The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree, and for reproduction defense, survival, energy storage and many other purposes. The first root produced by a newly germinated seedling is a taproot which goes straight downwards. Within a few weeks lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the tap root eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell root hairs. These are in immediate contact with the soil particles and can absorb water and nutrients such as potassium in solution. The roots require oxygen to respire and only a few species such as the mangrove and the pond cypress can live in permanently waterlogged soil