Another type of hydroponic transplant production system is the ebb-and-flood. This system is similar to the float bed pictured earlier in this series except that the water or nutrient solution is periodically pumped to another bed or holding tank so the transplant trays can dry. The ebb-and-flood system effectively manages water availability so the transplants do not grow to quickly and become overly succulent. When nutrients are added to the solution, the beds must remain covered at all times to reduce the growth of algae. The green color of the water is caused by algae in this picture. The white rails are constructed of PVC pipe and provide support for flats during "ebb" cycles so air can circulate under the trays and air prune the roots.