Marine Angiosperms There are a wide variety of marine angiosperms some, such an the mangals, saltmarsh grasses and seagrasses that will either partly or entirely submerged (or submerged dependent on the tides) as well as coastal plats growing in proximity to the oceanic environment but not directly inside of the saltwater. Angiosperms that will live partially submerged require adaptations such as salt glands, lack of stomates as well as adaptations for flotation, as necessary, including aerenchyma. Seagrasses Seagrasses live in the coastal waters of most of the worlds continents. They are the main diet of dugongs and green turtles and provide a habitat for many, smaller marine animals, some of which, like prawns and fish, are commercially important. They also absorb nutrients from coastal run-off and stabilize sediment, helping to keep the water clear. Seagrasses are unique amongst flowering plants, in that all but one genus can live entirely immersed in seawater. Members of the genus Enhalus are the exception, as they must emerge to the surface to reproduce; all others can flower and be pollinated under water. Adaptation to a marine environment imposes major constraints on morphology and structure. The restriction of seagrasses to seawater has obviously influenced their geographic distribution and speciation. Seagrasses can reproduce through sexual or asexual methods. In sexual reproduction, the plants produce flowers and transfer pollen from the male flower to the ovary of the female flower. Most seagrass species produce flowers of a single sex on each individual, so there are separate male and female plants. Figure 1: Seagrass structural detail. Differences in leaf shapes, leaf sheaths etc. are characteristics for taxonomic distinction. A number of environmental parameters are critical to whether seagrass will grow and persist. These include physical parameters that regulate the physiological activity of seagrasses (temperature, salinity, waves, currents, depth, substrate and day length), natural phenomena that limit the photosynthetic activity of the plants (light, nutrients, epiphytes and diseases), and anthropogenic inputs that inhibit access to available light for growth (nutrient and sediment loading). Various combinations of these parameters will permit, encourage or eliminate seagrass from a specific location. Seagrasses occupy a variety of coastal habitats. Seagrass meadows typically occur in most shallow, sheltered soft-bottomed marine coastlines and estuaries. These meadows may be monospecific or may consist of multispecies communities, sometimes with up to 12 species present within one location. 1
The depth range of seagrass is usually controlled at its deepest edge by the availability of light for photosynthesis. Exposure at low tide, wave action and associated turbidity and low salinity from fresh water inflow determine seagrass species survival at the shallow edge. Seagrasses survive in the intertidal zone especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g., reef platforms and tide pools), protecting the seagrasses from exposure (to heat, drying) at low tide. Potamogeton crispus curly leaf pondweed is an invasive species (it is native to Eurasia, Africa and Australia) now commonly found in the Chesapeake Bay area (predominant species is Zostera marina). Pondweed is an aquatic perennial with submersed leaves only. The leaves are large (8 to 30 cm long and 1 to 4 cm wide) and lance to oval shaped. The leaf margins fold toward each other at the tip, resembling the bow of a boat. The flowers, followed by fruit, generally occur near the growing tips, and are densely arranged on cylindrical spikes. The small fruits are oval to egg shaped, with a prominent keel along a portion of the outer rim, ending in a blunt protrusion called a beak. Being an invasive species, does that make Potamogeton of economical or ecological significance? Explain your answer! Root As a typical angiosperm the root has a characteristic arrangement including and epidermis, endodermis, cortex and stele. Figure 2: Potamogeton stem A) Examine a slide of a root of Potamogeton and find the characteristic rhizodermis (epidermis of the root), endodermis, cortex and stele. B) Why is aerenchyma found in the root? What does that determine about the environment? C) Compare the roots of Potamogeton and Juncus. 2
Stem Figure 3: Potamogeton stem (cs). Leaf A) Examine a slide of a Potamogeton stem and compare it to the Juncus stem. What are characteristic differences? How are these plants similar? B) Are stomata present in the epidermis? Why or why not? C) What does the Potamogeton stem structure tell you about the environment the organism lives in? How is it adapted to the environment? Figure 4: Potamogeton leaf. A) Examine a floating leaf of Potamogeton. How does it differ from the Juncus leaf? Describe the characteristic appearance of the leaf and its cells. B) Are stomata present? If so, where? How is this adapted to the lifestyle of this plant? C) The leaf you are examining is a floating leaf (what does that mean for the environment it occurs in?). How is it adapted to its environment? Name structures that constitute these adaptations. D) Examine the chloroplast appearance and shape. Compare it to the leaf of Juncus. How are these two leaves adapted to their environment and growth forms with regards to the chloroplast distribution? 3
