(i) Diffusion is unable to meet the oxygen requirements of multi- cellular organisms such as humans because diffusion is one of the slow processes. A lot of time is required for oxygen to circulate to all the cells of the body. Besides, in humans, oxygen requirement is more at cellular and tissue level. Therefore, through diffusion, it becomes difficult for oxygen to reach all the cells.
(ii) Diffusion, transpiration pull and capillary action are the three factors responsible for making water in the xylem move upwards to the highest point of the plant body.