What is a node?
A node is the point at which a leaf originates. Along the length of the petunia from the ground to the terminal point where the farthest growth from the ground is are nodes. Nodes in many plants are where you can start new plants by stripping off the leaves and planting the stem with a few nodes below the soil surface. It is from these nodes plants can develop roots. Such as rooting plants from cuttings. You can do this with many plants. I have never tried this with petunias since they are such a fleshy plant. Hope this is helpful.