Marsh grasses Juncus roemerianus Common rush. Found in anthropogenic (man-made or disturbed habitats), marshes, meadows and fields, shores of rivers or lakes, wetland margins. Perennial. Plants with a basal concentration of leaves. Young stems cylindrical. Leaves alternate and leaf sheaths with free margins. Leaf blades entire; flat, or rolled (channeled), or solid; when solid terete; linear; parallel-veined. Leaves with a persistent basal meristem, and in cylindrical versions an intercalary meristem. Ninety percent of the biomass of marsh plants, such as black needle rush, is not consumed by herbivores. Instead, marsh plant biomass is decomposed to microbial biomass. This microbial biomass is available to primary consumers, which initiate food webs leading to commercially important fishes and crustaceans. Figure 2: Native and non-native (introduced) Juncus distribution. Roots: Primary roots have not been the subject of as many, or such full studies, as has been the case of stems or leaves. They do, however, show a wide range of variation which is influenced both by environment, in terms of ecological adaptation, as well as by the genotype. Compared with stems and leaves, root fragments can be difficult to identify in the primary state. This is not entirely because they are relatively undescribed, but partly since there is, overall, less variation. In all except aerial roots and the non-anchored roots of aquatic plants, root hairs are usually present a short distance from the growing apex. These develop from the rhizodermis or root epidermis. Often the hairs arise centrally from the basal part of the cell; occasionally they arise from near one end. Again, whilst many root hair bases are level with other cells in the rhizodermis, in other plants they may be bulbous and protrude; they can be sunken into the outer cortical tissues. The cortex is sufficiently variable to be used to assist in identification. Unfortunately from that point of view, the various types of cell arrangement seem to have more ecological than systematic significance. Note that Juncus roots have a broad outer region, which is mostly parenchymatous. The cortex extends from just beneath the root epidermis to the endodermis and its overall composition is generally environmentally induced. The endodermis is the innermost layer of the cortex, and acts as the physiological boundary between the cortex and the enclosed vascular tissue within the stele. 4
Figure 6: Juncus root A. Examine a slide of Juncus root identifying the rhizodermis, endodermis and cortex. Find the vascular bundle (what is the phloem/xylem arrangement?), which forms the stele. B. Examine the aerenchyma that is part of the cortex. What would be the reason for having aerenchyma in the roots (it is generally found in monocots in the stem)? Stems: Juncus is light-weight due to having intercellular air spaces or aerenchyma, but in Juncus they are very large spaces, and in fact as much as half the volume of the stem is just air space, not cellular material. Notice all the debris in the intercellular spaces those are remnants of cells destroyed, as neighboring cells were pulled apart. The outside areas of the micrograph shows vascular tissues, and the white spaces arranged in circles are xylem vessels. The band between the inner aerenchymatous cortex and the vascular tissues is the endodermis. A) Examine a prepared slide of Juncus stem and find the epidermis, endodermis, aerenchyma, vascular bundles and distinguish between phloem and xylem (with vessel elements). B) Can you find chloroplasts in any portion of the stem? If so, what is their shape? What is the chloroplast to nuclear ration? How is that comparable or different to the previously discussed macroalgae (including chlorophyta, phaeophyta and rhodophyta)? If there is a difference, why would that occur? C) If chloroplasts are present what does that indicate for the stem? What does that mean for stomata? D) What does the continue occurrence of aerenchyma demonstrate further? 5
Figure 7: Juncus stem Leaf: Epidermis with simple stomata (absent in true seagrasses, which are also angiosperms) in the cortex vascular bundles with xylem (including vessel elements) and phloem without companion cells and a large central pith. Figure 8: Juncus leaf. A) Examine a prepared slide of a Juncus leaf and identify the characteristic structures, tissues. B) Where are the chloroplasts located? What is their shape? Is this shape expected/characteristic? C) Juncus can growth above 2 m in size with leaves as long as the stems that accommodate the flowers. How did the pith occur? What does that tell you about the overall growth of the leaves in Juncus? Be specific regarding available growth patterns. Juncus has been known to be of economical significance. Can you imagine how these marsh grasses may be economically important? Beach grasses Salicornia virginica glasswort or pickle weed, a maritime halophytic succulent. 6
Figure 9: Salicornia distribution in the United States. Stem Figure 10: Salicornia stem. A) Examine a slide of Salicornia. How does the stem compare to those of Potamogeton and Juncus. What are differences and similarities between the three plants? What does that mean to the functionality of the stem? B) Are stomata absent or present? Is that usual or unusual considering the environment these plants occur in? C) Where are vascular bundles found? How does that compare to other succulents? D) What is the environment that Salicornia grows in like? What are challenges of the environment? 7