The following page is from The Crop Circle Engima, Ralph Noyes, Ed., with photographs by Busty Taylor, Gateway Books, Bath, UK, 1990.
Grain Plant Physical Changes
Numerous articles exist on the physical changes in the grain plants subject to circle formation. This fact makes it easy to distinguish between circles created by celestial forces, and those that are man made.
Some of these are:
Anatomical Changes - Nancy Talbot
Anatomical Changes - W. C. Levengood (This is a PDF file.)
Anatomical Changes - 21st Century Radio