This is a set of 50 algae brushes for Paintshop Pro version 7 and up.
They were created from images in the public domain and can be used in any way you wish.
The original images are some remarkable photographs taken by Anna Atkins (1799-1871) and appeared in the book Photographs of British Algae.
If you would like to see the originals, they can be found in the New York Public Library's Flickr Commons collection.
Download link can be found on my art site.