophioglossales (p. 464), and Filicales (p. 646), how the stelar 

 structure, however various, is uniformly referable in origin to the monostele : 

 for it is seen in the young plant either to show a solid xylem-core, or a 

 medullated state not far removed from that condition. The frequent 

 occurrence of a like structure even in the mature axis of the early fossils 

 has also been shown : and from such observations it becomes apparent 

 how fully justified the opinion is that for the various types of the 

 Pteridophytes the non-medullated monostele was the original vascular 

 structure in the axis. 



It will probably be objected that in many of the Pteridophytes the 

 embryogeny does not bear this out ; and that what is apparent, especially in 

 the larger-leaved types, is that the vascular tissue of the shoot is initiated 

 by a simple foliar strand, which descends fro^i the first leaf continuously 

 to the root, and in fact that the axial system is in its origin little more 

 than a sympodium of leaf-traces. But before this objection is allowed to 

 have weight the condition in the smaller-leaved forms must be taken into 

 account, and the question examined as a whole rather than from one aspect 

 only. A comparison of those Lycopods, which are held to be relatively 

 primitive, shows that the cauline stele is initiated in the first stages of the 

 embryonic development ; this is seen with particular clearness in Fig. 

 190 c, D, E of Selaginella spinulosa, where the tissue formative of the 

 stele can be recognised as extending up to the broad apex of the axis 

 before any foliar strand is initiated. The same is the case in Lycopodiitm 

 Phlegmaria (Fig. 185 c, D) and L. annotinum^- and it is indicated also 

 in the imperfectly known embryology of L. Selago (Fig. 183). In 

 these plants the vascular condition from the very first establishment 

 of the embryonic shoot is the same as in the continued embryogeny 

 (compare Fig. 172, p. 331): -the stele is essentially cauline, and the 

 foliar strands insert themselves upon its periphery. This appears to 

 be the normal condition of small-leaved forms ; according to our 

 hypothesis these are themselves primitive, and the result of a com- 

 parison of the embryogeny in the two types would be that in larger-leaved 

 forms the cotyledon bulks more largely at first ; that the axis in the first 

 instance is correlatively reduced in size, and the cauline vascular core is 

 reduced with it. But, nevertheless, the examination of the embryogeny 

 has shown with constancy that the axis is pre-existent to all the other 

 parts of the embryo, though it may often be correlatively reduced, or its 

 development deferred where the cotyledon or the root is precociously 

 developed. The same view will hold also for the constituent tissues of 

 the axis, including the cauline vascular core. The condition where this 

 xylem-core is present is accordingly held to be the primitive state of the 

 embryo, that where it is reduced and even absent is held as the secondary 

 and derivative. But even in the latter cases, the stelar tissue asserts 



1 Bruchmann, I.e., l'I. 4, Fig. 17. 



