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An Impregnable Fortress

Note the size and offensive or defensive quality of the spines that protect this chestnut bur. No bird or squirrel could hope to penetrate such an array of bayonets. Such a specimen, obviously, would not be among those chosen to carry forward an experiment in breeding the spines off the chestnut bur.

An Impregnable Fortress

This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 4