Page from Darwin's notebooks around July 1837 showing his first known sketch of an evolutionary tree.
There are a number of useful resources on the web to help you understand the relationship between the different plant groups and explore their diversity.
The Tree of Life is a collaborative project that provides information about the diversity of organisms on Earth, their evolutionary history and their characteristics. The link above takes you to the main page or you can skip directly to the green plants.
The Green Plant Phylogeny Research Group (Deep Green) is a group that coordinates research into the phylogeny of the green plants. Their website contains a useful hyperbolic tree. Hyperbolic trees are one way to display complex trees clearly. I posted a link to the hyperbolic trees earlier when we talked about phylogenies.
The Green Tree of Life aims to resolve the primary pattern of evolutionary diversification among green plants. An overview of the results to date can be seen in this poster.