Cell walls provide structural support for the plant and also provide fibers and building materials for humans, insects, birds and many other organisms. Plants exhibit diversity in size and shape ranging from single cells to gigantic trees. Plants are a primary source of fiber, medicines, and countless other important products in everyday use. Plants, like animals, are subject to injury and death due to infectious diseases caused by microorganisms. Plants have unique ways to defend themselves against pest and diseases.
Water is the major molecule present in plant cells and organs.
In addition to an essential role in plant structure, development, and growth, water can be important for the internal circulation of organic molecules and salts. Plant growth and development are under the control of hormones and can be affected by external signals such as light, gravity, touch, or environmental stresses. Plants live and adapt to a wide variety of environments. Plants provide diverse habitats for birds, beneficial insects, and other wildlife in ecosystems. 2001 American Society of Plant Physiologists.