If anybody just wants to download the hi-res images, Internet Archive is your friend: https://archive.org/details/usda-pomological-watercolor-coll...
You can have fun with 'em since they're public domain (:
Note 1: The metadata, such as title, author, etc. seem to be missing. If anyone knows of a collection with all that included, let me know (it's not in the EXIF either, I spot-checked).
EDIT: aha! Here is metadata, which you can correlate to the image files: https://github.com/Wumms/pomological
Note 2: I saw this in the MARC catalog record:
Use of the images in the U.S. Department of Agriculture Pomological
Watercolor Collection is not restricted, but a statement of attribution is
required. Please use the following attribution statement: "U.S. Department
of Agriculture Pomological Watercolor Collection. Rare and Special
Collections, National Agricultural Library, Beltsville, MD 20705"
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