There are some other, bigger plants out and standing in the open, not protected by glass, but the majority of the little ones had their own little domes. "Owen had set up a way to regulate when which plants needed what amounts of water. I think they might even have temperature regulators, so that they could be kept at their own homeworld environment as well." He shrugged. "Owen had a lot of free time on his hands in the last few months he was with us, this kind of became his pet project. I repair the apparatus if it needs fixing, and water the ones that are out here as per his instructions but..." He shrugged, moving a hand to brush what basically was a huge, palm-tree like fern. "I don't really know a lot about what's in here."

((Sorry, just checked the only other episode that has the hothouse in it, and it has bigger plants, because apparently, they change the layout later on in the season. Unfortunately, taking a screencap to show would largely be a screencap of Ianto and Jack, er, getting walked in on while they were "taking care of business," so it would be largely unhelpful in the long run.))