Final answer:
The root cap protects extending roots from friction by producing mucigel, which lubricates the soil and allows for smooth growth into the earth. The epidermis also contributes to root protection but is not the main structure for friction reduction.
Step-by-step explanation:
The structure in plants that produces a substance to protect extending roots from developing friction is known as the root cap. The root cap secretes a slimy substance called mucigel which lubricates the soil, allowing roots to penetrate with less friction and reducing the chance of damage to the growing root tips. This protective mechanism ensures that the root maintains its structural integrity while growing into the soil, absorbing water and minerals, and firmly anchoring the plant.
It's important to note that the epidermis plays a crucial role in the protection and absorption processes of the roots. However, when it comes to protection against friction during root extension, it's the mucigel from the root cap that facilitates this process.