Mostly these are the bacteria that live around the vent, often forming a dense mat.

It grows rapidly and forms dense mats over still waters.

Her eyes sought the dense mat of leaves far above.

The branches tend to intertwine and form a dense mat.

It is a spreading plant which forms dense green mats on the ground.

It can be spreading with many narrow branches, or a small, dense mat.

The water made the dense mat curl into little whorls.

In large numbers, the plants make a dense mat on the water's surface.

It can form dense mats and even mounds that grow over low features.

It is a spreading ground cover that grows in a dense mat.