What is the need of special tissues or organs for transport of substance in plants?
Normal cells in plants cannot transport substances rapidly and efficiently. There has to be specialized cells and tissues to carry out this function. For example - Continuous tubular structures with empty spaces are required to transport water efficiently. In this case, water can rise up from roots to stem through capillary action. Also the walls of such tubes need to be lignified and impermeable to water. All this is fulfilled by the vessels and tracheids of xylem.