Water is vital for growth. Plants grow in two ways, cell division and cell expansion. Cell division creates more cells and cell expansion is the increase in size of the individual cells. Cells grow by taking up water. The cell swells by taking up water and pushing against the inside of the box causing it to swell and increase in size.
Young cells expand and produce new cell wall at the same time.
Eventually, the cell wall becomes so thick that the cell cannot expand anymore and growth stops. If water is reduced during growth, the final cell size is reduced. This means fewer, smaller leaves, smaller fruit, shorter, thinner stems and fewer roots. Drought stress results in a smaller, weaker plant. Just as we can look at leaves as organs that harvest light, roots are organs, which absorb water.