Why couldn't plants move onto land until an ozone layer formed?
No life either animal or plant could survive outside the water until the UV radiation (UVC) from the sun had been blocked by something. The Ozone layer provided a barrier through which the most energetic UV radiation could not pass.
The water would have afforded some protection before the Ozone layer had formed. The reason no life forms could exist with the UV radiation hitting them is because the radiation would damage their cell structure.
You may be familar with a water purification device for camping which has a battery powered UVC lamp which you place in the water to sterilize it. The UVC radation Kills any bacteria, virus or parisite. Sorry for spelling, David
The Earth was a pretty volatile place before the atmosphere formed, so I don't know how successful any theoretical life would have been back then anyway.
Plants need carbon dioxide gas in order to for photosynthesis to work. If there's no significant atmosphere, there's no way for them to capture energy from the sun and grow.
