LEAVES 



215 



habit of shedding their leaves are called deciduous. 

 In temperate regions all plants, except the few ever- 

 greens, shed their leaves in the autumn. As soon as 

 the temperature of the 'soil 

 is at the freezing point, the 

 plant can absorb very lit- 

 tle water. When the time 

 comes for shedding of 

 leaves a special layer of 

 tissue is formed at the 

 base of the petiole. This 

 tissue gradually cuts the 

 leaf off and at the same 

 time covers the wound 

 which is produced. The 

 brilliant coloration of au- 

 tumn leaves is closely asso- 

 ciated with the changes 

 which take place in the 

 leaves previous to their 



removal. The food materials are withdrawn from the 

 leaf before it is shed, and the gradual reduction of the 

 activity of the leaves and the breaking-down processes 

 which take place in the dying leaf produce the brilliant 

 colors. These processes may be caused by frost, but 

 not necessarily so. 



Respiration. All living things require oxygen. A 

 plant takes in oxygen largely through the stomates of 

 the leaves, but this function is not limited to the leaves. 



Fig. 86. A sycamore tree in 

 winter condition. 



