It just seems like there’s so much wasted real estate with all that trunk space, even in the canopy, there’s light shining through. (palm tree’s are what first got me thinking on the topic).
In: Biology
The trunk has to have space for vascular tissue (to move water up from the roots), structural fiber (so they don’t blow over in the wind) and bark (so they can’t be easily eaten by every hungry critter). It works better when there’s a specialized section for structure and other specialized sections for photosynthesis.
We do the same thing. Our bones do one job, our lungs do another. Our bones wouldn’t work as well if they had air sacs all through them. 🙂
Also – leaves have little holes on the bottom, like pores, that they ‘breathe’ through. If you put leaf-surface on the trunk, there wouldn’t be any way for it to do gas exchange because the back surface would be facing in. Leaves are a good design because they’re thin, and have a sun-facing side and a gas-exchange side.
[Edit: AND, leaves can pivot slightly to help them keep facing the sun. A photosynthetic surface bonded to a tree trunk couldn’t do that, so, less efficient.]
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