What kind of nut am I?

It grew on a medium sized tree in the garden.
The fruit was round and green
And I had no idea what it was.
I thought perhaps a nashi pear as some of the neighbours grow them.
Eventually the green skin began to split.
And when it fell to the ground I peeled it and discovered it was –
A walnut!

How ignorant can one be!

Post script - Interesting news release:22 April 2009
WASHINGTON (Reuters) - By eating walnuts, women could reduce their risk of breast cancer, researchers said on Tuesday.
"Walnuts contain multiple ingredients that, individually, have been shown to slow cancer growth including omega-3 fatty acids, antioxidants and phytosterols," Hardman's team wrote in a summary presented at the American Association for Cancer Research's annual meeting in Denver.

Photos: Walnuts from tree to table