Which of the following is NOT considered a level of transport systems?

Cellular absorption is not classified as a level of transport systems; rather, it refers to the process by which cells take in nutrients, water, and other substances from their external environment. This specific process occurs at the cellular level, but it is not a transport system in the broader sense used in plant biology.

On the other hand, individual cells, cell-to-cell transport, and the transport of sap within the xylem and phloem represent different hierarchical levels of transport systems in plants. Individual cells utilize various methods to move substances in and out. Cell-to-cell transport describes the movement of substances from one cell to adjacent cells, often facilitated by plasmodesmata. Lastly, the transport of sap within the xylem and phloem encompasses the complex and organized movement of water, nutrients, and sugars throughout the plant, identifying it as a key transportation system integral to plant functioning.