Vascular structures permit the distribution of fluids and nutrients inside living organisms. This, in turn, allows cells to specialize. If the leaves of a plant are making glucose, sending some of that glucose to the cells in the roots keeps the root cells alive and allows them to grow. In return, the root cells can gather water and nutrients that are deep in the soil, and can share these resources with the leaves. Ferns are the plants that developed vascular systems .
Some ferns still have rhizoids, but they also have roots.
This makes it possible for ferns to grow into large plants. Ferns do not have true seeds. They reproduce by alternation of generations. When you turn fern fronds over, you can often see little dots on the underside. These dots are groups of spores.