DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 103 



of detaching the leaf from the bough which ^bears it. Murray also 

 attributed the cause to a mechanical one, but although entirely- 

 opposed to Mustel, his interpretation is not less problematical. The 

 latter maintained that the sap, far from distending the leaf, is on the 

 contrary stopped in its upward course by the axillary bud : this 

 bud, compressing the tissue at the base of the petiole, at first 

 hinders the sap from getting to the leaf, and afterwards continuing 

 to deyelope, presses down the leaf, already suffering from loss of 

 nourishment, and causes first a rupture in the tissues, and lastly 

 the fall of the organ. 



Senebier thoroughly adopted these ideas of Murray (1798), and 

 defended them with ability. He set himself to combat the many 

 objections which arose against the theory, but the reasons which 

 seemed so strong to him, are not borne out by careful observation 

 of facts. 



It is in the writings of Duhamel and Vrolik bearing on the sub- 

 ject, that we have found the first accurate observations, based upon 

 an attentive study of the tissues, and the modification they undergo 

 in autumn ; the explanation only, is erroneous. Thus, Duhamel 

 described between the stem and the base of the leaf, a layer of 

 cellular tissue, which is disorganized by the influence of 

 cold ; in addition, he thought that the leaf stopping in deve- 

 lopment whilst the stalk continued to grow, a tension resulted 

 which ended in the separation of the two. This cellular tissue of 

 Duhamel really exists, but, as we shall see, it forms part of the 

 pulvinvs, somewhat elastic, always fixed to the stem, serving to 

 unite leaf and stem, and the elasticity of which, to some extent, 

 protects the leaf from being prematurely torn away from the stem. 



The effect of cold in separating the two is also undoubted, but 

 its mode of action is quite different from what Duhamel sup- 

 posed. 



Vrolik (1796) saw in the separation of the leaf an effect of re- 

 sorption. The resorbed tissue leaves a gap, which is the starting 

 point of the solution of continuity. This resorption is, nevertheless, 

 a fact, although exceptional, but our author has generalised too 

 much from it. Farther on we shall see in what cases it may be 

 observed ; anyhow, it appears certain that in many cases Vrolik 

 has mistaken a solution of continuity, which was no more than the 

 beginning of a mechanical separation between the leaf and its 

 support, for an effect of resorption. 



