For Stem Water Potential readings.
These bags are made of aluminized plastic material. This aluminized material allows light to enter the bag, thus creating an artificial night causing leaf potential balance with the xylem of the stem.
They have not a zip-lock, so it is easy to use them by using 2 metal clips to attach to the leaf. Bags are durable and can be re-used. They are normally lost in the orchard before they wear out.
The bags are made for taking Stem Water Readings. This method give you less variability between plants as it is more representative of the state of the all tree.