Submerged plants

Common Eelgrass, Zostera marina Photograph: Marieke van Katwijk

Completely submerged water plants like seagrasses and pondweed (Elodea canadensis) lack stomata. The leaves are mostly thin (e.g. Zostera marina, here left) or thread-like (round but with a small diameter; e.g. Syringodium isoetifolium). In general, a favorable ratio of surface (high) with respect to volume (small) is encountered. Fresh water plants have no water balance problems. They exchange gas optimally without risk of dessication through the thin cuticle which is air-permeable; the epidermis is also thin or event absent. Seagrasses by contrast have to struggle against the high salinity in their environment. The external cell wall is not so thin as in fresh water plants. The vascular system in water plants is reduced in general. Their buoyancy derives from large intercellular cavities that protrude into stems and roots and thus facilitate oxygen transport (Microscopical fluorescence and bright field views of aerenchym in a Zostera leaf). The intensity of light underwater is relatively low; a true palisade parenchyma layer is often lacking.
 

Seagrasses

Seagrasses are often confused with algae, but they are fundamentally different. While algae are primitive plants, seagrasses have true leaves, stems, and often rhizomes and a root system with which they can have hold in the soil and they develop flowers that are involved in sexual reproduction. Thus, seagrasses as flowering plants, can produce pollen, fruits and seeds under water. (Pollen is indicated by an arrow on the photograph of flowering Common Eelgrass, Zostera marina, here right; Photograph by Dr. J. D. Ackerman, used with permission).
The surface of leaves of seagrasses forms also a substrate on which all kinds of epiphytic algae can life (fluorescence view of chloroplasts in a Zostera leaf and epiphytes; stereo-anaglyph to be watched with a green filter for the left eye, and red one right, and a monochrome gif animation).
 

Research on seagrasses at the Radboud University Nijmegen
mouse-over the species names here below to see the corresponding macro view		Because of their great ecological importance as a nursery for fishes, crabs and other biodiversity, as sediment stabilizer and as forage for tropical dugongs and sea turtles, seagrasses and their environment became an important research object of the Departments of Environmental Science (website), Environmental Biology, Animal Ecology and Ecophysiology (website) of the Radboud University Nijmegen. One of the seagrass projects focuses on the transplantation of seagrasses (in Dutch) in the Eastern Scheldt (more about seagrasses in the Netherlands). Another example is the PhD research program of Marjolijn Christianen on the effect of nutrient loads and grazing by sea turtles on tropical seagrass (website). Overview of all seagrass projects RU and marine and estuarine research RU. Here below: Microscopic views of seagrasses from Indonesia studied by Marjolijn. Striking in these views are the open structure of the leaves and the abundance of chloroplasts. Photographs: M. Christianen and E. Pierson
1 Cymodocea rotundata	2 2. Cymodocea serrulata	3 Halodule universis	4 Halophila ovalis	5 Syringodium isoetifolium	6Thalassia hemprichii
zoom	zoom	zoom	zoom animation 1 animation 2	zoom	zoom

 

A fresh water plant: Canadian waterweed (Elodea canadensis)

 
The leaves of Elodea are marked by the presence of only two cell layers with photosynthetic activity. The chloroplasts of these cells exhibit a circulating motion pattern (view a larger display of this film of the cytoplasmic streaming; gif film; 731 KB)
 

Completely submerged fresh water plant: the Canadian waterweed (pondweed)
Tip of a branch	Top view of the branch tip	Leaf surface
Sectioned tip	Young initial leaf	Older initial leaf