Aquatic Plants of Pennsylvania. Timothy A. Block

Figure 2.3. Leaf venation with and without prominent mid-vein.

      A. leaves not strap-like, entire blade floating

      C. floating leaves compound, shaped like a four-leaf clover ....................................................................... European water-clover (Marsilea quadrifolia), p. 151

      C. floating leaves simple

      D. floating leaves deeply notched or lobed at base of blade, or petiole attached near center of blade

      E. petiole attached near the center of the blade

      F. leaves very large, roundish, often 30 cm or more across ......................................................................... lotus (Nelumbo spp.), p. 147

      F. leaves elliptical to linear-elliptic, never more than 20 cm across

      G. floating leaves linear-elliptic, to about 2 cm long .................................................................... fanwort (Cabomba caroliniana), p. 272

      G. floating leaves elliptic, 10–15 cm long ....................................................................... watershield (Brasenia schreberi), p. 161

      E. petiole attached at the summit of a deep notch or sinus

      H. flowers borne singly .................................................................. water-lily (Nuphar spp. and Nymphaea spp.), p. 153

      H. flowers in an umbel ................. floating-heart (Nymphoides spp.), p. 143

      D. floating leaves not deeply notched or lobed at base, petiole attached near base of blade

      I. floating leaves in a rosette

      J. leaf blades more than 2 cm long, petioles swollen in the middle .................................. European water-chestnut (Trapa natans), p. 149

      J. leaf blades less than 2 cm long, petioles not swollen ............................................................... water-starwort (Callitriche spp.), p. 229

      I. floating leaves borne singly

      K. veins of the leaf blade parallel ................................................................ pondweed (Potamogeton spp.), p. 186

      K. veins of the leaf blade netted

      L. floating or emersed leaves generally kidney-shaped, 3-to 5-lobed ................... water-crowfoot (Ranunculus spp.), p. 275

      L. floating or emersed leaves lance-shaped to elliptical, not lobed

      M. floating or emersed leaves toothed ....................................................................... Beck’s water-marigold (Bidens beckii), p. 278

      M. floating or emersed leaves entire ........................................................................... smartweed (Persicaria amphibia), p. 104

Key to Thalloid or Tiny Floating Plants

      A. plants without distinguishable leaves and stems

      B. plants appearing to consist entirely of repeatedly branching fine stems or stem-like structures ....... Charophyte algae (Chara spp. or Nitella spp.), p. 164

      B. plants consisting entirely of flattened stems or segments

      C. plants segmented, individual segments elliptical or rounded, solitary or in small clusters, or attached by strap-like stalks ............................................................................. duckweed, watermeal, and water flaxseed, p. 166

      C. plants forming branching colonies of flattened stems

      D. plants attached to rocks in fast-flowing water ................................................................ riverweed (Podostemum ceratophyllum), p. 180

      D. plants free-floating .................................................................. aquatic liverworts (Riccia spp. or Ricciocarpus spp.), p. 163

      A. plants with minutely leafy stems ...................................................................................... eastern mosquito fern (Azolla caroliniana), p. 178

Key to Submergent Leafy-stemmed Plants

      A. leaves scale-like, or stems leafless

      B. stems stiffly erect, leafless or with scattered, scale-like leaves ........................................................................ water-milfoil (Myriophyllum tenellum), p. 289

      B. stems flexuous, more regularly leafy ...................................................................................... aquatic mosses (Fontanalis spp. or Sphagnum spp.), p. 183

      A. leaves not scale-like

      C. submersed leaves alternate

      D. leaves with a basal sheath or sheathing stipules variously fused to the leaf base or free; flowers in a terminal or axillary spike; tepals 4, inconspicuous .............................................. pondweed (Potamogeton spp.), p. 186

      D. leaves without basal sheaths or stipules; flowers terminal; tepals 6, yellow and conspicuous ......................... water star-grass (Heteranthera dubia), p. 221

      C. submersed leaves opposite or whorled

      E. leaves not more than 0.5 mm wide .......................................................................................... pondweed (Zannichellia palustris), p. 220

      E. leaves more than 0.5 mm wide

      F. leaves whorled

      G. leaves mostly in whorls of 3............ waterweed (Elodea spp.), p. 233

      G. leaves mostly in whorls of 4–8

      H. leaf margins very finely toothed; leaves crowded, overlapping ............................................................ Brazilian waterweed (Egeria densa), p. 233

      H. leaf margins distinctly spine-toothed; leaves not overlapping ............................................................ hydrilla (Hydrilla verticillata), p. 185

      F. leaves opposite

      I. leaves with a narrow blade and a broader, sheathing base ................................................................................. waternymph (Najas spp.), p. 223

      I. leaves tapering or truncate at the base, broadest in the blade

      J. leaves linear, spatulate, or obovate, weakly 3-nerved ................................................................................ water-starwort (Callitriche spp.), p. 229

      J. leaves oblong to narrowly obovate, 0–1-nerved ....................................................................................... waterwort (Elatine spp.), p. 237