Assimilation is defined as simply the ability to
absorb something in a manner that it becomes part of
the physical makeup of whatever ingested it.

When Sasami took a header of the platform in the OAV 9
flashback sequence, the force of her impact with the
floor killed her.  But her physical essence (her
blood) and soul became mixed in with Tsunami's
quintessence.

Remember, the soul is quintessence as it manifests in
sentient life.  And by a couple of definitions I won't
bother you with here, the soul of a child is very
close to quintessence in its pure and uncontaminated
form.

Because of the accident, Sasami effectively became
apart of Tsunami (though whether the reverse is true
is uncertain), and because her biological functions
did not cease after death, she would "grow up" to
resemble Tsunami.  

The real question is to whether assimilation can be
reversed, or nullified in a manner that whatever was
absorbed can pass back into life.

It also appears that Tsunami isn't actually associated
with any tree, contrary to the impression created by
the Jurai, but is rather contained within the water
they derive sustenance from.  

No... That's not quite correct....

Whatever source there is for the water, is where you
will find Tsunami.

Peter