452 TRANSACTIONS AND PltOGEElJlNGS OK THK [Sess. Lxviii, 



The circumstance that the divisions taking place in 

 normal lenticellar meristem are not only more numerous 

 than those characteristic of most phellogens, but are 

 also more n)arkedly centripetal in direction, the greater 

 number of cells being formed towards the outer side of the 

 phellogen, may perhaps also be taken as at least partial 

 evidence in favour of the view that in these areas the inward 

 epidermic pressure is minimal, though obviously many other 

 causes must be active in determining such relationships. 



The question of the suberisation or otherwise of the 

 complementary cells relates rather to the functions per- 

 foruied by them than to the form which they exhibit, and 

 which must at least in part be looked upon as a mani- 

 festation of the stresses to which their initial cells were 

 subjected during division. The only compression stress to 

 which the cells of an approximately superficial meristem 

 like a normal hypodermic phellogen can be exposed during 

 the period of lenticellar formation must be the pressure 

 exerted by the elastic epidermis and cuticle ; a pressure 

 which along with its main cause, viz. secondary growth in 

 thickness, is at this season nearing, if it has not already 

 reached, its maximal value. The tangential tension due 

 to this cause often amounts, as pointed out by Pfeffer, to 

 as much as ten atmospheres, and as the radial pressure 

 resulting therefrom varies directly as this value, and 

 inversely as the square of the radius, it must in a narrow 

 stem reach very considerable dimensions. 



The relations of position which points of lenticellar 

 origin bear to other plant organs have also to be taken 

 into consideration, as on these must to a large extent 

 depend the pressures to which the lenticellar initials are 

 subjected. 



In the great majority of stems with superficial periderms, 

 the primary lenticellar divisions take place in cells lining 

 the air space below a stoma, or group of stomata. In 

 such a position there can be little if any radial pressure 

 on the dividing cells, as their free surfaces abut on a cavity 

 communicating with the atmosphere by an aperture, which 

 n)ust itself constitute a point of weakness in the epidermis. 



That the elements originating from these cells ultimately 

 rupture the stoma and tear the surrounding tissues, scarcely 



