differ from Wolffiella by having thick, nearly
globose fronds and flowers that are located
medially (as opposed to off-center) on the upper
frond surface (Landolt 1986).
As the lineage most distantly related to the
other Lemnaceae, it is tempting to interpret
Spirodela as having morphological features that
are ancestral for the family, yet it is important to
bear in mind that the genus also acquired
synapomorphic traits during its evolution. Spirodela species have fronds with the largest number of veins and roots, and at least some of their
roots do not perforate the associated scale-like
leaf. Landoltia punctata is a distinct duckweed
lineage, with fewer leaves per frond than Spirodela species, and roots that all perforate the
associated leaf (Landolt 1986).
There are several synapomorphies that help
define the clade of Lemna + Wolffia + Wolffiella.
Species belonging to these genera have a disporic
embryo sac (versus monosporic in Landoltia and
Spirodela), and they also differ from other
Lemnaceae by lacking dorsal/ventral scales.
Lemna species, in addition to their reduced
vein number and root number, differ from other
Lemnaceae by lacking root tracheids, guard cell
plastids, and anther pigment cells. Their crystal
cells consist only of raphides, without the additional druses that are produced by Landoltia and
Spirodela (Landolt 1986). Phylogenetic data
support the monophyly of section Alatae (L.
aequinoctialis and L. perpusilla), containing two
species with short, sharply pointed roots with
sheaths that are winged at the base. Lemna tenera
constitutes the sole species in section Biformes,
and it differs from most other Lemna species in
having elongate fronds that commonly grow
submersed. Obvious synapomorphies are lacking
for the sibling sections Alatae and Biformes.
Species in the monophyletic section Uninerves
(L. minuta, L. valdiviana, L. yungensis) share the
trait of having fronds with only one nerve each
(Landolt 1986, 1998). Sections Alatae, Biformes,
and Uninerves together are monophyletic. The
remaining Lemna species belong to section
Lemna, including the morphologically distinct L.
trisulca, which molecular data cannot sufficiently
distinguish from the rest of the section. A number
of morphological characters do distinguish section Lemna, however, including relatively long
(>3 cm) roots with sheaths that are not winged
and tips that are mostly rounded; fronds often are
dotted or completely red beneath, and ovules are
orthotropous.
Wolffia and Wolffiella are sibling lineages
assigned to subfamily Wolffioideae (Landolt
1986). Besides lacking roots and frond veins,
Wolffioideae have a great deal of morphological
features in common that distinguish them from
the rest of Lemnaceae, including the lack of a
floral organ prophyllum and crystal cells, and
having a single reproductive pouch (versus two
in the rest of the family), unilocular anthers
(versus bilocular), ribbed seeds (versus smooth),
a single stamen per flower (versus two), apical
anther dehiscence (versus transverse), and an
ovary inserted at the base of the stamen (versus
above the stamens).
Molecular data suggest that species of Wolffia
do not all comprise a clade, but they share quite a
few features, including thick fronds and flowers
that emerge from the middle of the frond. Turions are widely present in genus Wolffia, with the
exception of W. microscopica, whereas turions
otherwise are found in only a few Lemnaceae
species (S. polyrhiza, Lemna aequinoctialis, and
L. turionifera). The Wolffia species that are most
difficult to place phylogenetically also are distinct morphologically. In addition to being the
only Wolffia species without turions, W. microscopica (the sole species in section Pseudorrhizae) also features a conical appendage on the
lower frond surface. The two species belonging
to section Pigmentatae, W. borealis and W.
brasiliensis, have pigment cells in their vegetative tissue (visible on dead fronds). Wolffia australiana, which Landolt (1986) placed in section
Wolffia on morphological grounds, clearly is
separate from that group phylogenetically (Les
et al. 1997; Tippery et al. 2015) and remains
unassigned to a section. Section Wolffia, containing the largest number of species, resolves
consistently as monophyletic with strong support; species in this section uniformly have
globular to ovoid fronds lacking pigment cells
and conical appendages.
