One visible difference between lower epidermis and guard cells is that guard cells have thickened walls while lower epidermis cells have thin walls. The significance of this difference is that thickened walls of the guard cells allow them to change shape when they absorb water, which opens and closes the stomata present in the lower epidermis. This mechanism helps in the regulation of gas exchange and water loss in the plant. Thin-walled lower epidermis cells do not have this functionality as they are not involved in the control of the stomata.
