In general, plants need food in order to grow. The process by which plants make their own food is called photosynthesis. We can determine if light affects the growth of an Elodea plant by finding out the effect of light on its ability to produce its own food and maintain optimal growth.
Such experiments have been carried out using the Elodea plant before, and the results of one such experiment as presented in a research report by Dr Norman Herr indicates that light affects the rate at which the Elodea plant makes its own food and, by extension, its ability to maintain optimal growth.
The experiment concludes that excessive light may result in a ‘burn out,’ while too little light will not provide sufficient light for the plant to manufacture its own food and maintain optimal growth.