The black seaweed wrappers used in makimono are called nori. Nori is a type of algae, traditionally cultivated in the harbors of Japan. Originally, algae was scraped from dock pilings, rolled out into thin, edible sheets, and dried in the sun, in a process similar to making rice paper. Whereas in Japan, nori may never be toasted before being used in food, many brands found in the U.S. reach drying temperatures above 108 °F (42 °C).

Source: http://en.wikipedia.org/wiki/Sushi; Sept 28, 2010