The term 'root growth' can be summarized by the crossword solution 'NODULE, ' which is a six-letter word. Nodules are small, round structures that form on the roots of certain plants, such as legumes. They are formed when bacteria in the soil interact with the plant's root system, resulting in the formation of nitrogen-fixing nodules. These nodules help the plant to absorb more nutrients from the soil, allowing for increased root growth and healthier plants.