Dick Chambers wrote:
I know, of course, that edible fruit has evolved by natural selection. The
sweet-tasting fleshy fruits attract animals, who eat the fruit, spit out the
seeds, and inadvertently sow the next generation of the plant. This
evolution process may have started, millions of years ago, with a
not-very-satisfying fruit, and natural selection has caused the evolution of
the fruit into the sweet and fleshy forms we know today. The tastier and the
fleshier the fruit, the better it will attract animals to spread the seed.

Not necessarily. Take Trillium fruits. They're sort of fleshy -- but
the attraction to the disperser is in the aril on the seed. It's full
of fatty acids and sugars; and the ants (and yellow jackets) love them.
They carry the seed off, eat the aril and leave the seed behind. The
fruit serves not only as a vehicle for dispersal, but also to protect
the seed.


My question is:- Why did the prototype form of fruit ever exist in the
first place, many millions of years ago? Why did seeds ever have fleshy
fruit around them, however thinly and however bitter-tasting, from which the
evolution process could begin? Could the flesh of the fruit have been a
botanical form of placenta, separating the DNA of the mother plant from the
DNA of the seed? i.e. performing a similar function to that of the
placenta in the animal kingdom. Is there any evidence to support this
hypothesis? Does the flesh in (for example) the modern peach perform the
function of DNA-separation, in addition to its well-known function of
attracting an animal to eat it and spread the seed?

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.


What function does the flesh of the berry have in Deadly Nightshade? As the
berries are poisonous, the plant does not rely on animals to spread the
seed. The flesh of the berry would therefore seem redundant, unless it also
has some other function, such as a botanical form of placenta.


Just because the berries are poisonous to humans, doesn't necessarily
mean that they're poisonous to other critters. I believe that birds
are the dispersers in those Solanums.

Susan