Susan wrote

[ . . . ]

The fruit both protects the seed and acts as an aid to dispersal.
Gymnosperms (in case you don't know, the name means "naked seed") have
only seeds with no fruit. The pine seeds are attached to ovuliferous
bracts inside the pine cone. It's similar to maple fruits in that that
scale is a papery thing so they're wind dispersed.

DNA isn't an issue is plants because of the way that plants reproduce.
In lower plants, like ferns, it's easy to see what we call the
Alternation of Generations. The fern body is diploid sporophyte tissue
-- it makes the spores. The spores then germinate into a tiny
structure called a gametophyte which produced the eggs/ovules and
sperm. Fertilization takes place, and a new fern body is produced.
The concept is an old one (Foster & Gifford give 1862) and it's the
metaphysical equivalent of cats having puppies which in turn have
kittens. They didn't know haploid and diploid -- they only knew that
the progeny of a fern was not a fern, it was this little heart-shaped
thing -- and the progeny of *that* was another fern.

The change is more fundamental though than just presence or absence of
a fruit. It's the presence or absence of an ovary. In Gymnosperms,
the ovule itself sits on that scale; in Angiosperms, the ovules are
protected inside an ovary. In most cases, the fruit then, is made of
of ovary tissue. Fruits are kinda cool -- you have diploid parent
tissue (the "meat" of the fruit) and diploid progeny tissue (the
embryo) -- and then there's the triploid endosperm).

The flesh of the fruit may be composed of different types of tissue.
In a tomato, for instance, I know that the "meat" is placenta tissue.
(that tussue that connects the seed to the ovary wall is indeed called
placenta). In a bean, the placenta is that little bitty stub that
connects the bean to the ovary wall -- and the "meat" of the fruit is
just the ovary wall. In an apple, the "meat" of the fruit is
hypanthium tissue -- the ovary wall is that tough membrane that
surrounds the seeds. Sometimes, like in Coconuts, both coconut milk
and coconut "meat" are endosperm; popcorn is also endosperm.

Plants don't have an immune system like people do, so there's no issue
with mixing the different DNAs. (As an aside, it's not the DNA that's
the problem in humans -- it's the proteins on the blood that can be
problematic. During pregnancy, the mother's immune system is
"suppressed" for lack of a better word. There's one school of thought
that thinks that it might be an immune response that triggers labor,
but I digress). There is a plant pathogen response, but it's a really
complicated mixture of virulence genes in both pathogen and host plant.

[ . . . ]

Thank you, Susan, for the trouble you have taken in providing this
comprehensive reply. This was the level of detail I was looking for,
informing me exactly where the underlying assumptions of my original
question were incorrect. The incorrectness of my assumptions has already
been noted in less detail by Mr van Rijckevorsel's third posting - but at
least as a result of your reply I now know why they were incorrect.

Richard Chambers Leeds UK.