If you'd prefer hard copies of these watercolors, there's a recent apple "opus" that came out that includes 1400 of them: https://www.jakkawpress.com/. It's a six volume set, currently marked down to $150. I'm tempted.
"The lavishly illustrated hardcover set of seven volumes (each 500-600 pages, 8½” x 11”) comprehensively documents all apple varieties that have appeared in publications in the United States and Canada through the year 2000."
Not to detract from your wonder, but apples aren't true-to-seed... so each new seedling could potentially produce a named variety (although most end up trash trees either with undesirable fruit or susceptible to disease).
At first I thought these were cute little still lifes, but from the date range ... were these of scientific use during a period when color photography wasn't easily accessible? I'm aware that color photography did exist and was producing pretty good results in the early 20th century. By the late 30s/early 40s, would it be surprising to still be using watercolors to try to capture and convey images where subtle color differences are important?
Look at a modern-day plant or animal field guide. A lot of the time you'll find drawings, not photos. Getting color "correct" in photos isn't that much easier (esp in the past, digital workflows help), and drawings are flexible in presenting the right amount of detail, optimal perspectives and perfectly "representative" specimens. Of course with plants you don't have the problem of them running away and not sitting still properly, so you can pose photos well, but drawings are still done a lot.
Pome: "Botany. The type of fruit that is characteristic of the apple (Malus domestica), the pear (Pyrus communis), and related members of the family Rosaceae, which consists of a fleshy, enlarged receptacle enclosing a tough central core (the true fruit), formed from several united carpels and containing the seeds." Outside of botany, that usage is obsolete.
The definition of pomology is broader, however: "The branch of horticulture concerned with the cultivation of fruit."
The link above will give you thumbnails that you can view in browser.
I'm considering making a taxonomic tree (and then forget formal phylogeny and include the varieties) with the thumbnail images. Similar to another pet project of mine.
Is there a general term for this style of artwork where biological species are drawn from different angles / sections in old books? I always love them when I see them. I'm really interested learning more about it.
I think it would be very cool if the government made an NFT collection out of this and other collected works, in all seriousness. It's actually quite easy, and little to no programming is required.
This is also my first thought - USDA needs to get on the NFT game with these artworks esp since there's a historical element to these works. I'm sure there will be people who want to collect these.
These graphics have been digitized at a high resolution, so they can be infinitely, perfectly duplicated. As works of the US government they are inherently uncopyrightable, so there is no legal barrier to copying them either. Thus, these images are completely limitless. Why would you want to introduce artificial scarcity of such a resource?
I also uploaded the collection to Wikimedia Commons: https://commons.wikimedia.org/w/index.php?title=Category:USD...
And for several years have run a Twitter bot that posts one image every few hours: https://twitter.com/pomological/