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 
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Fic. 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 
