154 Our Field and Forest Trees 



trifling cause will split this cuttlng-off band. The 

 weight of the leaf itself helps to tear it away. 



By October the corky scale in each leaf-stalk 

 has gained its full thickness and has separated the 

 leaf from the branch almost completely. There 

 are still a few woody threads running from the 

 larger veins through the leaf-stem to the bough 

 and lightly tying the leaf to the tree. Frosty 

 nights and sunny mornings first freeze and then 

 thaw what little sap remains In these woody 

 threads, and thus help to break them. At last, 

 some cold night, a thin plate of Ice forms in the 

 absciss layer. Then the last ties are broken and 

 the leaf and the branch are entirely separated. 

 When the morning sun melts the ice, the leaves 

 shower from the boughs, however calm the air. 



And now Nature doctors the place which the 

 breaking of the leaf-stalk has laid bare. 



The broken ends of the woody threads are cov- 

 ered, In many trees, by a protecting gum. A little 

 later they are surrounded by the cork, growing 

 closely around them and over them, and the heal- 

 ing of the scar is complete. 



The falling leaves of the horse-chestnut leave 

 scars which plainly show the marks of Nature's 

 surgery. The cork seal, which Is very plainly 

 seen, has a horseshoe-shaped outline, and the ends 

 of the woody threads, overlaid by dark glistening 

 gum, suggest the horseshoe nails. 



