After the previous three systematic chapters, three habitat chapters follow, the first of which describes the environment between the reef crest and the shore, an area of relatively calm water dominated by Thalassia, a marine grass that reproduces sexually, producing seeds, but also by spreading rhizomes, horizontally growing roots. Thalassia is an important repository of carbon and a generator of oxygen. Though corals are found here too, and many other organisms of the reef, the back reef is an environment of plants like Penicillus, the shaving brush, and Acetabuleria, a large single-celled organism. It also serves as a nursery for reef fish. The animals and plants are all named as in the systematic chapters, and a couple of plants also found on the reef are put in this chapter, such as Halimeda, a calcareous alga that contributes much of the sand on the reef.