Ptilota sericea

When captured by Anna Atkins, this little seaweed was called Ptilota sericea (later Ptilota elegans). Today, we refer to it by the scientific name of Plumaria plumosa. The reddish-purple seaweed grows 2-3 inches long and is typically found in the North Atlantic, along both North American and European shores. 

Atkins created her cyanotype photograms by placing the specimen onto light-sensitive paper, and then exposing paper and subject to the sun for a period of time. She then developed the image using the cyanotype process, resulting in a rich blue background.