What are the bumps on this tomato stem? They are way off the ground so I doubt they are air roots.
What are they, what function do they have and why do they form?
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Those bumps are aerial surface hairs, a type of trichome. They are epidermal outgrowths. Therefore it's vines are of pubescent type.
In addition to facilitating better exchange of gases, it also facilitate the vining process, turning into roots wherever the plant is in contact with the ground and moisture, especially if the vine's connection to its original root has been damaged or severed.
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