What are those white and dark bumps on the underside of the penstemon leaves?
Those are the life stages of the of insects called whiteflies.
The adult whitefly looks somewhat like a tiny moth, but whiteflies are actually in the Family Hemiptera. They are closely related to aphids and scales. In fact the nymphs look a lot like flat scales. They are the oval lumps on the leaf surface.
The adult whiteflies lay eggs on the undersides of the leaves. (The eggs are above the adult whitefly in this photograph.) The eggs hatch into tiny nymphs with legs, called "crawlers." The crawlers find a place to feed, insert their mouthparts into the leaf, and suck juices from the plant. Usually there will be a number of nymphs feeding together on the underside of a leaf, giving it the appearance of sandpaper.
This particular species of whitefly has brown markings on its wings. It shows up only on penstemon plants in our yard and only in the spring. By summer it is gone. Another smaller species, the silverleaf whitefly, shows up later in the summer and fall on a wider variety of plants.
Whiteflies: another sign spring is here!