Photoperiodism is the influence of the day length on the flowering of plants. The day length or duration of light is referred to as the photoperiod. The photoperiodic responses are controlled by genes. According to the photoperiodic responses the plants are classified into long day plants, short day plants and the day neutral plants. The long day plants are the plants which flowers when the day length becomes greater than the critical day length. Some of the common examples are radish, sugar beet and spinach. The plants which flowers when the day length becomes shorter than the critical length are known as the short day plants. Some of the common examples of short day plants are Chrysanthemum, xanthium, sugarcane, tobacco and soybean. The plants which flowers after a period of vegetative growth which is regardless of the photoperiod. Common examples of day neutral plants are Cotton, Sunflower, Cucumber, Tomato and Pea.
Whereas the tropism is the movement of the curvature of the plant towards or away to an external stimulus which determines the direction of movement. There are different types of tropism such as the phototropism, geotropism, thigmotropism etc.