according to your important and innovative paper
in Aquatic Botany 50, 1995, and to the highly interesting
paper by Stockey et al. 1998 on Limnobiophyllum
from morphological features (but not pollen characters)
usually Lemnaceae are considered to be closely related to Pistia
(Araceae), and Limnobiophyllum (=Pandanaiidites?)
bridging the gap (e.g., Mayo et al 1998 in Kubitzki). However,
the pollen morphology differs greatly and significantly: Pistia pollen
is inaperturate and plicate, Lemnaceae pollen is spiny and ulcerate.
The latter facts are well known (again e.g., Mayo et al 1998 in Kubitzki).
Recent findings on pollen features reveal further strong arguments
against a close relationship of Lemnaceae and/or Limnobiophyllum
to Pistia and its Aralean relatives.
Pistia and its Aralean relatives always have inaperturate pollen with
uncommon stratification found so far only in Aroideae with an outer
quite unusual sporopollenin-less layer and an also uncommon spongy endexine
(e.g., Weber et al. 1998, 1999, also for references).
Lemnaceae have a "classical" spiny tectate-columellate ektexine and a compact
thin endexine, and a simple porus without an annulus.
Limnobiophyllum is similar to Lemnaceae, but differs in two points: another exine
stratification (massive tectum and footlayer, very low columellae)
and an annulate porus.
In sum I see a palynological relationship between Lemnaceae and Limnobiophyllum,
but a clear, palynologically unbridgeable gap between Lemnaceae/Limnobiophyllum
at one hand and Pistia (with its related genera of Aroideae) on the other hand.
Palynologically, Lemnaceae and Limnobiophyllum can only be related
to the aperturate Araceae, i.e. NOT to the Aroideae subfamily.
Only the large genus Anthurium with porate pollen has some spiny species,
but I think this is no good candidate for a relative to Lemnaceae.
Other spiny Araceae are inaperturate. No sulcate one is spiny.
Find a forwarded mail in this matter to Dr. Cross, to whom
I also send this mail.
I would be very glad to find your comments
With very best wishes