"In normal gardening lingo, 'perennial' is used to describe a low-growing herbaceous or evergreen non-woody perennial plant. However, this book takes a much wider perspective. For example, trees and shrubs are also perennials, and in fact there are many nice edible leaves from then - lime leaves, saltbush leaves and mullberry leaves, to name but three. Aquatic perennial plants are also included here, as are some bulbs, and one fern" -- P. 19.