As Cathy from My Little Kitchen so wisely suggested regarding my, er, unusual growth, I contacted the Cornell Agricultural Extension. Steve Reiners, Associate Professor in Horticultural Sciences, was kind enough to reply:

Dear Debbie,

I have to admit, I have never seen anything like that.

Seeds have an amazing way of finding ways to germinate. How the seed got in the ceiling in the first place is a good question. It may have worked it’s way in through a small hole, carried by the wind or rain. It may have been brought in by a mouse or squirrel to keep as a food source. It may have been sitting there for many years and it was not until it received the right amount of moisture could it germinate. I’m a little surprised that it grew downwards as normally it would grow upright as it searches for light. Perhaps there was already a hole in the ceiling that allowed light to come in and that’s why it pushed through.

As far as a health concern, the plant would not be a problem. The bigger concern is the conditions that allowed the seed to germinate. Warm temperatures with plenty of moisture is great for seeds but also great for mold growth. I am not an expert on mold in homes but I would be concerned about the possibility that you have mold in the ceiling and I would urge you to get that checked out by a professional. I have no idea if they would need to remove the ceiling to do that or just parts of it. There may also be lots of other seeds and plant material up there so that should be checked out.

Somehow, I doubt my landlady’s going to spring for an expert to come over here and check for mold–I’m pretty sure this whole building is a giant mold factory, given all the leaks we’ve had over the years (and how long it takes to get them fixed.)

Sigh.

On the other hand, here’s what the plant looks like now:

It’s dead! It’s dead! We’re just waiting for it to fall off. I’d cut it down, but I’m afraid it would wake up the roots or something and who knows what might happen. (OK, I guess I’m still not entirely convinced it isn’t some alien life form.)