A student is studying the relationship between a leafy plant and a species of beetle. He divided 20 plants into two groups. He planted each group in a separate planter box. He then released 50 beetles into one of the planter boxes. The beetles fed on the leaves and left a white substance around the plant, changing the pH of the soil. He recorded the average height of the plants at the end of three months. His data are shown below.

• How did the substance probably affect the growth of the plants in Group 1?
   
• Describe how changes in pH may affect the metabolic rates of cells.
   
• Describe how other environmental factors could affect growth in plants.