Characteristics of Seaweeds
The animals and plants that live on the seafloor or in the sediments are called benthos, and they include remarkably rich and diverse groups of organisms, among which are seaweeds, organisms that are members of a large group called algae (aquatic plants or plant-like organisms). Seaweeds—large, benthic, multicellular algae—are conspicuous members of coastal intertidal and subtidal communities. Because algae photosynthesize (convert sunlight into chemical energy) and contain chlorophyll pigments, they have been considered members of the plant kingdom, but most biologists view them as only plant-like, placing them in other categories. Algal body form, reproduction, accessory pigments, and storage products generally differ from those of land plants. Seaweeds have simple tissues, they do not produce flowers or seeds, and their pigments and nutrient-storage compounds vary from group to group.
Seaweeds grow attached to rocks, shells, or any solid object. They are attached by an organ known as a holdfast that anchors the organism firmly to a solid base or substrate. The holdfast is not a root—it does not absorb water or nutrients. Above the holdfast is a stem-like portion known as the stipe. The stipe may be so short that it is barely identifiable, or it may be up to 35 meters in length. It acts as a flexible connection between the holdfast and the blades, the alga’s photosynthetic organs. Seaweed blades are thin and are bathed on all sides by water. They serve the same purpose as leaves, but they do not have the specialized tissues and veins of leaves, because algae do not require that water be conducted from the ground, up a stem, and through the veins to leaf cells. The blades may be flat, ruffled, feathery, or even encrusted with calcium carbonate.
Seaweeds are not found in areas of mud or sand where their holdfasts have nothing to which they can attach. Sometimes during a storm, the seaweeds are dislodged, taking with them the rocks their holdfasts cling to. The seaweeds, carrying their rock anchors, may drift for some time before they sink back to the bottom. If they sink too deep for sufficient light to reach them, they die, but if their blades remain in the photic zone (the portion of the sea that is penetrated by light), they will continue to grow. Because benthic algae are dependent on sunlight, they are confined to the shallow depths of the ocean, where they are surrounded by water, dissolved carbon dioxide, and nutrients. They are very efficient at producing biomass (biological material) from non-living sources, exposing a large blade area to both the water and the sun.
In the sea, both the quality and the quantity of light change with depth. Light at the red end of the visible spectrum is quickly absorbed at the surface, and the blue-green light penetrates to the greatest depths. On land and at the sea surface, plants receive the full spectrum of visible light, and it is the green algae—with the same chlorophyll pigments as land plants and the ability to absorb both long and short wavelengths of light—that are found in shallow water. Algae found at moderate depths have a brown pigment that is more efficient at trapping shorter wavelengths of solar light. At maximum growing depths, the algae are red, for the red pigment can best absorb the remaining blue-green light. Both the brown algae and the red algae possess green chlorophyll, but the red and brown pigments mask its color so that the seaweeds appear red or brown.
Seaweeds provide food and shelter for many animals. They act in the sea much as the forest and shrubs do on land. Some fish and other animals, such as sea urchins, limpets, and some snails, feed directly on the algae; other animals feed on shreds and pieces as they settle to the bottom. Some organisms use large seaweeds as a place of attachment; some of the smaller algae grow on the larger forms. Algae that produce hard, shell-like calcareous outer coverings help provide the framework for tropical coral reefs.
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P1: The animals and plants that live on the seafloor or in the sediments are called benthos, and they include remarkably rich and diverse groups of organisms, among which are seaweeds, organisms that are members of a large group called algae (aquatic plants or plant-like organisms). Seaweeds—large, benthic, multicellular algae—are conspicuous members of coastal intertidal and subtidal communities. Because algae photosynthesize (convert sunlight into chemical energy) and contain chlorophyll pigments, they have been considered members of the plant kingdom, but most biologists view them as only plant-like, placing them in other categories. Algal body form, reproduction, accessory pigments, and storage products generally differ from those of land plants. Seaweeds have simple tissues, they do not produce flowers or seeds, and their pigments and nutrient-storage compounds vary from group to group.
1.Why does the author note that seaweeds “have simple tissues, they do not produce flowers or seeds, and their pigments and nutrient-storage compounds vary from group to group”?
A.To provide supporting evidence for a claim about the differences between certain types of seaweeds
B.To identify similarities between seaweeds and a particular category of land plants
C.To specifically illustrate an observation made about seaweeds in the previous sentence
D.To introduce some features of seaweeds that will be discussed in the following paragraph
2.According to paragraph 1, which of the following do algae have in common with land plants?
A.Their body form
B.Their method of producing energy
C.The way in which they store food
D.The way in which they reproduce
P2: Seaweeds grow attached to rocks, shells, or any solid object. They are attached by an organ known as a holdfast that anchors the organism firmly to a solid base or substrate. The holdfast is not a root—it does not absorb water or nutrients. Above the holdfast is a stem-like portion known as the stipe. The stipe may be so short that it is barely identifiable, or it may be up to 35 meters in length. It acts as a flexible connection between the holdfast and the blades, the alga’s photosynthetic organs. Seaweed blades are thin and are bathed on all sides by water. They serve the same purpose as leaves, but they do not have the specialized tissues and veins of leaves, because algae do not require that water be conducted from the ground, up a stem, and through the veins to leaf cells. The blades may be flat, ruffled, feathery, or even encrusted with calcium carbonate.
