The Growth and Fall of Leaves 103 



ferent structurally and physiologically from the leaves of decidu- 

 ous trees. The evergreens of temperate regions must be able 

 to withstand freezing and thawing, and also the dry winds of 

 winter, which cause water loss even when the ground is frozen. 

 Their usual transpiration rate is very low in comparison with 

 that of deciduous trees, and in the autumn they undergo changes 

 which reduce the water loss nearly to that of deciduous trees that 

 have dropped their leaves. 



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 advantages. 

 The advantages of the evergreen habit are: (i) that the leaves 

 can manufacture food even when the temperature is low ; (2) that 

 with their low water requirement, evergreens can withstand drier 

 conditions throughout the year ; (3) that the tree does not use 

 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 con- 

 serving 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 v/inter drought, because at that season the entire tree is 

 covered with cork. The disadvantages of the deciduous habit 

 are : (i) that the food-manufacturing season is only from 5 to 8 

 months, as compared 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. 



