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TEXTBOOK OF BOTANY 



fundamentally the same thing as the formation of a habit by 

 ourselves. It results from the fact that when living matter 

 has done a certain thing once in response to a stimulus, some 

 change takes place in it which makes it easier to do the same 

 thing a second time ; the third time it is easier than the 



second ; and after many repeti- 

 tions the act may become so 

 easy that it is done without any 

 external stimulus at all. 



265. Fall of Leaves. — In the 

 autumn the leaves of most of 

 our trees and other common 

 plants die and fall. If we ex- 

 amine a fallen leaf, we find that 

 the surface where it has broken 

 away from the stem or branch is 

 as smooth as though it had been 

 cut with a knife. The scar left 

 on the stem or branch by the 

 fall of the leaf is also smooth. 

 This means that the leaf has not 

 been torn roughly from its place 

 the leaves and leaflets fall. At of attachment, but that changes 

 a, two leaflets have fallen; at have taken place in the leaf- 

 stalk which resulted in its sepa- 

 ration from the shoot. As the 

 end of the growing season ap- 

 proaches, a change takes place 

 in the walls of a layer of cells 

 extending across the leaf-stalk 

 just where it is attached to the stem or branch. These walls 

 become softened, so that a shaking by the wind, or sometimes 

 a touch, is all that is necessary to break them and so to send 

 the leaf to the ground. The walls of the layer of cells on 

 either side of the softened region become permeated with cork. 



Fig. 149. — Portion of a branch 

 of horse-chestnut, showing how 



b, all the leaflets have fallen, 

 and only the petiole remains; 

 at f, the petiole also has gone, 

 and the leaf scar is shown with 

 traces of the five vascular bundles 

 that ran from the branch into 

 the leaf. 



