Today there is a lovely nature study book on Homeschool Freebie of the Day. It is a guide for the little ones about looking at leaves and learning their parts - it is very friendly and has very short lessons.
http://www.wholesomechildhood.com/August10/25.html
Leaves and Flowers: Plant Studies for Young Readers by Mary Spear, 1893
Edited to add: This is also available through google books
