This is common with all stem plants, some are worse than others. Eileen gave the reasons, mainly light. Stem plants are fast growers, and thus require more light (in intensity) than most rooted or rhizome plants. As with all plants, they grow toward the surface, and if left alone (not pruned regularly) will continue to grow across the surface where the light is strongest. Lower leaves will die off naturally as the plant is better able to photosynthesize through the leaves in the stronger light.
Stem plants thus require more maintainance. During the regular partial water change, pull them up, trim off the lower portions (height of top portions depends upon how high up in the tank you want them at that time, remembering that they will quickly grow up further) and plant the top portions, removing a few of the lower leaves as this part of the stem goes in the substrate to anchor the plant. When the stems are again at the surface, repeat. This keeps them looking fresh.
Most of the stem plants do not have a root system at the base but develop roots all along the stems. On some species, the portion of the stem in the substrate will develop slightly longer roots, and this occurs with any portion that is placed in the substrate.
Light also has an effect on colour with red plants, along wth the nutrients (iron and others) as Eileen mentioned. Red leaved plants generally require brighter (more intense) light. This occurs with rooted plants like the red-leaf forms of Echinodorus (swords) and Cryptocoryne as well.
P.S. Nice aquarium lionhead, I like your aquascape.