What is root cap? Which any two variations are found in this?

What is root cap? Which any two variations are found in this?

The apex of each root is covered by a cushion of thin-walled cells known as root cap.
Variations:
(i) Multiple root cap is present in Pandanus.
(ii) In aquatic plants, the root apex is enclosed within a sac-like structure called root packet, e.g., Pistia, Eichhornia, etc.