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 be- 

 come 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 

 most annual plants (337), the older leaves become use- 

 less from this clogging and die before the stem is fully 

 developed, and in most perennials the leaves endure 

 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 those that are not 

 used by {he plant, but have been deposited with them 

 during transpiration (125). 



