THE LEAVES. 87 



leaves lose their green color, and their cells becoming gradually 

 and entirely choked up with mineral matter, the sap no longer 

 circulates through them. They absorb oxygen from the air, 

 and the result of their different degrees of oxidation is seen in 

 all that variety of autumnal tint, which casts such a charm over 

 the dying landscape. 



Whilst these chemical changes are taking place, nature is at 

 the same time preparing to effect the mechanical excision of the 

 leaf from the plant. 



Now, at first, all leaves are contiguous with the stem. As 

 they grow, an interruption of their tissue takes place at the 

 base of their footstalk, by means of which a more or less com- 

 plete articulation is formed. This articulation is produced by 

 the continuation of the growth of the stem after the leaf has 

 attained its full growth, which it generally does in a few weeks. 

 The growth of the leaf being completed, all its functions lan- 

 guish in consequence of the increased deposition of mineral 

 matter within its cells, and the base of the petiole or footstalk 

 being no longer able to adapt itself to the increasing diameter 

 of the stem, a fracture between the base and stem necessarily 

 ensues ; the excision advances from without inwards, until it 

 finally reaches the bundles of woody fibre, which are the main 

 support of the leaf. 



Whilst, however, nature is forming a wound, she is at the 

 same time making provision to heal -the same j for the cuticle 

 or epidermis of the stem is seen to grow over the surface of 

 the scar, so that when the leaf is detached the tree does not 

 suffer from the effects of an open wound. The provision for 

 separation being thus completed the leaf is detached by the 

 growth of the bud at its base, by the force of the winds, or 

 even by its own weight. Such is the philosophy of the fall of 



