We prefer for roots to be sent to us wrapped in paper rather than plastic - Ideally in a wet tissue (newspaper) and in a not completely closed plastic bag e.g. in a well labeled paper envelope.
 

Normally we are able to identify roots of 1-2mm in diameter.
 

A root length of 5cm or longer is ideal.
 

It is advisable to label every sample with details including your reference code or sample address, date of collection, trial pit number and depth where applicable.