There is no Darwinian explanation. Plants make chemicals. Very
recently, we discovered that some of those chemicals have "neat"
effects, which can be bad in terms of fitness. But assuming for the
sake of your question that the decrease in fitness is substantial, what
do you expect? Evolution is generally a slow process. It takes time to
weed nastiness out.
Slainte,
-xx- Damien R. Sullivan X-) <*> http://www.ugcs.caltech.edu/~phoenix
Oh, a blacksmith courted me, nine months and better.
When he'd fairly won my heart he wrote me a letter.
With his hammer in his hand he looked so clever
And if I were with my love I would live forever.