Motor cells are present in the upper epidermis of leaves of many monocots to:

Question

Accepted Answer

The correct option is 3: Check transpiration by reducing the surface area of the leaf

The phenomenon described is a remarkable adaptive strategy found primarily in monocots, especially grasses, to cope with water stress. The large, colourless cells present in the upper (adaxial) epidermis are specifically known as Bulliform cells or Motor cells. These cells have thin walls and much larger vacuoles compared to the surrounding epidermal cells, making them highly sensitive to changes in water availability. When the leaf is well-hydrated, the bulliform cells are turgid, and their turgor pressure causes the leaf to remain fully expanded, maximising light absorption. However, under conditions of low water availability (drought or high temperature/wind), the motor cells rapidly lose turgor. The collapse of these large, flaccid cells creates tension that pulls the leaf inward, causing it to curl or roll tightly. This rolling mechanism dramatically reduces the total surface area of the leaf exposed to the atmosphere, thereby minimising water loss through stomata and surface evaporation (transpiration), effectively checking excessive water consumption for survival. This structural adaptation is a critical mechanism for survival in xerophytic conditions or during temporary drought.

The incorrect option is 1: Increase the surface area of the leaf

This statement is factually incorrect and contradicts the actual effect of motor cell action during water stress. The entire purpose of the turgor-driven collapse of these cells is to *reduce* the surface area of the leaf. An expanded leaf, facilitated by the turgid motor cells, is the state of maximum surface area, which occurs under optimal conditions. If the leaf were to increase its surface area during water scarcity, the rate of transpiration would increase drastically, leading to rapid desiccation and death of the plant. Therefore, the function of the rolling mechanism is precisely the opposite of increasing the surface area; it is a mechanism for controlled *decrease* in area to prevent dehydration.

The incorrect option is 2: Store large amounts of water

While motor cells possess large central vacuoles, which do contain water, their primary biological function is *not* long-term water storage for the plant. They are designed for a rapid, turgor-driven *movement* (rolling/unrolling) of the leaf in response to internal water fluctuations. True water storage is typically the role of ground parenchymatous tissue found in specialised water-storage organs, such as the fleshy stem or leaves of succulents. Furthermore, their thin walls and high water sensitivity make them ill-suited for the structural resistance needed for effective long-term storage; their design is optimised for quick changes in volume to enact a mechanical response.

The incorrect option is 4: Bear unicellular trichomes

Motor cells are large, specialised, water-sensitive cells that are an integral component of the epidermal layer; they are not themselves trichomes. Trichomes are hair-like outgrowths, which can be unicellular or multicellular, whose roles typically involve protecting the leaf surface, such as reflecting light, acting as a physical barrier, or passively slowing air movement to reduce transpiration. Motor cells are primarily involved in the active, reversible *mechanical movement* of the leaf to regulate transpiration, a function entirely separate from the protective or passive roles of trichomes.