1. Water is an essential and major constituent of protoplasm. In fact bulk of protoplasm is water. All the cell organelles maintain their structure and function in the presence of adequate quantity of water.
2. Distribution of various dissolved substances from one cell to another and from one part to another can take place only in the medium of water.
3. Water is an excellent solvent. It also acts as a reagent in various reactions in plant cells. In photosynthesis and respiration water plays an important role.
4. Breakdown of various chemical compounds takes place only in the medium of water. During germination, complex food substances stored in the seed breakdown into simple constituents. This is possible only when water is present.
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5. During metabolic reactions, the inner temperature of the plant rises. Water stream flowing throughout the plant body acts as a coolant and maintains the temperature.
6. Adequate amount of water is necessary in cells and tissues to maintain the turgidity of the plant body.
7. Enzymes, without whose mediation, virtually, no biological reaction can take place, function only in a hydrated medium.
8. Plants take in their nutrients from soil only in a solution form. Soil water thus is very essential for the survival of the plants.
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9. A network of thin layer of water surrounding each cell plays an important role in the movement of dissolved substances.
10. In lower plants water is essential for the mobility of gametes.
11. Dehiscence of sporangia and dispersal of spores (hygroscopic reaction) is aided by atmospheric water.
12. Water also helps in pollination and dispersal of fruits in many of the higher plants.