Why do tree’s only photosynthesize with their leaves?

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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
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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.]

Keep in mind that your tree trunk is a generally vertical shape, and light is generally falling vertically. This means that the most efficient way to capture light would be to expand horizontally, hence branches and leaves. Second, the more surface area you have, the more light you can capture. You will see some small animals that carry algae in their cells for this purpose, but it does not occur in larger animals. This is because the surface area to volume is low in a larger animal, but higher in a smaller animal. That is, a small animal can receive sufficient nutrients from its guest algae to make it worth doing, but a larger animal will never be able to.

Now if you go cut into a tree, you will notice that the upper-most layer just below the bark is actually green and contains chloroplasts. If they were exposed, they could photosynthesize. As I understand it, trees are able to sense the change of light this way and decide when they should produce leaves as the season change.

Palo Verde trees have green trunks and branches that contain chlorophyll and, thus, photosynthesize. There are a few other desert plants that have similar strategies to minimize water loss as well. Whereas a cactus isn’t technically a tree, they are quite large and their trunks and branches are green and photosynthesize as well.

Something I think the others are missing is why trees have trunks *at all*. Animals compete with each other for food, right? Well, plants do, too. In their case, the food is sunlight. There are different strategies to get sunlight. If a plant grows *out* with big, broad leaves it will catch a lot of sun. However, if another plant grows *up* it can grow over the broad plant and then grow out, in which case the tall plant is catching sunlight and casting shade over the short, broad plant.

Trees have evolved to grow tall, towering over lower plants so that the trees have access to sunlight without worrying about other plants growing over them and shading them. It also puts the soft, tasty leaves high above potential predators so few things can eat the leaves. But that means they have to have tough, strong trunks to hold up the weight of the rest of the tree. But that also leaves them with a particular vulnerability, which is a big open trunk full of nutritious sap that tons of animals would love to get a hold of.

So trees evolved a countermeasure, which is bark. Bark is thick and tough and essentially inedible. It isn’t living tissue and there’s no nutritional value in it. It’s armor to keep things from burrowing into the tree trunk. An insect trying to get to the sap has to first waste precious time and energy burrowing deep through the bark. Woodpeckers have needed to evolve some pretty astounding adaptations to smash their way through bark. When trees first evolved to produce lignin and bark, there was nothing that could chew through it at all.

All that bark gets in the way of photosynthesis. Chloroplasts won’t do anything buried under the bark, and putting them outside the bark would defeat the purpose of having bark at all, since it would put living tissues where predators could get to it.