Circulatory Systems Single-celled organisms are in constant contact with their environments, obtaining nutrients and oxygen directly across the cell surface. The same holds true for small and simple plants and animals, such as algae, bryophytes, sponges, cnidarians, and flatworms. Larger and more complex plants and animals require methods for transporting materials to and from cells far removed from the external environment. These organisms have evolved transport systems. Circulatory systems function to transport materials to and from cells throughout an organism. Organisms ranging from plants to animals have different nutritional requirements. Due to these differences, various species have evolved distinct circulatory processes to assist with their specific transport needs.
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circulatory systems, vascular plants. Vascular, Circulatory systems function