Leaf Coloration and the Fall of Leaves 71 



winds of winter, which cause transpiration even when the 

 ground is frozen. This suggests that their usual transpiration 

 rate must be very low in comparison with that of the decidu- 

 ous trees. 



Evergreen versus deciduous habit. In temperate regions, 

 where there are great changes in temperature and moisture, 

 the deciduous and the evergreen habit each has certain ad- 

 vantages. The advantages of the evergreen habit are: (i) 

 that the leaves can manufacture food even when the tem- 

 perature is low ; (2) that with their low water requirement, 

 evergreens can withstand drier conditions throughout the 

 year ; (3) that the tree does not waste so much material each 

 year in the construction of a complete set of new leaves. 

 The disadvantages of the evergreen habit are: (i) that the 

 heavy cuticle and compact tissues which aid in conserving 

 water interfere with rapid photosynthesis ; (2) that the lower 

 rate of food manufacture prevents rapid growth ; (3) that the 

 leaves lose in efficiency by their longer service on the trees. 



The advantages of the deciduous habit are : (i) that the 

 leaves, being renewed each year, are more efficient organs of 

 food manufacture ; (2) that the leaves, with less cuticle and 

 with tissues less compact, are better fitted for rapid food 

 manufacture ; (3) that the total leaf area may be much 

 larger than in the case of the evergreens ; (4) that the trees 

 are better fitted to withstand the winter drought, because 

 at that season the entire tree is covered with cork. The dis- 

 advantages of the deciduous habit are: (i) that the food- 

 manufacturing season is only from 5 to 8 months, as com- 

 pared with from 8 to 10 months in the evergreens ; (2) that 

 each year a large amount of food material is needed to make 

 an entirely new set of leaves. 



