Delphinidin

Water, even after air has been bubbled through it, contains less than one-thirtieth the amount of free oxygen found in the air. Accordingly, plants growing with their roots in water may not have enough oxygen available for normal respiration in their root cells. Some swamp plants, such as the black mangrove (Avicennia nitida) and the yellow water weed (Ludwigia repens), develop special spongy roots, called pneumatophores, which extend above the water’s surface and enhance gas exchange between the atmosphere and the subsurface roots to which they are connected.

The woody “knees” of the bald cypress (Taxodium distichum), which occurs in southern swamps, were in the past believed to be pneumatophores, but there is no conclusive evidence for this theory.