Aspirin/willow water: an organic gardener's dream?

Gordon Haverland wrote:
Salicin is found in the Salix genus, as well as in the Populus genus.  It is used as a plant hormone in many genii (sp? sorry, I never took Latin).

Genera was the word you were looking for.

And this brings me to the question I was going to ask: what about tropical climates where neither willow nor meadowsweet grow? I looked up salicylic acid on Wikipedia, and here is what stood out to me:
"Some results have been consistently reported. Meat, poultry, fish, eggs, oils, dairy products, sugar, cereals, and flour all have little to no salicylates. Measurable levels of salicylic acid have been found in fruits, vegetables, herbs, spices, nuts, and teas."

So then the question becomes, at what "measurable" level does it become enough to use for this purpose? There is so much primary research yet to be done.