If Carol Lerner were asked to list three reasons why she started writing and illustrating books for children, she would probably say the Morton Arboretum, Joshua, and Jesse. The Morton Arboretum in Lisle, Illinois, on the outskirts of Chicago, is one of the few institutions in the country that offers instruc-tion in botanical illustration on a regular basis. Over the years, Ms. Lerner had taken courses in botany, the local flora, birds of the Midwest, and other aspects of nature, but she hesitated to join the illustration class because the members seemed very accomplished. "Finally I gave it a...