THE PLANT AND THE SEASONS 23 



of union between the stalk and the stem, a special 

 arrangement of cells is formed. This has been 

 called the layer of separation, and consists of a 

 number of cells running at right angles to the 

 tissue of older formation. The greater the age 

 of the leaf the more markedly does the line of 

 separation appear, and as the processes of assimila- 

 tion begin to slacken and the moisture evaporates, 

 the severance of the organ is only a matter of 

 time. Finally, the presence of organic acids so 

 weaken the cells that the leaf will perhaps fall by 

 its own weight ; in any case the first gust of wind 

 will be sufficient to complete the fracture. 



When the stalk has become detached it does 

 not leave a raw and bleeding wound behind, such 

 as would be produced by the forcible pulling of 

 the leaf from the branches. The very layer of 

 cells which brought about the separation has also 

 ensured the welfare of the twig from which the 

 leaf has been removed. A thin veil is spread 

 over the point of cleavage, so that there is never 

 any open surface exposed to the air at all. There 

 are few more beautifully contrived processes in 

 the natural world than this fall of the leaf, with 

 which the tree passes into its winter rest. And 

 so the leaves, often brightly coloured as the result 

 of the chemical change which has taken place in 



