Question

What organism was the ancestor of land plants? What two traits allowed plants to successfully colonize land?

Answer 1

http://en.wikipedia.org/wiki/Evolutionary_history_of_plants (to confirm my recollection)

Ancestor was an alga.
The second part is more interesting: So you have an alga - what's that like? (think seaweed or kelp)
What are land plants like (think of all types; mosses, ferns, trees, grass).
What's different?
What special challenges to land plants have that aquatic algae don't?