Why use lanolin on plants in the control group?

In this botany class, we use lanolin on the control group but for the gibberellin on the experimental group. We are testing the effects of the hormones of gibberellin. but why use lanolin on the control?|||I would guess it is to keep the gibberellins off from the Control's leaves.|||the lanolin will provide a similar barrier to environmental influence that the gibberellin will provide, hence you can attribute any effects of gibberellin to the hormonal influence, not the mechanical influence which will also be shown by the lanolin on the control group.