3.The word “identifiable” in the passage is closest in meaning to
A.functional
B.accessible
C.recognizable
D.measurable
4.The word “flexible” in the passage is closest in meaning to
A.secure
B.suitable
C.vital
D.bendable
P3: Seaweeds are not found in areas of mud or sand where their holdfasts have nothing to which they can attach. Sometimes during a storm, the seaweeds are dislodged, taking with them the rocks their holdfasts cling to. The seaweeds, carrying their rock anchors, may drift for some time before they sink back to the bottom. If they sink too deep for sufficient light to reach them, they die, but if their blades remain in the photic zone (the portion of the sea that is penetrated by light), they will continue to grow. Because benthic algae are dependent on sunlight, they are confined to the shallow depths of the ocean, where they are surrounded by water, dissolved carbon dioxide, and nutrients. They are very efficient at producing biomass (biological material) from non-living sources, exposing a large blade area to both the water and the sun.
5.According to paragraph 3, storms can endanger the survival of seaweeds by
A.covering them in mud and sand
B.separating them from their holdfasts
C.forcing them to drift on the sea surface for long periods of time
D.transporting them to areas below the photic zone
6.According to paragraph 3, a seaweed may continue growing after it is dislodged if the seaweed
A.attaches itself directly to the ocean floor to keep from drifting
B.has blades that are reached by sunlight
C.uses its holdfast to carry a rock anchor while drifting
D.finds an area on the bottom of the ocean with non-living sources of biomass
P4: In the sea, both the quality and the quantity of light change with depth. Light at the red end of the visible spectrum is quickly absorbed at the surface, and the blue-green light penetrates to the greatest depths. On land and at the sea surface, plants receive the full spectrum of visible light, and it is the green algae—with the same chlorophyll pigments as land plants and the ability to absorb both long and short wavelengths of light—that are found in shallow water. Algae found at moderate depths have a brown pigment that is more efficient at trapping shorter wavelengths of solar light. At maximum growing depths, the algae are red, for the red pigment can best absorb the remaining blue-green light. Both the brown algae and the red algae possess green chlorophyll, but the red and brown pigments mask its color so that the seaweeds appear red or brown.
7.According to paragraph 4, which of the following is true of green, brown, and red algae?
A.They contain green chlorophyll pigments.
B.They receive the full spectrum of visible light.
C.They are found mainly in shallow water.
D.They do not easily absorb blue-green light.
P5: Seaweeds provide food and shelter for many animals. They act in the sea much as the forest and shrubs do on land. Some fish and other animals, such as sea urchins, limpets, and some snails, feed directly on the algae; other animals feed on shreds and pieces as they settle to the bottom. Some organisms use large seaweeds as a place of attachment; some of the smaller algae grow on the larger forms. Algae that produce hard, shell-like calcareous outer coverings help provide the framework for tropical coral reefs.
8.Which of the following is NOT mentioned in paragraph 5 as one of the ways seaweeds support other organisms?
A.By being a source of food for many animals
B.By providing a base for other algae to grow on
C.By attaching to the ocean bottom and absorbing nutrients
D.By supplying building materials for reefs
P3: ■Seaweeds are not found in areas of mud or sand where their holdfasts have nothing to which they can attach. ■Sometimes during a storm, the seaweeds are dislodged, taking with them the rocks their holdfasts cling to. ■The seaweeds, carrying their rock anchors, may drift for some time before they sink back to the bottom. ■If they sink too deep for sufficient light to reach them, they die, but if their blades remain in the photic zone (the portion of the sea that is penetrated by light), they will continue to grow. Because benthic algae are dependent on sunlight, they are confined to the shallow depths of the ocean, where they are surrounded by water, dissolved carbon dioxide, and nutrients. They are very efficient at producing biomass (biological material) from non-living sources, exposing a large blade area to both the water and the sun.
9.Look at the four squares [■] that indicate where the following sentence could be added to the passage.
Seaweeds are found in many parts of the world where there is seawater, but not in all types of habitats.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
10.
A.Seaweeds are similar to land plants in most ways but have more complex storage and reproduction systems.
B.The seaweed’s holdfast serves as a kind of root that conducts water and other nutrients up to all parts of the plant.
C.Most algae grow at the surface, and those that live at lower depths must receive both long and short wavelengths of light to grow.
D.Seaweeds have specialized structures that allow them to remain attached to one place, where they can receive the light that they need.
E.Seaweeds depend on sunlight for survival and grow only within the photic zone of oceans, where their pigment color is best adapted to the depth at which they live.
F.Many ocean organisms are dependent on seaweeds for food or for a place of attachment upon which to grow
