A true graft hybrid is a plant that has genetic material from both
parents in all cells and which has arisen from cells right at the
graft union. True graft hybrids are very rare, because you have to get
actual mixing of cell contents and then development of a shoot from
these mixed cells. I don't know if lemons and oranges would do this.
IF it happened, the resulting hybrid would bear something intermediate
between an orange or a lemon, not oranges AND lemons.

BUT, citrus varieties and species are often widely compatible as
grafting scions and stocks, so it would be possible to have a whole
tree full of, say, different orange varieties. Not sure if oranges
and lemons are compatible as scion and stock, but I'd bet both are
graftable to trifoliate orange, which is widely used as a stock for
citrus. If they are, then you could have a tree that bears both
oranges and lemons. A bit of research from your author should turn up
whether both share a compatible stock and if so, when it was first
use--and make sure that the horticultural terms used in the book are
accurate.

Monique Reed

Iris Cohen wrote:

I received the following from an author in New Zealand. Maybe some of you can
give a more definite answer.
From
I am a New Zealand novelist and my third novel is
about to be published. In it I refer to a graft hybrid citrus
tree created by one of my characters, an American
orange-grower of the 1890s. He creates a tree that
produces both oranges and lemons. I am very much
hoping you might be able to tell me whether, in theory,
this is possible!
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)