It contains P-proteins, which help assimilate transport, and in roots calose, detectable by resorcin blue, is deposited along its sieve cells, while the stem and leaves generally consist of sieve tubes and sieve plates.  
In young plants it is formed by activity of the apical meristem.  In older plants its secondary variety is differentiated by the vascular cambium.  It contains P-proteins, which help assimilate transport, and in roots calose, detectable by resorcin blue, is deposited along its sieve cells, while the stem and leaves generally consist of sieve tubes and sieve plates.  Responsible for the conduction of food materials in plants, FTP, identify this plant tissue considered the analog to xylem.
In young plants it is formed by activity of the apical meristem.  
phloem