group of cells protecting the apical meristem at the root tip.

Merriam-Webster Online Dictionary
root cap (noun)
a protective cap of parenchyma cells that covers the terminal meristem in most root tips
root cap (Wikipedia)

The root cap is a type of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap protects the growing tip in plants. It secretes mucilage to ease the movement of the root through soil, and may also be involved in communication with the soil microbiota.

Root tip magnified 100×. 1. Meristem 2. Columellae (statocytes with statolithes) 3. Lateral part of the tip 4. Dead cells 5. Elongation zone

The purpose of the root cap is to enable downward growth of the root, with the root cap covering the sensitive tissue in the root. Also, the root cap enables geoperception or gravitropism. This allows the plant to grow downwards (with gravity) or upwards (against gravity).

The root cap is absent in some parasitic plants and some aquatic plants, in which a sac-like structure called the root pocket may form instead.

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