DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 105 



grows by its base ; there, is formed the growing tissue, but this 

 tissue can also gi^e place to a corky production, for upon the leaves 

 of most of our trees we notice a demarcation, easily seen by the 

 naked eye, * the articulation,' which points out the part where the 

 leaf will separate from the stem at its fall. 



'* As far as I have been able to observe, myself, we find on the stem 

 all the cicatrices covered with a layer of cork, which leaves nothing 

 free save the woody cells of the vascular bundles. Schleiden had 

 already pointed out the fall of the leaf as the result of the stop- 

 page of the passage of li(iuids. I thought I had found in the sub- 

 erification of the base of the leaf of DraccEna the cause of that 

 interruption of osmose ; and I wish to draw the attention of 

 observers to an act so important in the life of the plant." (Der 

 Tod der Pfianzenzelley c. xii., p. 245.) 



These ideas formed the point of departure in our researches ; 

 the results which we shall unfold farther on will tend to show how 

 far Schacht's opinion may be admitted, and what are the 

 physical and chemical influences which must intervene, before we 

 can arrive at a solution applicable to the whole of the facts ob- 

 served. 



77. — Means of Observation. 



Since the early part of the autumn of 1868, our micrographical 

 studies have been entirely directed to the solution of this question, 

 resumed the following year, and suspended for certain kinds of 

 difficult observations during the autumn of last year (1871) ; our 

 observations bearing upon the majority of the trees and shrubs of 

 our climate. 



It was upon the bushes in our thickets and the taller trees of 

 our woods and public walks that we began our work ; in these 

 we have been able to follow the phenomenon step by step, by 

 making different sections of the leaves of the same species, and sub- 

 mitting them to those re- agents which can throw light on the 

 nature and successive modifications of the component tissues. 



In the histological examination of the petiole, a transverse 

 section is enough to give a clear idea of the relative disposition of 

 the tissues. As for the pulvinus, recourse is almost always had to 

 a longitudinal section, perpendicular to the cicatrix, in order to 

 recoGfnize the tissue which covers the latter. A cross-section of 



