Whether Lemnoids have a single
flower or a composed inflorescence cannot be determined at this point.
Traditionally “Lemnaceae” have
been interpreted as having an inflorescence. I emphasize here
”interpretation”, and the extremely reduced Wolffia is not the right starting
point for a comparison with Amphophallus.
One reason for the
inflorescence interpretation is the un-equal development of stamens in Lemna
and Spirodela: Stamen-carpel-Stamen. This sequence is unusual
for flowers, where the organs of one identity tend to develop together, but not
unusual for inflorescences, if each organ (or a group of not more than two) is
considered a strongly reduced flower. The other reason is just
tradition – or burden – from the past, when Lemoids were compared with Pistia
(which is now rejected, see below).
1) Blurred Distinction:
Buzgo & Endress (2000) and
Buzgo (2001) described that strong unidirectional development of flowers
can lead to a mixed up sequence of flower organ development; particularly in
reduced inflorescences. Buzgo et al (2006) described that the distinction
of meristem identity of flower and inflorescence can be blurred in reduced
flowers, and the debate whether reduced structures represent a flower or
inflorescence is not serving anybody.
2) Phylogeny and Ancestral
Looking at the molecular
phylogeny of Araceae (Cabrera et al. 2008), we see that the Lemnoideae clade
inserts at the second node of the grade of basal Araceae, between the basal
most node to the sister clade with Orontioideae and Gymnostachys, and the third
node to Pothoideae (+Anthurium) and Monsteroideae. That is, Lemnoideae
are “surrounded” by clades with bisexual and pethal-bearing flowers! It
is plausible that the ancestor of all Lemnoideae also had bisexual,
petal-bearing flowers. It is not necessary (not parsimonious) to
assume that the structures in Lemanoideae represent several distinct flowers.
3) The Pistia Legacy
The reason for sticking with
the interpretation of Lemnoid reproductive structures as inflorescence is a legacy
or burden from the past, when Pistia was often proposed to be related to
Lemnoids (Engler, Stockey et al. 1997). This relation of Pistia to
Lemnoids has been rejected now several times (rev. Cabrera et al. 2008; also
Renner et al. 2004), and no longer bears significance for the interpretation of
the flower in Lemnoids. Just drop it!
As a conclusion, the structure
may well represent a single flower, possibly as the result of a strongly
reduced inflorescence (point 1 above), with the tubular membrane in some
representing a bract (spathe) or a perianth organ (since it lacks an axillary
meristem, “perianth” would be appropriate).
Below some Literature – I hope
Matyas Buzgo, PhD
Dept. of Biological Sciences,
One University Place
Shreveport, LA 71115, USA
(318) 797 5120 office
(318) 797 5222 fax
+ Arber, A. 1919. The
vegetative morphology of Pistia and the Lemnaceae. Proc. Roy. Soc. London 91 B:
+ Bogner, J. 2000. Friedrich
Hegelmaier (1833-1906) and the Lemnaceae. Aroideana 23: 4-8
+ Buzgo, M. 1994. Inflorescence
development of Pistia stratiotes (Araceae). Bot. Jahrb. Syst. 115 (4): 557-570
+ Buzgo, M., Endress, P.K.
2000. Floral structure and development of Acoraceae and its systematic relationships
with basal angiosperms. Int. J. Plant Sci. 161 (1): 23-41
+ Buzgo, M. 2001. Flower
structure and development of Araceae compared with alismatids and Acoraceae. Bot. J. Linn. Soc. 136 (4):
+ Buzgo, M. Soltis,
D.E., Soltis, P.S., Kim, S., Ma, H., Hauser, B.A., Leebens-Mack, J. Johansen,
B. 2006. Perianth
development in the basal monocot Triglochin maritima (Juncaginaceae). - In:
Columbus, J.T., Friar, E.A., Porter, J.M., Prince, L.M., Simpson, M.G. (eds),
Monocots: Comparative Biology and Evolution, (excluding Poales). ALISO 22:
107-125 (Claremont, CA, USA: Rancho Santa Ana Botanic Garden, ISSN:0065-6275)
+ Cabrera, L.I., Salazar, G.A.,
Chase, M.W., Mayo S.J., Bogner, J., Davila, P. 2008 Phylogenetic Relationships
of Aroids and Duckweeds (Araceae) inferred from coding and non-coding plastid
DNA. Am. J. Bot. 95 (9): 1153-1165
+ Caldwell, O.W. 1899. On the
life-history of Lemna minor. Bot. Gaz. 27: 37-66
+ Landolt, E. 1980a.
Biosystematische Untersuchungen in der Familie der Wasserlinsen (Lemnaceae). Veröff. Geobot. Inst. ETH Rübel 70 (1):
+ Landolt, E. 1980b.
Biosystematic investigations in the family of duckweeds (Lemnaceae), vol2. The
family of Lemnaceae - A monographic study. Veröff. Geobot. Inst. ETH Rübel 71:
+ Landolt, E. 1986. The family
of Lemnaceae - a monographic study. Vol. 1 of the monograph: Morphology,
karyology; ecology; geographic distribution; systematic position; nomenclature;
descriptions. Veröff. Geobot. Inst. ETH Rübel 71 (2): ??
+ Landolt, E. Kandeler, R. 1987.
Biosystematic investigations in the family of duckweeds (Lemnaceae), vol. 4.
The family of Lemnaceae - A monographic study. Veröff. Geobot. Inst. ETH Rübel 95 (1):
+ Renner, S.S., Zhang,
L.-B. 2004. Biogeography of
the Pistia clade (Araceae): Based on chloroplast and mitochondrial DNA
sequences and Bayesian divergence time inference. Syst. Biol. 53 (3): 422-432
+ Stockey, R.A., Hoffman, G.L.,
Rothwell, G.W. 1997 The fossil monocot Limnobiophyllum scutatum: Resolving the
phylogeny of Lemnaceae. Am. J. Bot. 84 (3): 355-368