Re: What is the definition of a 'true nut'?

Botanically, a nut is an indehiscent (not opening at maturity) dry fruit with one seed and a thick, hard pericarp (the shell). A true nut also has a cup at its base (Stern, 1991). Another definition of nut is a dry indehiscent fruit that develops from a compound gynoecium (a compound pistil) in which all but one ovule degenerates during development and the pericarp is hard at maturity (Mauseth, 1995).

Examples of true nuts are acorns (in oaks), chestnuts, hickory and hazelnuts. The term nut is also used to refer to various hard-shelled fruits or seeds. For example, a peanut is botanically a legume. Pine nuts, pistachio nuts, cashew nuts, and brazil nuts are really seeds. Coconuts, walnuts, pecans and almonds are drupes.

Botanical fruit classification in most introductory college textbooks is rather inexact and is based on the work of famous botanist Asa Gray (Foster and Gifford, 1974). For example, some botanists call a walnut a true nut, others call it a drupe.

References

Foster, A.S. and Gifford, E.M. Jr. 1974. Comparative Morphology of Vascular 
Plants. San Francisco, W.H. Freeman.

Mauseth, J.D. 1995. Botany: An Introduction to Plant Biology. Philadelphia: 
Saunders College Publishing.

Stern, K.L. 1991. Introductory Plant Biology. Dubuque, Iowa: Wm. C. Brown.