In February the Smithsonian Institution announced they are releasing almost three million images to the public domain. Contents from 19 museums plus libraries, research centers, and the National Zoo. All free for the download.
And, unlike the British Library illustrations that are free on Flickr, searches seem to work pretty well. (Except that whatever I search for I always get some plant diagrams. What's that about?)
Here are a few examples related to our favorite subject. Enjoy.