Most seaweeds (apart from the giant kelps) have no vascular system because 
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they need no strengthening tissue, being supported by the water around 
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them. Seaweeds have a thick cuticle that covers a thin photosynthetic layer. 
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The inner layers are formed from densely packed, thick-walled filaments, 
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with a few thin-walled medullary cells between them. These cells may serve 
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to transport water through the plant.