NOVELTY - The apparatus (1) has a light source e.g. LED (10), and a fuel cell e.g. direct methanol fuel cell (3), which are mounted on a mobile carriage (7), where the light source is driven by power produced by the fuel cell. Light emitted by the light source and a reaction product such as carbon dioxide, heat or water, from the fuel cell are directed toward a plant (2) in an operative state of the apparatus. A control unit (5) comprises a position indicator such as global positioning system module or local positioning module, for determining position of the apparatus.    USE - Multistage thermal convection apparatus e.g. two-stage thermal convection apparatus, for influencing growth of a plant i.e. grass on sport fields.    ADVANTAGE - The apparatus combines the mobile carriage with the fuel cell, thus ensuring high degree of flexibility and mobility for the apparatus. The apparatus can be positioned at an arbitrary position without considering any cables or hoses. The apparatus ensures that the reaction products of the fuel cell are used to improve growth of the plant. The fuel cell generates low voltage direct current (DC) power, and the LED operates on the low voltage DC power, thus ensuring power conversion in a simple, cheap and efficient manner. The LED does not produce radiated heat, so that the LED can be placed close to the plant. The control unit continuously controls the apparatus, the plant and environment of the plant, thus enhancing efficiency of the apparatus. The apparatus can be fit in palm of a user's hand for use as a portable apparatus, and is simple to operate with low cost polymerase chain reaction (PCR) amplification device.    DETAILED DESCRIPTION - The fuel cell is a formic acid fuel cell, a formaldehyde fuel cell or an ethanol fuel cell. The LED is a UV-LED or a mercury lamp. An INDEPENDENT CLAIM is also included for a method for influencing growth of a plant.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional side view of a lighting device.    Multistage thermal convection apparatus (1)    Plant (2)    Direct methanol fuel cell (3)    Control unit (5)    Mobile carriage (7)    LED (10)