2 Tiny Plants with Enormous Potential: Phylogeny and Evolution …
27
globose fronds and flowers that are located
medially (as opposed to off-center) on the upper
frond surface (Landolt 1986).
As the lineage most distantly related to the
other Lemnaceae, it is tempting to interpret
Spirodela as having morphological features that
are ancestral for the family, yet it is important to
bear in mind that the genus also acquired
synapomorphic traits during its evolution. Spirodela species have fronds with the largest number of veins and roots, and at least some of their
roots do not perforate the associated scale-like
leaf. Landoltia punctata is a distinct duckweed
lineage, with fewer leaves per frond than Spirodela species, and roots that all perforate the
associated leaf (Landolt 1986).
There are several synapomorphies that help
define the clade of Lemna + Wolffia + Wolffiella.
Species belonging to these genera have a disporic
embryo sac (versus monosporic in Landoltia and
Spirodela), and they also differ from other
Lemnaceae by lacking dorsal/ventral scales.
Lemna species, in addition to their reduced
vein number and root number, differ from other
Lemnaceae by lacking root tracheids, guard cell
plastids, and anther pigment cells. Their crystal
cells consist only of raphides, without the additional druses that are produced by Landoltia and
Spirodela (Landolt 1986). Phylogenetic data
support the monophyly of section Alatae (L.
aequinoctialis and L. perpusilla), containing two
species with short, sharply pointed roots with
sheaths that are winged at the base. Lemna tenera
constitutes the sole species in section Biformes,
and it differs from most other Lemna species in
having elongate fronds that commonly grow
submersed. Obvious synapomorphies are lacking
for the sibling sections Alatae and Biformes.
Species in the monophyletic section Uninerves
(L. minuta, L. valdiviana, L. yungensis) share the
trait of having fronds with only one nerve each
(Landolt 1986, 1998). Sections Alatae, Biformes,
and Uninerves together are monophyletic. The
remaining Lemna species belong to section
Lemna, including the morphologically distinct L.
trisulca, which molecular data cannot sufficiently
distinguish from the rest of the section. A number
of morphological characters do distinguish section Lemna, however, including relatively long
(>3 cm) roots with sheaths that are not winged
and tips that are mostly rounded; fronds often are
dotted or completely red beneath, and ovules are
orthotropous.
Wolffia and Wolffiella are sibling lineages
assigned to subfamily Wolffioideae (Landolt
1986). Besides lacking roots and frond veins,
Wolffioideae have a great deal of morphological
features in common that distinguish them from
the rest of Lemnaceae, including the lack of a
floral organ prophyllum and crystal cells, and
having a single reproductive pouch (versus two
in the rest of the family), unilocular anthers
(versus bilocular), ribbed seeds (versus smooth),
a single stamen per flower (versus two), apical
anther dehiscence (versus transverse), and an
ovary inserted at the base of the stamen (versus
above the stamens).
Molecular data suggest that species of Wolffia
do not all comprise a clade, but they share quite a
few features, including thick fronds and flowers
that emerge from the middle of the frond. Turions are widely present in genus Wolffia, with the
exception of W. microscopica, whereas turions
otherwise are found in only a few Lemnaceae
species (S. polyrhiza, Lemna aequinoctialis, and
L. turionifera). The Wolffia species that are most
difficult to place phylogenetically also are distinct morphologically. In addition to being the
only Wolffia species without turions, W. microscopica (the sole species in section Pseudorrhizae) also features a conical appendage on the
lower frond surface. The two species belonging
to section Pigmentatae, W. borealis and W.
brasiliensis, have pigment cells in their vegetative tissue (visible on dead fronds). Wolffia australiana, which Landolt (1986) placed in section
Wolffia on morphological grounds, clearly is
separate from that group phylogenetically (Les
et al. 1997; Tippery et al. 2015) and remains
unassigned to a section. Section Wolffia, containing the largest number of species, resolves
consistently as monophyletic with strong support; species in this section uniformly have
globular to ovoid fronds lacking pigment cells
and conical appendages.
2 Tiny Plants with Enormous Potential: Phylogeny and Evolution …
27