How can you tell a vein from other structures in the leaf?

You can tell the vein from the others by holding the leaf up to a light. This makes the leaf a bit see through and the veins will be clearly visible as they do not become semi-transperent. You can also put paper on top of the leaf and rub gently over the paper with crayon to see the veins. They will make darker or lighter lines on the paper depending on if you have the leaf facing up or down.

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Sugar + oxygen > carbon  dioxide + water + energy  c6h1206 + 6o2 > 6co2 +6h2o + energy  and the numbers after the element symbols are small.