Morphologie, Teratologie, Befruchtung, Cytologie. 231 



This thickening of the sides of the gap is due chiefly to the increase 

 of the centripetal xylem, though additional centrifugal wood is also 

 produced. The inner centripetal xylem forms the tirst bridge and 

 projects outwards into the gap still existing in the centrifugal xylem. 

 A little further out the centrifugal xylem closes over this pro- 

 jection and completes the xylem of the main stele, so that at this 

 level the vascular „bulge" may be regarded as a rudimentary 

 branchstele with centripetal and centrifugal wood. A little higher 

 up this vascular projection begins to subside and the axial endo- 

 dermis is restored. 



Adventitious branches occasionally arise, originating as G wy n ne- 

 Vaughan originally suggested, by the Stimulation of vestigial buds 

 to development. The xylem of these adventitious branches Starts by 

 the development of two groups of „accessory" xylem on each side 

 of the leaf-gap and outside the cauline xylem. The inner axial 

 xylem passes into this gap joining up the two masses of outer or 

 accessory xylem and forms a narrow band lining the concavity of 

 the arc of outer xylem. The latter spreads round the mass until 

 the centrifugal xylem is completed, parenchyma appears between 

 the elements composing the internal xylem. The inner and outer 

 xylem, the endodermis and probably the phloem of the branch are 

 continuous with those of the stem. In some cases the accessory 

 xylem, which passes out to form the outer xylem of the branch is 

 more developed in the main stem; it maj' arise lower, extend round 

 the Stele and persist for some distance above the departure of the 

 branch. This accessory xylem is clearly a secondary formation, 

 though no regulär secondary meristem is formed. 



The juvenile condition was examined both in young rhizomes 

 developing as lateral branches and in young sporophytes arising 

 from embryos. In both cases the xylem was more or less solid, 

 the central part of the stele not containing a definite pith, but 

 consisting of tracheides mixed with parenchyma. In view of the 

 fact that in j'^oung rhizomes that arise laterally as branches the 

 central tracheides are continuous with the inner xylem of the main 

 Stele the stele of the young plants of both kinds is regarded not as 

 centrarch, but as mesarch. The progression from the juvenile to 

 the adult type leads to increase in size accompanied by definite 

 and permanent medullation, the pith being apparentl}'- intrastelar. 



In some cases a stele of the adult type shows a reversion to 

 the juvenile form; this is associated with small size and less effi- 

 cient nutrition and is clearly produced by condensation. 



The inner xylem and pith are held to show a rythmical rela- 

 tion, the pith being relativel}^ large at the departure of the leaf- 

 trace and the inner xylem encroaching on it above. In small rhi- 

 zomes the pith may be interrupted at some of the nodes by the 

 inner xylem Alling up the centre of the pith. 



The author holds that some portion of the stele is purely cau- 

 line, thus differing from Campbell, who believed that the axial 

 Stele is built up of the extension of the leaf-traces and perhaps of 

 the root traces as regards its ventral portion. At the same time the 

 author recognizes a segmental construction of the rhizome of Hei- 

 minthostachys, holding that there are two dorso-lateral and one 

 ventral series of segments. This he does, however, without giving 

 up the morphological unity of the shoot. 



Dr. Paul Bertrand and Kids ton and Gwynne-Vaughan 

 agree in comparing the departing leaf-trace of the Osmundaceous 



