DR. K. LEDEGANCK ON THE FALL OP LEAVES IN AUTUMN. Ill 



all flow of sap is stopped, and the leaf, without dropping off, dies 

 from dessication. Such is the mode of formation of this new tissue, 

 the presence of which should be considered as the predisposing 

 cause of the fall of the leaf, viz., to intercept the passage of liquids, 

 and to destroy the cohesion between the leaf and its support whilst 

 awaiting a more energetic cause to achieve the definitive separation. 

 Such, in our opinion, is the part performed by the peridermal 

 tissues. 



As for the more active cause, we find it principally in the action 

 of cold. Suppose a sudden fall of temperature, and notice the 

 behaviour of the two kinds of tissues in its presence ; it may be 

 predicted that either one or the other will contract under the 

 influence of cold, but the tissue at the base of the petiole, spongy, 

 full of air, and elastic, will contract to a greater degree than that of 

 the pulvinus, which presents different physical characteristics, and 

 of which the change in bulk will be hardly appreciable. 



These unequal shrinkages will result in the rupture of a certain 

 number of cells at the junction of the two organs, and let but a 

 slight shock come, and the separation will be complete But let 

 the fall of temperature pass + 4 C* the shrinkage will not con- 

 fine itself to an unequal retraction of the tissues ; it will produce a 

 true movement in opposite directions, in consequence of the dilatation 

 of the dense and succulent parenchyma of the pulvinus opposed to 

 the ever growing contraction of the spongy tissues of the leaf- 

 stalk. 



Thus the fall of temperature gives rise to congelation, and the 

 expansion accompanying the solidification of the juices in the pul- 

 vinus becomes so great that all cohesion is destroyed, the rupture 

 takes place at once, and the leaf drops. 



These physics of the question explain the large increase in the 

 number of fallen leaves after a sharp night in autumn, the least breath 

 of air then bringing down the disorganized leaf; they likewise ex- 

 plain the sudden and complete stripping of trees by the early frosts, 

 when the leaves falling by their own weight sometimes form, even 

 round the foot of the tree itself, a layer of considerable thickness. 



VII. — Physiological Conclusions. 



The peridermal tissue, which is only found on the exterior parts 

 of plants, naturally a bad conductor of heat, and impermeable by 



* 39 2F Fahr., maximum density of water. 



