The Burbank "Flats"
Mr. Burbank usually Plants seeds in boxes like these, eighteen inches square and four and one-half inches deep, the boards at the bottom being spaced to permit free drainage. He calls such a box a "flat." The boxes should be made of durable wood, such as redwood, cypress, or locust.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 4