Selaginella rupestris, the northern selaginellla sometimes locally known as ledge spike-moss or rock spike-moss, is a species of spike-moss occurring in dry rocky places in eastern North America, including one locality in Greenland. It has a wide but sporadic range. In the absence of water, it rolls into a ball for which, it is also known as bird nest moss. Again, when it comes in contact with water, it opens up. (Wikipedia)