350 TEAXSACTIOIS'S AND PROCEEDINGS OF THE [Sess. lxiv. 



may be met with as an occasional variation, probably 

 induced by the action of environmental diherences, but 

 under ordinary conditions one or two layers of cork are, as 

 above mentioned, in general laid down before the root 

 reaches to within a distance of two or three cells from the 

 actively dividing phellogen. When, however, this position 

 has been approximately attained, the phellogenetic divisions 

 in the cells immediately opposite the root entirely lose 

 their centripetal character, and there is initiated a series 

 of centril'ugal divisions, which, commencing opposite the 

 apex of the root, extend laterally in the phellogen, and 

 eventually result in the formation of a lenticular mass 

 of secondary cortex, with its greatest thickness in the 

 centre opposite the root, but becoming gradually thinner 

 till it disappears in the circumference of a circle, whose 

 diameter is in general about three times that of the 

 root in front of which it originates. 



This secondary tissue is entirely composed of thin-walled 

 parenchyma cells, somewhat rectangular in outline, and with 

 but a few small intercellular spaces between their slightly 

 rounded angles. 



Considering the relation between the position occupied 

 by the apex of the root, and the period at which the 

 development of this tissue takes place, as well as its 

 exceptional character, it is difficult to avoid the conclusion 

 that its formation must be looked upon as the external 

 evidence of the response made by the actively dividing 

 cells of the phellogen to the pressure exerted on them by 

 the elongation of the root. 



The rapidity with which these centrifugal divisions 

 succeed one another in the line of the rootlet's advance 

 is somewhat remarkable, and in general exceeds that of 

 the normal centripetal divisions, taking place at the same 

 time in the unaffected portions of the phellogen. Indeed, 

 the time required for the completion of the wliole lenticular 

 mass of tissue, which in its centre often reaches a thickness 

 of twelve or thirteen cells, is frequently less tlian that occu- 

 pied by the deposition of a single layer of cork. 



This rapirl localised deposition of secondary cortex by 

 tJie phellogen is the primary cause of the formation of 

 protuberances on the surface of the stem, and these, at 



