Why are most of the animals we see symmetrical in appearances (e.g. humans have left-right symmetry) but not plants and trees?
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One of the most basic differences between animals and plants is in their body plan.
Animals (with the exception of the the very most primitive of them) have what you could call a “closed” body plan. They have a set and tightly coordinated genetic program of development by which a fertilized egg divides and differentiates to form a body. This body has a lot of specialization, so it needs to be made the same way each time. If you accidentally leave out the liver, for example, you’re screwed. Many animals have left-right symmetry because they descend from a symmetric common ancestor, but others like jellyfish are radially symmetric instead. Symmetry sort of naturally emerges from development, as the same developmental program gets executed in different directions.
Plants have a different strategy and use an “open” body plan. They are much less specialized in their parts and don’t closely coordinate their entire bodies. Roots go down and shoots go up, but beyond that pretty much everything is under local control with no overall plan. If a part is exposed to sunlight, it makes leaves. If it is under strain, it grows thicker. If it has enough nutrients, it grows longer. Since there is no centrally executed body plan, their appearance depends more on local conditions and life history than anything else. Neat symmetries don’t happen in this way of growing.
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