THE STUDY OF LEAVES. 225 



substances are probably useless and are got rid of in the 

 falling leaves and the bark." 



What is the cause of the change ? Two facts may be 

 noted. There is in general the regularity of its occur- 

 rence at each returning autumn, and there is the great 

 diversity as to the particular time for the same kind of 

 tree in different parts of the country or in different situa- 

 tions. There is in fact much individual variation as to the 

 time and rate of progress of the changes involved in the pro- 

 duction of the autumn tints. 



The period over which autumn leaf changes extend 

 may reach from August to November. Teachers might 

 collect evidence on this point. There is no doubt that the 

 inducing cause is seasonal failing light and temperature 

 perhaps, or frost, and that the immediate cause is chemical 

 change with the transference of some of the products to 

 other parts. 



Can we attribute any significance to the colours in the 

 life of the tree ? Are they of any direct service to the tree 

 in its external relations ? We know of nothing. They 

 are the outward manifestations of an inward process. 

 They delight the eye of man, but do not seem as colour 

 to serve the tree any more than the brilliant pigments of a 

 deep-sea starfish serve that animal in the darkness of its 

 natural environment. 



The Fall of the Leaf. 



The mechanism of the fall may be described. At the 

 base of the leaf there grows a layer of cork tissue which 

 cuts off supplies between leaf and stem. Behind this part 

 towards the leaf there usually grows a layer of soft tissue 

 which liquefying enables the leaf with a slight breath of 

 wind to detach and fall to the ground. 



N. S. 



15 



