Hi Sherwin,
I just read Becca's reply and I have some problems with her answer,
especially that last paragraph.
I think both of you are being a little careless with the use of the word
"species". All peaches, for example, are the same species. In the wild,
plants cross pollinate with other (often genetically slightly different)
members of the same species. Sure, there are natural hybrids, but those
are really mistakes of nature.
We humans have selected (or created by selective breeding) the best of
the species, which we propagate, usually by grafting. If you have two
golden delicious trees standing side by side, they are, for all
practical purposes, the same exact tree.
There is more than one species of plum and the two main species do not
pollinate each other. European plums are quite self fertile but Japanese
plums, as I recall, are not. All apple cultivars, generally, can cross
pollinate with the other cultivars. One exception would occur if you had
one that bloomed very early and a second that bloomed very late. They
would just miss each other. The other exception that comes to mind is
the fact that some apple cultivars are triploids and produce sterile
pollen. They can't pollinate anything, not even themselves.

OK, let me finally get to the heart of your question.
Let's say you want to grow a Stanley Plum or a Redhaven peach. Both are
fairly self fertile. If your catalog says to plant two of the same
cultivar for even better pollination, they are just trying to sell more
trees! They might be right if they recommended a different European plum
or a different peach, since then there could be some improvement.

The only possible mechanism that I can think of that could make two
identical trees pollinate better than one is as follows. Suppose you had
an Elberta peach and it was the only fruit tree for a mile in any
direction. If you had another Elberta growing next to it, there would be
twice as many flowers and just maybe that would attract more bees. More
bees would pollinate more flowers and might improve fruit set. That's a
bit of a stretch and it would only matter if your area had a low bee
population.

Steve




Sherwin Dubren wrote:
I have often seen descriptions of certain fruit trees, specified as self
fertile, that their productivity can be increased by having two or more
trees of the same variety nearby. I can't understand what is the
genetic difference between pollen from the same tree and pollen from an
identical species nearby. Is there some other mechanism at work here?

Sherwin Dubren