This micrograph to the right shows a sieve plate from a metaphloem sieve element from the external phloem of a large stem vascular bundle. Individual sieve plate pores can be seen in this micrograph. The pores contain  proteins and the pore themselves are lined with callose.

 

Sieve plates function to regulate passage of substances between adjacent sieve tube members and perhaps more importantly, to ensure that the plastids and other organelles contained within their sieve tube members, remain located within the cell