Species descriptions were drawn from our own extensive experience with the aquatic plants of Pennsylvania and from references and sources listed in the back of the book. References that pertain to a single species are cited in the description of that plant. All species descriptions are accompanied by line art and photographs for further help in confirming the identity of a specimen. Most of the photographs are by the authors; the few exceptions are noted. The botanical illustrations by Anna Aniśko were drawn from live material or herbarium specimens under the direction of the authors.
Distribution maps for each species were prepared using herbarium specimen records from the Pennsylvania Flora Database (2010) plus records from a 2001–2002 study of glacial lakes of northwestern Pennsylvania (Grund undated) and additional observations we made during an eight year study of lakes of eastern Pennsylvania 2000–2007.
Species with similar patterns of growth are grouped together for ease of comparison. A taxonomic list of the plants treated in the book is included in the Appendix.
Acknowledgments
Funding to support costs of publication was received from:
William L., III and Elizabeth P. McLean
Sandra L. McLean
John A. H. and Elizabeth J. Shober
Mt. Cuba Center, Inc., Rick Lewandowski, Director
University of Pennsylvania, Business Services Division
We are also grateful to the following:
The Wild Resource Conservation Program of the Pennsylvania Department of Conservation and Natural Resources, which funded our field work on aquatic plants through annual grants for the years 2000–2007;
Pennsylvania Natural Heritage Program botanists Steve Grund and John Kunsman for access to their collection data;
Bonnie Isaac, Herbarium of the Carnegie Museum of Natural History in Pittsburgh, James Bissell, Cleveland Museum of Natural History, Alina Freire-Fierro, Herbarium of the Academy of Natural Sciences of Philadelphia, and Daniel F. Brunton for access to specimen information from their collections;
Barbara Lathrop, Pennsylvania Department of Environmental Protection Clean Lakes Program, who collaborated with us on vegetation and water sampling at many lakes;
Chris Firestone, Bureau of Forestry, Pennsylvania Department of Natural Resources, who reviewed an early version of the manuscript and provided many helpful suggestions;
the owners and managers of numerous lakes who provided access to us for the purpose of conducting surveys of aquatic plants;
and, the John and Janet Haas Family, and the McLean Contributionship for their ongoing support of botanical research at the Morris Arboretum of the University of Pennsylvania.
Chapter 1. Evolution and Ecology
EVOLUTION AND FORM OF AQUATIC PLANTS
It’s not always easy to decide what to include in a list of aquatic plants. We treat 194 macrophytes (large plants) here, including 178 flowering plants, 2 horsetails, 2 ferns, 5 quillworts, 3 mosses, 2 liverworts, and 2 Charophyte algae. Phytoplankton and filamentous algae are not included. Macrophytes range from plants that grow with their roots under water but most of their stems, leaves, flowers, and fruits above the water surface—the emergent flora—to species that are completely submergent.
The difficulty in choosing what to include arises because of the flexibility of some species, which can grow as emergents for part of the time, but survive long periods without standing water. In selecting the species to include it was our intention to focus on plants that typically grow completely beneath the water surface, floating on water, or with at least their roots in standing water. This does not exclude occasional periods of low water when plants that are normally aquatic may be stranded on a muddy shore.
Most of the aquatic plants treated in this book are flowering plants. They reproduce sexually to produce seeds which are contained in a matured ovary (fruit). These plants also have specialized vascular tissues which make possible the distribution of water and dissolved nutrients throughout the plant body, along with an external cuticle to retain moisture. These are features that allowed plants to expand from their aquatic origins to colonize land early in their evolutionary history. Algae have continued to diversify in aquatic environments including both fresh and salt water, and, in fact, dominate some aquatic habitats.
Continuing evolution of the land flora resulted in secondary adaptation to aquatic environments. This “return to water” has occurred in all major groups of land plants: bryophytes, quillworts, horsetails, ferns, and flowering plants (see Figure 1.1 on page 2). Thirty-six families of flowering plants are represented in this book, including members of the basal angiosperms, monocots, and dicots (Table 1.1).
Table 1.1. Representation of Land Plant Lineages in the Aquatic Flora of Pennsylvania (see Appendix for a full list of species)
| TAXONOMIC GROUP | FAMILIES | SPECIES |
| Bryophytes | ||
| liverworts | 1 | 2 |
| mosses | 3 | 3 |
| Quillworts | 1 | 5 |
| Horsetails | 1 | 2 |
| Ferns | 2 | 2 |
| Flowering plants | ||
| basal angiosperms | 2 | 6 |
| monocots | 14 | 110 |
| dicots | 20 | 62 |
Figure 1.1. Evolutionary relationships of major groups of aquatic plants.
Plants have evolved a variety of growth forms to take advantage of the full range of aquatic habitats. Emergent plants occupy lake and stream margins where their roots can be under water but the stems and leaves are largely above the surface. Rooted plants with floating leaves such as water-lilies (Nymphaea and Nuphar spp.) and watershield (Brasenia) are limited to water depths to about 1.5–2 m. The leaf blades are attached by long petioles to rhizomes imbedded in lake or streambed sediments.
Free-floating plants such as the duckweeds (Lemna spp.), watermeals (Wolffia spp.), and water flaxseed