I would say a lot of it depends on what type of plants you want to grow...
If its low light requirement plants like Anubias, i'm guessing that direct sunlight should be enough to keep it healthy and slowly growing... but if its any of the more light dependent plants, then sunlight alone will almost definitely not work... not to mention you won't be able to control the amount of lighting your plants get each day, as well as the direct sunlight could cause your tank to overheat if your house is hot during summer...