Q.What is present on the surface of the leaves which helps the plant prevent loss of water but is absent in roots?

The surface of leaves is coated with a waxy layer called the cuticle, which checks (reduces) the loss of water from the leaf through transpiration. This same waxy layer is specifically absent in roots, since roots need to take up water rather than prevent i …

The epidermis of above-ground organs like leaves is coated on its outer face with a thick, waxy secretion known as the cuticle.

• Because it is waxy and largely impermeable, the cuticle acts as a barrier that reduces uncontrolled evaporation of water from the leaf surface, supplementing the more regulated water loss that happens through the stomata. …

Method: A Cost-Benefit Argument From First Principles

Rather than naming the cuticle directly and moving on, derive both its existence on leaves AND its absence on roots from a single cost-benefit principle: a structure only evolves/persists where its benefit exceeds its cost for that specific organ.

State the benefit of a waxy covering: a waterproof wax layer reduces uncontrolled evaporation of water from a surface — clearly useful wherever a plant needs to CONSERVE water.

Apply the cost-benefit test to leaves: leaves are directly exposed to sun, wind, and open air — exactly the conditions under which uncontrolled water loss (transpiration) is a real risk. The benefit of a waterproofing layer here is high, so a cuticle is worth having, and indeed the leaf epidermis secretes one.

Apply the same test to roots: a root's entire job is the OPPOSITE — it must ABSORB water and minerals from the soil, not conserve water within itself. A waxy, waterproof layer on the root surface would work directly against this job, blocking the very water uptake the organ exists to perform. Here the "benefit" of waterproofing is actually a cost (it would sabotage the organ's function), so no cuticle develops, and the root epidermis (epiblema) is left bare, well-suited instead to producing absorptive root hairs. …