While running through a leafy park days ago, I passed dozens of tree species of different sizes and ages, and I was suddenly struck by how 1 similar they were. Wide trunks split into smaller branches in the same sort of way. You could guess the height of a tree just by looking at how wide its trunk is. We would notice 2 if someone drew a tree with branches that were too wide or a trunk that was too thin. What makes a tree look like a tree? Maximising the sunlight hitting the tree’s leaves is obviously critical for the tree’s 3 . But the size and shape of the branches is largely 4 by a different standards: a tree must be strong enough to stay standing 5 , possibly for centuries. Imagine an oak tree, with branches reaching 6 off the main trunk. Each branch has to support its own considerable weight, so the wood in its lower half is slightly compressed and the top half is being 7 . That top layer of a branch takes the weight of the branch. And a particular type of wood has grown there to do that job. It’s called reaction wood, because it formed as reinforcement (增强) to take the increasing weight as the branch grew. The trees that reinforce on top the branches are the “lollipop” ones: 8 branching species like oaks. But there is another way of solving that problem. Some trees grow their reinforcing reaction wood on the 9 instead, so the branch is pushed up, rather than pulled from above. It works, but not as 10 , so these trees can’t support large side branches. The ones that reinforce underneath have a thick trunk with much smaller side branches, and they grow to be conical—some examples of these trees are pines and firs. | 
