Aquatic Plants of Pennsylvania. Timothy A. Block
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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 (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 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 from those encountered on land. Water is denser than air and provides support through greater buoyancy. Consequently, submergent and floating aquatic plants have less need for tissues that provide the stiffening that allows terrestrial species to stand erect. At the same time roots, stems (including rhizomes), and petioles of aquatic plants usually have porous, gas-filled columns of tissue (aerenchyma) that contribute to buoyancy and allow diffusion of carbon dioxide and oxygen throughout the plant.
The vascular tissues of submergent aquatic plants are also greatly reduced. Consequently, these plants tend to be limp and to collapse in a soggy heap when removed from the water. In order to prepare herbarium specimens, it is necessary to float the plants in a tray of water, slide a piece of mounting paper under them, and carefully lift them out of the water so the stems and leaves are spread in a lifelike manner as they are when floating in a column of water.
Terrestrial plants have evolved various coverings (cuticle, surface hairs, etc.) to prevent excess water loss through evapotranspiration, a problem not encountered under water. Submergent aquatics typically have thin leaves with little or no cuticle; as a result they shrivel quickly when removed from water. Floating leaves, on the other hand, typically have a cuticle on surfaces exposed to the atmosphere.
In order to resist the force of moving water, aquatic plants have evolved strong anchoring features such as rhizomes that are buried in lake and stream bottom sediments, or various forms of holdfasts by which species like water moss (Fontanalis spp.) and riverweed (Podostemum ceratophyllum) cling to rocks and other surfaces. Flexibility is another important characteristic in water plants; many species have long flexible stems or leaf petioles that can move with the water currents or waves. The petioles and peduncles of water-lilies are longer than the distance from the water surface to the rhizome to which they are attached, to allow for wave action; in addition, the petioles are firmly attached near the middle of the leaf blade. The rounded shape and smooth margins of the leaf blades also reduce resistance. The underwater leaves of many species such as the water-crowfoots (Ranunculus spp.), coontail (Ceratophyllum sp.), bladderworts (Utricularia spp.), and water-milfoils (Myriophyllum spp.) are finely divided, thus offering less resistance to water currents and also increasing the surface area for absorption of carbon dioxide.
Stomata are pores that allow for the movement of gasses such as carbon dioxide and oxygen in and out of leaves. In terrestrial plants stomata are typically on the lower leaf surfaces; floating leaves of aquatic plants have their stomata on the upper surface, providing access to atmospheric carbon dioxide. Submersed leaves have few or no functional stomata because carbon dioxide and oxygen dissolved in water enter the leaf, or in some cases, the roots, by diffusion.
Emergent aquatics are more like terrestrial plants in many ways; however, their roots must be able to withstand constant inundation and the stress of wave action. Many are rhizomatous, providing a network of anchoring structures. Porous tissues (aerenchyma) that allow oxygen to diffuse through stems, rhizomes, and roots are often present. Even the leaves of emergent species show adaptations such as blades with an arrowhead shape that offer less resistance to the forces of wind and water.
Some emergent species have evolved leaf surfaces that shed water. This characteristic is most notable in American lotus (Nelumbo lutea), goldenclub (Orontium aquaticum), and northern mannagrass (Glyceria borealis). The hydrophobic quality is the result of tiny rounded, wax-covered projections called papillae that cover the leaf surface (Neinhuis and Barthlott 1997). Engineers in Germany have used the leaf surface of lotus as a model