Wherefore poison in apple seeds?

[Spoke]

My protective mother went to great lengths to impress upon me as a child that apple seeds were poisonous. Science confirms my mother's wisdom. (http://museum.gov.ns.ca/poison/roses.htm)

So how come? I mean to say, from an evolutionary perspective, why would apple seeds be poisonous? Was it part of a grisly scheme by the apple trees to trick our forebears into eating their fruit and then nourishing their seeds on the rotting remains of our poisoned carcasses? (shudder)

Or maybe it was just a seed distribution mechanism. The tree's "thought" being that we humans would take the apple away, eat the delicious fruit, and then toss the seed-filled core somewhere far away from the parent tree. Is that it?

Also, are there any plants that really do fertilize their seeds with the bodies of animals foolish enough to eat them?

[Yeah]

Some years ago I noted that when horses eat apples, they eat the core, seeds and all without obvious ill effects. Since then, I have eaten apple cores. Even thought I have eaten MORE THAN ONE SEED at a time, I have never notices "anxiety, confusion, dizziness, headaches & vomiting."

According to The Chemical Institute of Canada/L'Institut de chimie du Canada (http://www.chem-inst-can.org/ncw/articles/efood.html
"Apple seeds contain cyanide but you would have to eat kilograms of apple seeds to do yourself any harm."

[DrFidelius]

Just a WAG, but chemicals in the seeds to make them bitter or otherwise distastful would discourage frugivores from eating the seeds. Remember, wild apples are not much larger than rose hips, and were most likely eaten by small animals (rodents and birds). They were not tasty enough or abundant enough to attract the attention of animals large enough to not notice their level of poison.

[sethdallob]

The answers above are close: I wrote my piece on fruit reproduction here:

http://boards.straightdope.com/sdmb/showthread.php?threadid=63622

Seeds contain just a small amount of poison; a little once in a while won't do much - if you do it excessively though you are asking for trouble. The poison is there in case some animal got the bright idea to break the seed open and eat it's contents. Horses (and humans) can't break up seeds, so it, erm, gets some free fertilizer on the way out.