Aquatic Plants of Pennsylvania. Timothy A. Block
or leaves absent
B. leaves absent
C. stems jointed; tiny, scale-like leaves in whorls at the nodes ................................................................... water horsetail (Equisetum spp.), p. 88
C. stems not jointed, not branching in whorls
D. inflorescence appearing lateral ....... bulrush (Schoenoplectus spp.), p. 47
D. inflorescence terminal ................. spike-rush (Eleocharis spp.), p. 111
B. leaves present
E. leaves all basal
F. leaves expanded at the base and containing spore sacs ...................................................................... quillwort (Isoetes spp.), p. 245
F. leaves not expanded at the base, lacking spore sacs
G. leaves mostly not more than 10 cm long ................................................................... pipewort (Eriocaulon spp.), p. 243
G. leaves mostly more than 50 cm long ................................................................ flowering-rush (Butomus umbellatus), p. 76
E. at least some leaves cauline
H. perianth ............................................. showy iris (Iris spp.), p. 90
H. perianth not showy, reduced to a few bristles, or absent
I. individual flowers of the inflorescence subtended by 1 or 2 bracts
J. stems mostly triangular at the base; each flower subtended by 1 bract
K. flowers in spikelets with 2-ranked scales ......................................................... three-way sedge (Dulichium arundinaceum), p. 121
Figure 2.2. Leaf expanded at the base, containing spore sac (×3/8).
K. flowers in spikelets with spirally arranged scales
L. spikelets with 1 perfect flower and 1 achene ............................................. twig-rush (Cladium mariscoides), p. 123
L. spikelets with many perfect flowers and many achenes .................................. bulrush (Scirpus ancistrochaetus and Schoenoplectus spp.), p. 47
J. stems round at base; each flower subtended by 2 bracts ....................................... grasses (Echinochloa, Glyceria spp., Leerzia oryzoides, Torreyochloa pallida, Zizania aquatica), p. 79
I. individual flowers of the inflorescence not subtended by bracts
M. flowers perfect ................ bayonet rush (Juncus militaris), p. 102
M. flowers unisexual
N. flowers in a dense, elongate spike; leaves flat .......................................................... cat-tail (Typha spp.), p. 71
N. flowers in spherical heads; leaves keeled, at least at the base .................................... bur-reed (Sparganium spp.), p. 61
A. emergent leaves broad, their blades less than 7 times longer than broad
O. plants woody at the base; branches arching over the water’s edge ............................................................ water-willow (Decodon verticillatus), p. 137
O. plants herbaceous throughout
P. leaves very large, roundish, often 30 cm or more across .................................................................................... lotus (Nelumbo spp.), p. 147
P. leaves never more than 20 cm across
Q. leaves compound
R. leaves opposite ............ beggar-ticks/bur-marigold (Bidens), p. 42
R. leaves alternate
S. plants to 2 m tall; leaflets toothed ................................................................ water-parsnip (Sium suave), p. 128
S. plants seldom more than 0.5 m tall; leaflets entire ..................................................... watercress (Nasturtium officinale), p. 141
Q. leaves simple
T. leaves opposite ....................... beggar-ticks/bur-marigold (Bidens), p. 42
T. leaves alternate
U. leaf blades lobed or truncate at the base
V. leaves kidney-shaped, mostly broader than long .................................................................. mud-plantain (Heteranthera spp.), p. 96
V. leaves longer than broad
W. leaf blades shallowly lobed, cordate, or truncate at the base
X. inflorescence a spike ............................................................................. lizard’s-tail (Saururus cernuus), p. 94
X. inflorescence a panicle .................................................................................... water-plantain (Alisma spp.), p. 129
W. leaf blades deeply lobed at base
Y. inflorescence a dense fleshy spike enclosed in a sheathing bract .............................................................................. arrow-arum (Peltandra virginica), p. 29
Y. inflorescence not enclosed in a sheathing bract
Z. flowers purple ............................................................................. pickerel-weed (Pontederia cordata), p. 100
Z. flowers white ........ arrowhead (Sagittaria spp.), p. 31
U. leaf blades tapering at both ends
AA. leaves parallel-veined; inflorescence a thick spike of yellow flowers ...................... goldenclub (Orontium aquaticum), p. 77
AA. leaves net-veined; inflorescence various
BB. emergent leaves toothed or lobed .......... mermaid-weed (Proserpinaca spp.), p. 273
BB. emergent leaves entire
CC. emergent leaves 2–5 mm long
DD. flowers unisexual, petals absent ......................... water-starwort (Callitriche terrestris), p. 229
DD. flowers bisexual, petals white .................................. Nuttall’s mud-flower (Micranthemum micranthemoides), p. 14
CC. emergent leaves more than 5 mm long
EE. inflorescence spike-like .................................................................... smartweed (Persicaria spp.), p. 104
EE. inflorescence not spike-like
FF. flowers yellow; fruits 2 cm or more long ............ water-primrose or primrose-willow (Ludwigia spp.), p. 132
FF. flowers pale violet to white or inconspicuous
GG. flowers small, inconspicuous, radially symmetrical; leaves ovate to obovate....water-purslane (Ludwigia palustris), p. 136
GG. flowers pale violet to white, 2-lipped; leaves linear to lanceolate ................. water-willow (Justicia americana), p. 137
Key to Floating-leaf Plants
A. leaves strap-like or ribbon-like, mostly submerged,