Reproduction by rhizomes is typically done when the parent plant is under stress. Wetland environments put stress on plants because there is so little oxygen. Click to enlarge. Click to enlarge. Some trees in wetland areas develop knees .
Knees are a part of the root system that rises above the water.
Knees were once thought to help bring oxygen from the air to the plant roots. However scientists today are unsure how the knees are used. Some plants live in fresh water and some live in salt water. The ones that live in salt water not only have to deal with being flooded, but also must deal with salt, which is harmful to many plants. Spartina alterniflora, a common salt marsh cordgrass, is specialized to live in this high salt environment.