Aquatic Plants of Pennsylvania: A Complete Reference Guide

Chapter 1. Evolution and Ecology

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 Block-Rhoads_Chap01_Final.indd 1 3/21/11 1:30 PM 2 E V O L U T I O N A N D E C O L O G Y 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 (Spirodela punctata) are independent of water depth, but winds and waves usually push them toward the lake or stream margins except on small ponds, where they may cover the entire surface. Rooted submergent species such as waterweed (Elodea spp.) are limited to depths where light penetration is sufficient to support photosynthesis. This can vary from less than 1 m in very turbid water to 3–4 m or more in exceptionally clear lakes. Other submergent species such as Eurasian water-milfoil (Myriophyllum spicatum), hydrilla (Hydrilla verticillata), and many pondweeds (Potamogeton spp.) typically produce a long stem that only branches when it approaches the water surface. It has been shown that low light promotes shoot elongation and inhibits branching. High light availability has the opposite photosynthetic ancestor vascular plants Bryophytes Charophytes green algae Figure 1.1. Evolutionary relationships of major groups of aquatic plants. Block-Rhoads_Chap01_Final.indd 2 3/21/11 1:30 PM E V O L U T I O N A N D E C O L O G Y 3 effect, inhibiting further increase in stem length and stimulating branching. The result is to place the bulk of the leaves just below the water surface where photosynthesis can be most efficient. Modifications for Life Under Water An aqueous environment poses challenges different...