Food carrying tubes and Water carrying tubes

Food carrying tube is Phloem.
Water carrying tube is Xylem.

Why plants need nitrogen?

All plants need nitrogen to make amino acids, proteins and DNA, but the nitrogen in the atmosphere is not in a form that they can use. Other plants get the nitrogen they need from the soils or water in which they live mostly in the form of inorganic nitrate (NO₃-). Nitrogen is a limiting factor for plant growth.