Supercooling inhibits the formation of ice within the tissue by ice nucleation and allows the cells to maintain water in a liquid state and further allows the water within the cell to stay separate from extracellular ice. Cellular barriers such as lignin, suberin and the cuticle inhibit ice nucleators and force water into the supercooled tissue. The xylem and primary tissue of plants are very susceptible to cold temperatures because of the large proportion of water in the cell. Many boreal hardwood species in northern climates have the ability to prevent ice spreading into the shoots allowing the plant to tolerate the cold. Supercooling has been identified in the evergreen shrubs Rhododendron ferrugineum and Vaccinium vitis-idaea as well as Abies, Picea and Larix species. Freezing outside of the cell and within the cell wall does not affect the survival of the plant. However, the extracellular ice may lead to plant dehydration.