plant is to seed as
tree is to sapling
flower is to bud.
Plant and seed are related in a cause-and-effect relationship. A plant typically arises from a seed, which acts as the starting point of growth. To find another analogous relationship, we need to identify a similar cause-and-effect pair.
One possibility is "tree is to acorn." A tree grows from an acorn, just as a plant grows from a seed. Another option could be "flower is to bulb." A flower blooms from a bulb, which is the equivalent of a seed in certain flowering plants.
So, to summarize, the relationship between plant and seed is one of cause and effect, where the seed is the starting point for the growth of a plant. Similarly, a tree grows from an acorn and a flower blooms from a bulb.