MAY. 89 



Some trees lose their leaves without much previous 

 change of tint. My walnut trees are still green below, 

 but their upper branches are already bare, and apples, 

 pears, and plums similarly show little change of colour. 

 The leaves of horse chestnut and of the oak turn brown 

 before they drop, and the latter keeps its foliage for a very 

 long time. The leaves of maples and poplars, "turning 

 yellow, fall and float adown the air," and the elms become 

 nearly golden, while the birch, beech, and chestnut are 

 reddish or even like old gold in the glory of their hues. 

 Down our road we have watched the rowan fading almost 

 to crimson, while a favourite dome-shaped lime tree has 

 turned its foliage here and there to yellow. 



The fall of the leaf at the approach of winter is a 

 phenomenon almost confined to trees of the northern 

 hemisphere, and appears to be associated with the recur- 

 rence in the vast land areas of the north of glacial epochs, 

 and with the modifications which plants of that hemisphere 

 have acquired to enable them to stand the severe alterna- 

 tions of temperature to which they are subjected. I do not 

 refer so much to the British Islands, where the climatic 

 conditions are modified and tend to be equalised by the 

 Gulf Stream, but to the great continental masses of Asia 

 and Europe and of North America. For instance, in lati- 

 tude 53 N., which passes through the middle of England, 

 a maximum temperature of 90 F. is rarely reached 

 there, while the minimum seldom touches F. In the 

 same latitude in North America, a summer heat of 90 F. is 

 common, while in winter 50 (or 82 of frost) is reached 

 regularly. Similar conditions prevail in Russia and Siberia. 

 It is apparent that plants which are to live through such 

 severe winter cold must become dormant for a portion of 

 each year. The vital functions are almost arrested for 

 a time. 



The leaves of trees are not only the principal feeding 

 organs of the plants, but by the evaporation of water from 

 their surfaces they cavise a continuous flow of sap to ascend 

 from the roots through the stems and branches. But the 

 less water there is in a plant which is to be subjected to a 



