110 DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 



ceous, or subfrutescent (our observations extending to about 100 

 species), if the cicatrix be carefully examined, you will find it con- 

 stantly made up of one or more rows of peridermal cells, in com- 

 plete continuity with those of the stem. If, sometimes, it is difficult 

 to show the existence of such tissue in course of formation, on the 

 scar at least, its demonstration offers no difficulty, and the accord- 

 ance of all our observations on this point give it an indisputable 

 value ; there may be doubt as to the mode of transformation, there 

 can be none as to the result. 



According to our notes, the peridermal tissue appears in the 

 line of demarcation ; either by the gradual transformation of 

 the exterior layers, following a centripetal course, and leaving the 

 vascular bundles only intact, this being what we term layer suheri- 

 fication ; or else, by the transformation of the cell wall of certain 

 pre-existing cells, or by deposit of suberous matter in their in- 

 terior, an alteration which we term diffuse suberification (or suberi- 

 fication by infiltration). 



The first case prevails in leaves distinctly articulated, such 

 as the Elm, Lime, and Hazel. The second may be seen in plants 

 having non-articulate or incompletely articulate leaves — for instance 

 the Oak and Hornbeam, which may frequently be seen with leaves 

 attached during the whole winter, till in the following spring 

 they are thrown off; it is rather by a rupture of the tissues than 

 by true disarticulation. Those leaves having incomplete articula- 

 tion are distinguished from those possessing the complete process, by 

 the very gradual manner in which the transition takes place 

 between the pulvinus and petiole. In these the green cellulose is 

 not abruptly interrupted at the level of the exterior constriction ; 

 there cannot be seen any central parenchyma, thick and juicy, in 

 close contact (adosse directeinent) with a loose and air-containing 

 tissue. The transition is gradual, and rises to a certain height 

 in the petiole. In autumn no distinct layer betrays the position 

 of the constriction, but the use of tests, points out a deposit of 

 corky matter in many places in the central parenchyma and 

 mesophyllum. These are, at first, scattered groups of from two 

 to four contiguous cells, and may be clearly distinguished from 

 the neighbouring ones. Later on these groups increase in size, 

 and number, and appear most numerous in the proximity of 

 the constriction, especially in that part answering to the pulvinus. 

 From the time when these altered cells attain a certain limit 



