Gibasis geniculata (Tahitian bridal veil), honestly, is kind of annoying, and I don't blame any retailer for not stocking it if they don't want to. We had a few hanging baskets of it at the ex-job when I started working there, and they were forever getting tangled in one another, or other plants. The stems broke easily but remained tangled, so unless we put a lot of very careful, nitpicky work into it, they'd always have dead stems running through the mass of foliage, which didn't look good. The flowers were fine, I guess, and it was actually a pretty easy plant to grow, but it was messy, inconvenient, and not nearly pretty enough to make up for the hassle.
Is the problem with Gloriosa spp. and cvv. (glory lily) that they're super-poisonous? Maybe. They're in a lot of the books, but I've never seen one for sale up here. Perhaps the growth habit is awkward: it's hard to tell from the photos I've seen (which are mostly close-ups of the flower, for obvious reasons). Maybe they don't flower reliably indoors. Maybe the United Nations is using their black helicopters to keep them out of the state of Iowa, because something-something one-world government.
I don't have any idea. It seems like if it had ever been a good idea to grow them in the house, it would probably still be a good idea to grow them in the house. And certainly the flowers are lovely. I'm baffled.
In each of the lists so far, there's been a Big Mystery plant, a plant that's widely described among houseplant books of a certain age, but has completely disappeared from the houseplant trade (at least in my personal experience). In the first list, it was Aucuba japonica, which turns out to be widely available, just not where I am or for the uses I would put it to. In the second, Geogenanthus poeppigii. My questions about that one were met with utter silence, so I conclude that either no one cares, or no one knows (perhaps because no one cares).
That plant for this list is Grevillea robusta (silk oak). Where did they all go? And why? Were there ever very many in the first place?
I know there are Grevilleas still being sold for outdoor use. Danger Garden grows a few of them (G. juniperina, G. lanigera, G. victoriae, etc.), and I'm pretty sure Far Out Flora has at least mentioned them once or twice as well. So it's not like the genus has suddenly gone extinct.
I've seen some references to G. robusta having skin-irritant properties similar to those of poison ivy, which could something to do with it. (The older books don't mention this, though.) Though that doesn't seem to be stopping anybody from growing them outdoors. Maybe they're another victim of the changing standards in new-home construction? If they can be grown outdoors in the Northwest U.S., that suggests that they probably don't mind cold too much. I don't know.
Haemanthus albiflos (elephant's tongue, shaving-brush plant) and other Haemanthus spp. are probably not helped by their slow growth rates, but by most accounts H. albiflos is pretty easy to grow,1 and the flowers may not be gorgeous but they're at least interesting. If there's a market for Philodendron hederaceum, I'd think there would be a market for Haemanthus.
The only guess I can come up with is that maybe it's not sold very often because it's a passalong?2 That's how I wound up with mine; maybe that's how everybody gets them.
I've seen Heliconia psittacorum 'Bright Lights' for sale a couple times at the ex-job, and I assume other Heliconia cvv. (lobster claw, false bird of paradise) are probably available further south. (The main appeal of H. psittacorum is, I think, that it's relatively small for a Heliconia; many of them get big.) Both I and WCW3 have tried them indoors, and neither of us was particularly satisfied. We both got spider mites; my plant also seemed to want more humid air than it was getting, and was either getting underwatered or overwatered but it would never tell me which.
So I wouldn't try this again, but it did do surprisingly well for several months after I first got it, so Heliconia might still work for some people. Even if it isn't a great choice for your average plant-buyer, the inflorescences can last for a long time, so I'd think retailers might still be able to display them profitably. I don't have any guesses as to why they don't.
It is entirely possible that I see Howea forsteriana (kentia palm) for sale all the time and just don't realize it. I can't really distinguish between Chrysalidocarpus lutescens, Howea forsteriana, and Chamaedorea cataractum, in large part because I've never been able to directly compare all three. In fact, I've only actually seen Howea forsteriana tagged as Howea forsteriana once. This is despite several books' claims that they're either the best or second-best palm to have indoors. (Chamaedorea elegans generally being the other one.4)
The reason is that they germinate and grow so slowly that it's not really cost-effective for growers to produce them. People will buy the much faster-growing Ravenea rivularis (majesty palm) just as readily, so Ravenea can be sold for less than half the cost of Howea. Raveneas are also, of course, very poorly suited to indoor conditions, but the long-term fate of the plant is irrelevant to the growers (they already have your money either way), so they don't factor it in.
