Where can I find the history of heirloom plants? For example, where and when did "Amish Deer Tongue" lettuce originate?
Knowing the history of the plant helps me connect to it and the people who saved its seed.
Tracking down the origins is half the fun, TheTick. It can often be a detecting job that ranks up there with any Sam Spade story. I know a group of people who have spent several years, for instance, trying to track down the original Golden Queen forebear.
There are numerous resources, both modern and old. For instance, among the modern references are Will Weaver's books ("Heirloom Vegetable Gardening" and "100 Vegetables and Where The Came From").
Somewhat older are the "X of New York" volumes, such as "Beans of NY," "Apples of NY," etc.
Reaching way back, we have volumes such as Fearing Burr's "Field & Garden Vegetables of America," first published in 1863. And "Thomas Jefferson's Garden Book" And a whole host of others.
Older agriculural reports, such as the USDA's "Yearbook of Agriculture" often provide insights.
Old seed catalogs are a never ending resource. Unfortunately, they are a big-time collector item, and are sometimes cost prohibative.
Understanding human population dynamics is often an important part of the equation. Without knowing something about that, for instance, there is no way to understand the widespread incidence of bi-color tomatoes in eastern Kentucky.
Don't discount anecdotal evidence. Often it provides the only clues to origination. For example, everything we know about the origins of Cherokee Purple tomatoes is anecdotal in nature until Craig LeHoullier obtained and named it.
Now, a partial answer to your specific question. Deer Tongue lettuce, which dates to at least the 1740s, is also known as "Matchless." The Amish Deer Tongue is a later variety, said to be less prone to bolting. I don't know when it was introduced under that name. There is also a Red Deer Tongue.
That information was obtained in only a couple of minutes from references on hand, most of which were named above.
Tick: Your inquiry was a few years ago, but I thought I'd respond because I also find it fascinating to track down the origin of old heirlooms, particularly heirloom fruit. The research and discovery is one of the great reasons why I started The Heirloom Orchardist. For instance, on February 29, we posted a rare account on the origin of the Kane Spitzenberg apple, an obscure cousin to the famous Esopus Spitzenberg. Often times you can't research the information on origins, you just stumble on it.
Here is a link that might be useful: The Heirloom Orchardist