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 to design a paint that sheds water and dirt.
Variability in Form
Aquatic plants are notoriously variable in form, which can make identification challenging. Variation in water depth is a major cause. For example, common bur-reed, which is normally an emergent plant with stiffly erect leaves, will grow in deeper water, but does not flower, and the leaves are less rigid and bend over and float at the tip. Several arrowheads (Sagittaria graminea and S. rigida) grow vegetatively as short sterile rosettes of narrow, pointed leaves in deep water. Sagittaria graminea can even flower under water, but in the absence of flowers it is impossible to tell the rosettes of these species apart visually.
Plants like the water-crowfoots (Ranunculus spp.), false-mermaid (Proserpinaca spp.), and most of the water-milfoils (Myriophyllum spp.) and pondweeds (Potamogeton spp.) have both underwater and floating or emersed leaves. The floating or emersed leaves are simpler and sturdier compared to the submersed leaves of the same plant. In addition, many of the pondweeds vary as to whether floating leaves are produced. Species such as Potamogeton bicupulatus and Potamogeton diversifolius can grow as submergents in deeper water, but often produce floating leaves in shallow water.
Reproduction
Sexual reproduction—Aquatic angiosperms (flowering plants) may produce their flowers under water, at the water surface, or elevated above the water surface. Pollination strategies vary accordingly. For species that hold their flowers above the water surface, insect or wind pollination can proceed as for terrestrial plants. Underwater flowers, such as those of the waternymphs (Najas spp.) and some pondweeds must depend on water currents to transport pollen, much like wind-pollinated plants on land. For annuals like the waternymphs (Najas spp.) and waterworts (Elatine spp.) seed production is essential to maintain populations from year to year. The pollination success rate for perennial species is not quite as critical, since the plants usually live from year to year. In addition, most aquatic perennials also have effective asexual or clonal growth mechanisms (see below).
Some aquatic plants have evolved unique pollination strategies utilizing the water surface as a stage. Species in the Frog’s-bit Family (Hydrocharitaceae) are especially interesting in this regard.
Water-celery (Vallisneria americana) has one of the most unusual pollination methods. Water-celery is dioecious, meaning that male and female flowers are produced on separate plants. Male flowers are released from an inflorescence at the base of the plant to float freely to the water surface where they are dispersed by wind. Meanwhile female flowers on long peduncles just reach the water surface where their tips produce a dimple that attracts the floating male flowers to where the pollen can be transferred directly from anther to stigma (Figure 1.2). We have seen lakes white with male flowers at the leeward end from a large blooming population of water-celery (Figure 1.3). Once pollination has occurred, the peduncle coils tightly, pulling the developing fruit farther under the water surface where it can mature.
The waterweeds (Elodea canadensis and E. nuttallii) and hydrilla (Hydrilla verticillata), also in the Frog’s-bit Family, have similar mechanisms. Pollen is released onto the water surface from male flowers, explosively in the case of hydrilla, where it is carried by wind or water currents to the stigmas of the female flowers which are positioned at the surface.