Plant structures are constrained by evolution. Even though evolution can produce new forms, it has to build on pre-existing structures. Let's look at the structures that evolved in plants over time.
Cuticles: The cuticle is a waxy layer that covers aboveground plant parts. Cuticles prevent drying out, which was an important adaptation for surviving on land where water isn't touching every surface of the plant body.
Transport systems: Xylem and phloem are the tissues that conduct water and nutrients around a plant body. When plant ancestors lived in water, they didn't need special transport systems because every cell was close to water and nutrients. But when plants moved to land, the lack of a transport system made it hard to get water to all the cells, and limited the height a plant could achieve. Early land plants developed tubular cells that water could pass through, and fossils have been found with tube-like cells that are similar to those in modern mosses. Eventually, stems got thicker and sturdier, which allowed transport of materials and also supported plant height.
Roots: Roots anchor plants to the ground and also absorb nutrients from the soil. The earliest roots from the fossil record are 408 million years old, showing that early vascular plants developed roots at least a few feet long. Scientists think that early root systems probably weren't very different from aboveground plant parts, but over time specialized for living underground.
Leaves: Early land plants did all their photosynthesis in their stems. After 40 million years of doing this, someone got the bright idea to grow leaves. No, not really, because remember evolution isn't directional. But leaf-like structures began developing in two different forms, and we can still see the different forms in modern plants.
One type of leaf that evolved was small and grew directly from the stem. These are called microphylls. Lycophytes had (and still have) microphylls, which look more like thin stems than leaves. Microphylls only have one strand of vascular tissue. The other leaves were larger and attached to the stem with a petiole. These are called megaphylls, and are more similar to the typical leaves we think of today. They also have branching vascular tissue, letting the leaf be broad. All flowering plants have megaphylls, as do ferns.