Sieve element plastids (SE plastids) are a plastid type ocurring in sieve elements only. As other plastids, they derive from proplastids. They may contain starch and/or proteins. They are classified based on their components. It has been recognized that their content is highly plant family specific and they have been used for taxonomic classification (see literature by Dr. Dietmar Behnke, University Heidelberg, Germany). Besides their structure, little is known about their function. However, given the importance of other plastid types for plants, one can expect that they play a major role in phloem function.