Apr. 1904.] BOTANICAL SOCIETY OF EDINBURGH 451 



In most phellogens two distinct kinds of tissue formation 

 may be recognised, viz. that characteristic of normal cork 

 formation and that which gives rise to Lenticels. 



The initial divisions which set up these two types of tissue 

 formation take place under very different external conditions. 

 . Those phellogen initials which will ultimately give rise 

 to typical cork cells undergo their first divisions beneath a 

 continuous epidermis and cuticle which exerts a high radial 

 stress on the dividing elements, while, on the other hand, a 

 Lenticel initial may be regarded, from a purely anatomical 

 point of view, as a phellogenetic element in which division 

 is relatively unhampered by external mechanical forces. 

 That this merely mechanical aspect of the case is neces- 

 sarily complicated by internal physiological processes, such 

 as conditions of intercellular nutrition, transpiration, etc., 

 as well as by external, environmental, and probably also 

 phyllogenetic relationships, will be denied by no one. 

 But, as even the approximate solution of such a problem 

 as the causation of lenticellar formation is for the present 

 quite unattainable, I desire to direct attention solely to 

 those mechanical relationships of position and pressure w 

 which undoubtedly play an important part in the origin of[| 

 these structures. 



As Stahl pointed out, the first corkcambial divisions 

 take place in cortical cells occupying positions in which 

 Lenticels ultimately arise. The first cortical cells to 

 re-enter the meristematic condition are therefore those 

 which give rise to Lenticels, indicatmg that the environ- 

 ment of tliese elements is better suited for the promotion 

 of cell division than is that of normal phellogen initials. 



A consideration of the form exhibited by individual cells 

 resulting from meristematic division within a lenticellar 

 area leads to the conclusion that one of the factors in this 

 environment, whicli makes for cell division, is a reduction 

 of external pressure. This view is also supported by the 

 irregularity of the meristematic divisions from which the 

 rounded, loosely aggregated, complementary cells are formed. 

 While in other portions of the phellogen there is but one 

 initial layer ; the meristem underlying. a Lenticel is usually 

 composed of several radially disposed cells, all undergoing 

 tanuential division. 



