Jan. 1900.] BOTANICAL SOCIETY OF EDINBURGH 351 



least during the earlier portion of the first year, owe 

 almost their entire elevation to it, as the comparatively 

 slow elongation of the root confines it for a considerable 

 period within tlie primary cortex, where it can exert 

 but little direct influence on the height of the protuberance 

 above it. Later in the season, however, it begins to press 

 heavily on the internal surface of the secondary cortical 

 tissue, and soon penetrates into it, pushing before it the 

 upper and outer layers, and thus greatly increases the 

 elevation of the papilla. 



The centrifugal divisions of the phellogen continue even 

 after the apex of the root has pierced the inner layers of 

 the lenticular mass of tissue, but at this period the rate of 

 elongation of the root generally exceeds to a considerable 

 extent that of the formation of secondary cortex, with the 

 result that the former gradually penetrates deeper and 

 deeper into the latter, and, in the majority of cases, 

 entirely pierces it before the end of the second year, 

 though instances are not wanting in which some layers 

 of secondary cortex are still recognisable between the apex 

 of the root and the cork cells covering the outer end of the 

 papilla, as late as the end of the fourth year, so that the root, 

 it would appear, may stop short of reaching the phellogen. 

 This, however, is not of common occurrence; and in most 

 cases when the papilla is fully mature, the root apex may 

 be seen to be covered only by a few cells of cork, or even, 

 owing to the abrasion of the cork layers, to be entirely 

 without protection. 



The Lenticels. — Towards the end of the first year one or 

 two typical lenticels appear, as above mentioned, on the 

 lateral flanks of the superficial protuberances. 



These arise generally to right and left of the root apex, 

 and apparently owe their origin to a return to the centripetal 

 mode of division taking place in certain circumscribed 

 patches of the phellogen, which, however, instead of laying 

 down ordinary cork tissue proceed to deposit the loose, 

 rounded complementary cells so characteristic of lenticels. 

 As the remaining portions of the phellogen covering the 

 protuberance continue to divide in a centrifugal direction, 

 the lenticellar areas become depressed below the rest of 

 the dividing layer, while their rapid formation of comple- 



