Soil of coral atoll is basically limestone, so few species of plants can live there; those that survive are hearty.
Roots must be long enough to reach the water.One of seeds that most often finds its way to an atoll, perhaps borne by the wind, is the coconut seed which is why coconut palm trees are common growing on atolls.
Mangrove trees whose roots make it look like a giant spider, stand in shallow water protecting the leeward side of a reef. It sends out green podlike roots that drop from the tree and hang until they become anchored. This is why it can also withstand tropical storms. The mangrove helps build the island because its roots reduce water's flow across the reef and help accumulate sand.
Droppings of transient birds enrich the soil making it capable of supporting other plants.