In the filling-up of ponds, the activity of vegetation is in cases second to the physical factors of erosion and deposit. In open pools, anchored plants with floating leaves are often confined to a zone somewhat separated from the shore, their approach to the shore line being prevented by silt which is swept in, especially where the latter is shifted by wavelets The physical factors in that zone thus exclude the organic. Plants of the shore line in such cases have running stems similar to those of sand binders, which enable them to escape death by burial. The vegetation of the large open morainal pool, though undrained, may be purely hydrophilous. About the time of the formation of the floating mat the conditions appear to become xerophytic. The marginal ditch which surrounds pond islands and atolls is in this region, at least, formed only in the woods, where a dense felt of humusvegetation protects the ground from erosion. Fallen leaves and other organic materials swept from the forest floor into the edge of the pool tend to smother the vegetation which might grow there, and thus is produced a belt of open water, surrounding an island, or if the pond is larger a ring of vegetation.