0-48 TEANSACTIONS AND PROCEEDINGS OF THE [Sess. lxiv. 



fibres is situated nearly, if not quite, opposite the end of 

 the rhizogenetic medullary ray, and sometimes in direct 

 contact with it. 



As the papilla increases in length, these fibres become 

 curved outwards before it ; and when, owing to their ends 

 being securely fixed in the tissue above and below, the 

 resistance of the long, tough elements becomes too great to 

 allow of further displacement, they cut into the apex of 

 the root, forming therein a deep, narrow groove, at the 

 bottom of which they may be recognised, though in a 

 somewhat crushed condition, even when the root has 

 reached a stage of comparative maturity. 



As the groove so" formed extends to the extreme apex of 

 the root rudiment, and cuts through both root cap and 

 periblem, the conclusion seems unavoidable that no cells 

 external to these fibres can have any part in the origin of 

 the root papilla, thus excluding at least the endodeimis 

 from all participation in the formation of this structure. 



The remaining thin- walled cells of the peri cycle un- 

 doubtedly divide, but as regards the part which they play 

 in the formation of the root 1 am at present unable to 

 make any definite statement, though, from a consideration 

 of the arrangement of the cells in median longitudinal 

 sections through somewhat older root rudiments, it seems 

 improbable that they do more than give rise to the root 

 ca]>, wliile the cortex appears to arise from the divisions of 

 phloem parenchyma cells on the flanks of the medullary 

 ray, from which the central cylinder takes its origin. 



In the younger outgrowths no difl'erentiation into root 

 cap, cortex, and central cylinder is observable, but all 

 three may be distinctly recognised in median longitudinal 

 sections, at a stage so early that no trace of the existence 

 of a root rudiment can be perceived on the surface of the 

 stem. The characteristic dot on the radial walls of the 

 endodermis of the root does not, however, a])pear till much 

 later, not indeed till the superficial protuberance is almost 

 fully formed and the xylem of the central cylinder is 

 beginning to undergo lignification. It is, however, easily 

 recognisable at the end of the first year in median 

 longitudinal sections which have been double stained in 

 Magdala-red and Malachite-green, the latter of which 



