I just find it odd to stumble into a Lush cave while spelunking; there are plenty of trees, grass, and flowers - but somehow they perform photo synthesis with no sunlight - or any other light.

So my suggestion is that the Lush Caves generate with a glow-stone stalactite hanging from the ceiling - or perhaps some other type of light source. Not only would this provide a reason why the plants can grow, but would bring welcome relief from the underground darkness.

They do in 1.11.

They do?
I've seen crystals in those caves, but they don't seem to change the lighting...

MC 1.11.2
Forge 2259
ActuallyAdditions r106
Optifine 1.11.2_HD_U_B7

They do, lighting updates in Minecraft are just stupid and unreliable :/

So light emitting blocks and items rarely work as the developer intends?

Of course, Optifine seems to make my iron tools into light sources that interfere with normal vision - so maybe I need to adjust Optifine's settings (or find a replacement for it).