Three common groups of plants are the angiosperms (flowering plants), gymnosperms (conifers and cycads) and ferns. What is the main difference between these three groups of plants?

The main differences between angiosperms, gymnosperms, and ferns lie in their reproductive structures and processes.

1. Angiosperms: These are flowering plants. They are the most diverse group of plants and are characterized by having seeds that are enclosed within a fruit, which is a mature ovary. The seeds develop after the fertilization of an egg cell in the ovule by pollen. Angiosperms also have broad leaves and can be either deciduous (shedding leaves annually) or evergreen.

2. Gymnosperms: These include conifers, cycads, and ginkgoes. Gymnosperms produce seeds that are not enclosed within a fruit. Instead, they are often found exposed on the surface of cone scales. The word 'gymnosperm' actually means 'naked seed'. Unlike angiosperms, gymnosperms do not produce flowers and their leaves are usually needle-like.

3. Ferns: Ferns are different from both angiosperms and gymnosperms as they do not produce seeds. Instead, they reproduce via spores. These spores are typically found on the underside of the fern's fronds (leaves). Ferns are also characterized by their large, feather-like fronds.

So, in summary, the main difference between these three groups of plants is the way they reproduce: angiosperms produce seeds within a fruit, gymnosperms produce seeds not enclosed within a fruit, and ferns reproduce via spores.