454 TRANSACTIONS AND PKOGEEDINGS OF THE [sess. lxviii. 



the cortex of a young lateral rootlet, in that region where 

 the phellogen of the old root crosses the cortical tissue of the 

 branch in order to establish communication with the corre- 

 sponding layer of the latter. All the tissue primarily outside 

 this phellogen, whether on the old root or the young one, is 

 ultimately exfoliated. In this case, however, unlike that of 

 the stem, the normal phellogenetic divisions precede slightly 

 the formation of the lenticellar meristem, with the result that, 

 though the external cortex is not entirely thrown off at the 

 period when the lenticellar initials begin to divide, its cells 

 have already lost their turgidity and begun to wither, owing 

 to the formation of internal cork. The primary cortex of 

 the young root is, moreover, separated from that of the 

 parent by a cylindrical fissure extending from the surface 

 of the latter to its phellogen, and it is on the ring-like 

 area lying between the base of this fissure and the central 

 cylinder of the young root that the Lenticels arise. 



The divisions taking place in this area are tangential to 

 the old root, and due, as above mentioned, to the necessity 

 of uniting the two phellogens so as to provide a continuous 

 cork covering, but do not arise in a definite pericycle, the 

 cells of which have in this region been utilised at an earlier 

 period for the production of the lateral root. For the same 

 reason there is no endodermis above the dividing cells, 

 and the only radial pressure to which they are exposed is 

 that set up by the rapidly withering cortex of the branch 

 root, a stress which in any case cannot reach large dimen- 

 sions, and is further reduced by the presence of the 

 cylindrical fissure above referred to. The cells of the 

 normal phellogen, covering the remainder of the parent root, 

 are on the other hand subjected to the radial pressure of 

 an elastic endodermis together with that of the other 

 cortical layers. 



The surface in which the cortex of the young root comes 

 in contact with the phellogen of the parent has been 

 already referred to as a ring lying between the central 

 cylinder of the young root and the cylindrical fissure 

 separating the two cortices. 



In the majority of cases the lateral rootlet is considerably 

 the thinner of the two, and where this is so, the superficial 

 curvature of the parent may, for the present purpose, be 



