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You're definitely missing something; your information is incomplete. You've mentioned evaporation (from the soil, presumably) but not transpiration. Plants need water, it's the stuff of life - they need it for the process of transpiration and carbon exchange, they need it to keep plant tissues hydrated via the xylem and phloem, they need it for the transport of nutrients and without it, photosynthesis doesn't take place. Think of the xylem and phloem system as vaguely like our veins and arteries - if you deprived a human being of water and other fluids for a week, dehydration and death would quickly follow. And the same is true of plants; in fact, I would consider water as number one on the list of a plant's requirements, above all else, though obviously, some plants have adapted to store fluid so their requirements are lower (cacti for instance). Expecting plants to carry out all the processes of life without water is like expecting a non electric car to run without petrol (or gas, depending where you are). See here UCSB Science Line - Why do plants need water?, but there are loads of other links online giving more complicated scientific information on this subject.