Others acquire nitrogen by capturing insects in specially modified leaves. The xylem is essentially a one way street from the roots to the leaves for water and nutrients. Much of the time leaves are the source of sugars produced in photosynthesis and other plant structures, roots, flowers or fruits are the sinks. Sinks may be sites of storage or sites of growth and respiration. In the spring the roots of trees or other underground storage structures of perennials are the source and the developing buds are the sinks.
Sieve elements are living cells.
The water flows through the sieve tube carrying solutes with it. At the other end sink cells unload solutes, maintaining a turgor gradient along the sieve tube that dictates the direction of flow. Potassium, magnesium phosphate and organic nitrogen can also move in the phloem, but calcium is excluded. All rights reserved.