The roots have large number of root hair .why?

Root hair cells. Plants absorb water from the soil by osmosis. ... Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption.

Though they are small in structure, roots hairs have a very big job. Plants grow root hairs on the tips of their roots to increase surface area underground, which helps the plant absorb more water and nutrients.
That is why roots have a large number of root hair

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