in an environment with very high salinity such as mangrove swamps and semi-deserts, water uptake by plants is a challenge due to the high salt ion levels. besides that, such environments may cause an excess of ions to accumulate in the cells, which is very damaging. halophytes and xerophytes evolved to survive in such environments. some xerophytes may also be considered halophytes, however, halophytes are not necessarily xerophytes. the succulent xerophyte "zygophyllum xanthoxylum", for example, have specialised protein transporters in their cells which allow storage of excess ions in their vacuole to maintain normal cytosolic ph and ionic composition.