I don't know how to fix this situation. I can recommend that people ask for Howeas specifically, but I don't have first-hand experience that they're any better than Chrysalidocarpus or Chamaedorea, and I don't know how to tell the difference anyway. I could get first-hand experience, and learn how to tell the difference, if any producers ever sent them to the stores for me to look at, buy, and grow, but the producers can't do that because the consumers wouldn't buy them. The consumers might buy them, if only they knew that they wanted them. I could tell the consumers they wanted to buy them, if I were able to distinguish between the three species and had first-hand experience growing them. Wheeee!
I've seen Jatropha podagrica (Buddha belly plant) at Wallace's once; we never got any Jatropha spp. at the ex-job, though they were on the availability lists occasionally. Pretty sure I've never seen Jatrophas for sale anywhere else, though the QCBC has J. multifida (coral plant, Guatemalan rhubarb), as you can see in the above photo, so I've at least seen the two most cultivated species.
I've never grown one personally; I understand they probably need more sun than I could offer. Other than that, I really don't have much to go on. As far as I can tell, temperature wouldn't be an issue. Maybe slow growth? Unappealing form?
Holy crap, some Lachenalia spp./cvv. (no common name?) have some damn beautiful flowers. (There's a red one! And a turquoise-blooming one! And one with four-colored flowers!) And although one of the books which mentions Lachenalia has been in my possession for about a quarter-century, I had somehow never been consciously aware of them before now.
Now that I've noticed them, read up on them, and come to appreciate how pretty some of them are, I will have to forget them all again. Both books that mention Lachenalia agree that they need temperatures much colder than I'd ever be able to provide in the house (60-65F during the day, 40-45F at night). Which is a pretty obvious explanation for why I never see Lachenalia for sale.
Lapageria rosea (Chilean bell flower) is another plant I know only from the Kramer book. Or almost, anyway -- Talking Plants did a post about Lapageria not too long ago as well, though he doesn't mention it being cultivated indoors.
Kramer calls it "a challenge," but doesn't include many specifics about the ways in which it might be challenging, and says it's "difficult to bloom," but also says "can tolerate abuse if necessary and still bloom." So no clues there. It's another climber, which is possibly significant: beyond that I don't even have a guess.
I've seen Manettia luteo-rubra (firecracker plant, candy corn plant) for sale here as an outdoor annual, but not very often, and I didn't find them very impressive. I don't know of anyone who's tried to grow it indoors. The two books that mention it don't agree on the care it needs, though neither one makes it sound particularly difficult. Maybe it's just not interesting enough for people to care?
I understand Haemanthus is pretty easy indoors, and although I never did it personally, I'd bet Gibasis is very easy as well. I might be interested in Jatropha, in theory, even if I probably don't have the light it wants. And I would really like to try Howea at some point, if only to see what all the fuss is about, but it looks like the only way that's ever going to happen is if I start them myself from seed.
- Hedychium gardnerianum (kahili ginger, ginger lily): have never seen.
- Hoya australis / bella (wax plants): I'm pretty sure I've seen H. bella around here occasionally; I don't recall H. australis, though.
- Ixia spp./cvv. (corn lily): have never seen.
- Ixora (West Indian jasmine, jungle geranium): the ex-job has them occasionally, I think. I have trouble distinguishing between Ixora and Pentas, for some reason, but I'm pretty sure they've had both.
- Justicia brandegeana and carnea (shrimp plants): I've seen J. brandegeana once at the ex-job; not sure if I've run into them elsewhere or not.
- Kohleria cvv. (kohleria): have never seen in retail. The ones I've tried to grow haven't done well.
- Limonium (sea lavender, statice): the ex-job had one once, as an outdoor annual (?), but I think that was the only time I've seen any for sale at all, much less as a houseplant.
- Liriope (lilyturf): sometimes available around here as an annual or tender perennial. One from the ex-job survived for me for a while indoors, but it never grew much, and spent its last year or so looking decidedly unhappy.
- Lycaste spp. (lycaste): have never seen.
- Malpighia coccigera (dwarf holly, miniature holly): may have seen as a faux bonsai, but the pictures that come up in Google don't look particularly familiar.
- Malvaviscus arboreus (turk's cap mallow, nodding hibiscus): have never seen.
- Masdevallia spp./cvv. (masdevallia): have seen at Orchids and Moore, the specialty orchid-seller in Iowa City, and possibly at the Quad Cities orchid shows (can't remember if any of those were for sale or not), but that's it.
2 (A plant that is so easy to propagate that offsets or cuttings or whatever get distributed from gardener to gardener directly, without ever being part of the commercial trade.)
3 (Remember her? I haven't seen WCW in forever, sadly.)
4 Which has not been my experience -- my best palm has been either Chamaedorea metallica or Rhapis excelsa, depending on the criteria you want to use for "best." But that's off-topic.