NATURE'S SOIL-CONSERVING OPERATIONS 



177 



little patches of mosses or other low herbage, but nearly all 

 the levels are overspread with leaves, and under the leaves is 

 leaf -mold. Here is humus in the making. Let us examine 

 the bed of leaf mold. On top, the leaves are well preserved, 

 and show clearly by their form on what kinds of trees they 



FIG. 69. A skeletonized leaf of cottonwood. 



grew. Some leaves, such as those of oaks, that contain much 

 tannin 'are resistant to decay, and those of two seasons may 

 remain unrotted. But other leaves, such as those of elm, 

 decay so quickly that they will not outlast the first winter. 

 In some, such as those of maple and cottonwood (fig. 69), the 

 veins resist decay so much longer than the blade that the 

 leaves become beautifully skeletonized. In the lower strata 



