What should be regarded as the single most significant structure leading to rapid success of the so-called higher plants?

Answer 1

seed germination-----------> NovaNet Answer

vascular bundles

Higher order plants refer to plants have vascular system(think of them as tubes that moves water to their leafs), which allows them to draw water and other nutrients from the ground using xylem and phloem. this is oppose to simpler plants like moss and algae which are called lower order because they evolved very early and are some of the first plants to come out of the oceans and go on land. They cannot live outside of very wet places though because they can't absorb water simply through their "leafs".

Answer 2

Seeds. [i edited that one last as well and i am 150% sure that answer is correct.]

Well lets say flowers as well, without one you don't have the other. "Early" plants were either spore or cone bearing species, many of which still exist today (Cycas, Encephalartos etc.).

For this reason Angiosperms are considered more advanced than Gymnosperms (which still carry many of the traits of "early" plants)