This is just an educated guess. As the purple pigment, caused by anthocyanins, are on the bottom side of the plant, their purpose could probably be to direct the light.
Specifically, a purple pigment on the bottom of the plant would absorb certain wavelengths near the purple spectrum, and reflect other wavelengths back toward the portion of the plant with chlorophyll. In this way, light does not simply pass through the leaf, instead it is reflected back in towards the chlorophyll to maximize the amount of light being used for photosynthesis.
Dorsal means back as in the back area. Example~ Many dolphins have a pronounced beak and all have a central dorsal fin and sharp teeth for feeding, mainly on squid and fish.
Real size is image size/magnification, that will be 0.25 micrometers
