Note: This email was sent to the wrong address yesterday and I just
now noticed it so am forwarding it to the list.
In my experience, Z. grandiflora is seed-sterile and
pollen-fertile. The question is whether the species is
self-incompatible, or if something is wrong with the embryo sacs, or
even if there has been a change in the gametic DNA imprinting so that
it is not able to produce endosperm. A clearing study with methyl
salicylate or an optimized mixture of butyl benzoate and dibutyl
phthalate could easily determine if the embryo sacs are mature and
structurally normal at anthesis, but it would require a specialized
differential interference-contrast or confocal microscope. Such a
study could also determine if fertilization and incipient endosperm
divisions occur before the ovary aborts.
Also in my experience, Z. macrosiphon flowers are similar to
Z. grandiflora, except for the stigmatic placement. This apomictic
species sets maternal seed abundantly and does not offset like Z. grandiflora.
Charles Crane