Larry Jaques wrote:

My favorite book is by Dick Onians. (Huntley, $1.28 at Amazon)
http://tinyurl.com/8327srm


On pp. 32-33 of this book, he provides the basis of the two definitions
for the terms softwood and hardwood that krw and I were discussing in a
subthread he The historically "common" one, and the "biological" one.
These papes contain the most readable distinction between softwood and
hardwood that I have seen.