World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds


The organisms that we call seaweeds are protists called algae.  They are not true plants.

     Although  seaweeds are built up of many cells connected together, each cell takes in nutrients and water and expels wastes.  The water provides support and nourishment for the cells, so they do not need  vascular structures (tubes for moving liquids and nutrients around inside themselves). Every cell must be in contact with the water to get what it needs. .

     Seaweeds look something like the land plants that we are familiar with, but they are constructed differently.

     The part that looks like a leaf is called a blade.
A blade is a ribbon of algal cells that looks like a long streamer.  It is smooth and flexible, and it photosynthesizes.  

     The float is filled with gas.  It helps to hold the blades up near the light.   Some seaweeds have many floats.

     The stipe looks like a trunk to us, but seaweeds do not need trunks.  The water supports the seaweeds and stipe serves to hold the organisms to their anchors.  It must be strong and flexible.  Sometimes the stipe branches or can continue on to form a strengthening midrib for the blade. 

     Holdfasts  hold the seaweed to the rocks.  They look a bit like roots, but they do not absorb water and nutrients for the entire plant.  They just grip the rocks.  A large specimen of  seaweed with large floating blades can be pulled strongly during a storm.

     Now compare the parts of a land plant with the parts of a seaweed. Although some parts look similar, they function differently.

    True leaves photosynthesize, but they do not absorb water.  In air, they are fighting dehydration all the time.  The leaves of land plants have a waxy covering called a cuticle that reduces water loss.

     The trunk supports the land plant and holds its branches up to the light.  It must work against gravity.  Trunks have vascular tissue that allow water and food to be distributed to all the cells in the plant.

     The roots gather water and nutrients.  They also anchor the plant.

     Land plants use flowers to make seeds.  These flowers use pollen to fertilize their seeds and grow coverings for their seeds to protect them from drying out.

Seaweed Information Menu

Design Seaweed Page

Top of Page 

Header by Viau seaweeds from Olympic National Park, Washington
© 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 .