Wow! What a lot of great brainstorming over the weekend! I would venture to say that The Garden Professors have the smartest students in the world.
On to the answer…or answers. First, the phenomenon. It’s called paraheliotropism – literally, a movement to protect (the leaves) from the sun (yes, Trena, it is a tropism!). This is the opposite of another phenomenon called heliotropism, or solar tracking. Sunflowers famously do this, as do a number of arctic species that collect solar warmth for the benefit of their pollinators. (An aside: if you have never watched David Attenborough’s The Private Life of Plants you must add it to your Netflix queue. Right now.)
But our saxifrage (thanks, Holly! I’m such a taxonomy imbecile) is reducing solar exposure by positioning its leaves in parallel to the sun’s rays. This is a reversible movement and helps reduce photooxidative stress, leaf temperature, and water loss. It’s an important strategy as the newly emerging leaves are actively expanding. If turgor is reduced by high temperature or water loss, so is the final size of the leaf.
Finally, these rapidly expanding leaves have relatively thin cuticles (if they were thicker the leaves wouldn’t be able to expand as well). The cuticle gives further protection to the leaf from water loss due to heat, drought, wind, or even late season freezing events (thanks for that addition, John!). The cuticle will mature after the leaf has reached its full size.
So, as Foy suggested, this is a way for leaves to "harden off" and reach full size before exposing themselves to the sun. Aren’t plants cool?
And you are all such great participants! Group hug! Now, back to work.