That REALLY depends: 1. What`s your final goal with these tissues?; 2. what kind of plants (herbs, woody plants)? Depending of matter, I don`t know. I suggest you to see any article which approach your problem.
Juliano and Hanan are right! The question is not correct. But you may put these stems and roots in a polietilene bag and then - in a refrigerator at + 4 degree of Centigrade.
I agreed with Tatiana. The material can be stored in a plastic bag in a refrigerator at 4-6 C degrees. I was usted to keep a lot of herbaceous plant root systems for at least one month in the refrigerator and they were in good conditions to carry out root length measurements and strontium readings too.