Wetland plants have to be able to withstand flooding, drought, ice, waves, brackish (water with some salt content), and often very acid or alkaline soil conditions. They may have salt pumps like cord grass that removes salt absorbed from brackish water, or have thick rhizome roots that withstand flooding. They may have aerenchyma (bundles of air exchange tubes) in their stems. These allow oxygen from the air to get to the all areas of the plants where it is needed during respiration. In areas where soil has air pockets of oxygen in between soil particles the limited oxygen needed for plants comes through the roots. The aerenchyma also transport carbon dioxide from respiration up out of the root area and to the atmosphere.