Bean seeds were planted and put on a sunny windowsill. As the plants grew, their stems bent toward the window. This bending was most likely caused by an

Step-by-step solution

The plant hormone, auxin, is more distributed on the side away from the unilateral illumination causing cells to grow faster in the darker side, which in turn, causes the plant to bend toward the light. If the distribution were equal, the plant would grow just upwards. Neuro-transmitters are chemicals secreted by multicellular animals and are used in transmitting impulses in the nervous system.