World Builders™                                                                     Session Five  --  Seaweeds




Seaweeds and True Land Plants


Holding On Reproduction Unit 5 Home

Photosynthesizers began their evolution in the sea. It is thought that chloroplasts, the bodies in plant cells that do photosynthesis, were once small independent bacteria.

 Chloroplasts moved into larger cells, and those larger cells provided a nurturing environment for the chloroplasts. In return, the chloroplasts used water, carbon dioxide, and sun light to make sugars. By capturing the energy of the sun, the chloroplasts made energy available for plant and seaweed growth, and, through these autotrophs (organisms that create their own food), for all the animals that live on the planet.

    Life in water is much easier for seaweeds than life on land is for plants. In water, seaweeds are supported, kept moist, and carried to new locations.  Photosynthesizers lived in the oceans for many millions of years before venturing onto the land.

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  Supporting Structures

     Water plants can be delicate and ribbon-like or rather rubbery. Water plants are supported by the water that they live in: they are buoyant. They do not need structures to hold them up, because the water supports them. They do have to deal with currents in the water, so some of them are rather tough (like kelp stalks, for instance).

     In this picture of an ocean plant you can see the tough, rubbery stalk. Above it there are small oval structures like little balloons. These oval structures are filled with gas and tend to float in the water: they help to hold the stalk up. Above the ovals are the long, thin ribbons of plant tissue that float towards the surface, towards the light. The cells in these ribbons of plant tissue contain chloroplasts that make nourishing sugars from sunlight, water, and carbon dioxide.

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Roots and Holdfasts

     Seaweeds do not need roots, because they have water around them all the time. The individual cells just help themselves to what they need. However, some water plants have evolved holdfasts to allow them to cling to rocks. Holdfasts do not transport anything, they just hold the base of the plant in one place.

Vascular Structures

     Plants on land have vascular structures, internal tubes that allow water to go to every part of the plant, and that allow the food energy to go down to the roots as needed. Seaweeds may be ribbon-like and only two big cells thick: in a double layer of cells, every cell has access to the water around it, and vascular structures are not needed.   

Reproductive Strategies

   Seaweeds do not have seeds as land plants do. They can reproduce themselves by simply breaking apart, or they can release eggs and sperms to make spores, tiny one-celled capsules that can float away and grow into seaweeds by themselves. Seaweeds do not have flowers.

Seaweed Information Menu

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 Photographs from a Corel CD-ROM : for viewing only, not for downloading.   More Information.
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .