The Leaves. 85 



and, therefore, hardiness. In the apple, the large- 

 leafed varieties are, as a rule, hardier than others, prob- 

 ably because their vigorous roots supply the needed 

 water during the dry season, thus enabling the tree to 

 mature healthy wood and buds which can pass severe 

 winters unharmed (174). 



Crops grown for their leaves, as cabbage, lettuce, to- 

 bacco, etc., are especially liable to be curtailed by 

 drought, and hence should be given the culture that 

 best promotes soil moisture, as abundant surface tillage 

 and liberal manuring (231). 



125. Leaves are usually Short-Lived because they 

 become clogged with those mineral matters taken up 

 with the soil water which are not used by the plant 

 (63) and which do not pass off in transpiration (74). 

 In mcst annual plants (337), the older leaves become 

 useless from this clogging and die before the stem is 

 fully developed, and in most perennials the leaves en- 

 dure but a single season. In the so-called evergreen 

 plants, in which the leaves are usually very thick and 

 are often well protected against evaporation by a very 

 strongly developed cuticle (64), the leaves rarely live 

 more than a few years. 



126. The Manurial Value of Leaves, that mature on 

 the plant, is usually small, since the more valuable fer- 

 tilizing materials they contain pass into the stem before 

 the leaves ripen (170). The mineral matters contained 

 in largest quantity by leaves are these that are not 

 used by the plant, but have been deposited with them 

 during transpiration (125). 



