The Phyto-Philes - Pregnant Onion
Created | Updated Feb 9, 2014
Pregnant Onion
Willem says: 'The Phyto-Philes are for plant lovers of every size and shape, colour and flavour. As with my Colours of Wildlife column, I'll be featuring one species per article, illustrated with sketches, paintings, and/or photos. Over time I hope to be showcasing the amazing diversity of weird and wonderful plants that occur in South Africa, while also from time to time looking at the flora of other countries. While featuring many spectacular species, I'll not be neglecting the smaller, more humble kinds that are nevertheless fascinating in their own right.'
This time I have a plant that is weird in an endearing way. The Pregnant Onion, Ornithogalum longibracteatum, bears babies and gives birth to them! It has a nice fat bulb – and unlike most other bulbs, this one grows on top of the soil rather than in it. The bulb is covered by a thin, papery sheath called a tunic. In time you will see little bumps forming on the surface of the bulb. They'll become bigger and bigger … clearly something is growing just under the surface of the bulb. If you draw back the tunic, you'll see that the bumps are formed by tiny little baby bulbs growing there! In time, these will detach and drop to the base of the plant, where they will then send out roots and grow leaves to become new pregnant onion plants that will in turn bear babies of their own. It's very strange to see a plant 'giving birth' in such a mammalian fashion!
Actually, this is not so strange when considering the huge diversity of plant reproduction strategies. It is one form of vegetative reproduction, a kind of reproduction without sex found in many plants. (Remember, 'sex' when speaking of plants just means pollination – the pollen bearing the 'male' sex cells which merge with the female sex cells in the ovule, and so the genes of the 'father' and 'mother' are combined.) There are plants that can form new plants from almost any of their parts: some plants can generate new individuals from roots; some from branches or twigs – which make them easy to grow from cuttings; some from leaves – like the Adromischus umbraticola. These plants have special cells that are 'undifferentiated' … not dedicated to becoming specific tissues … instead, they can form many different tissues, indeed, all the different tissues needed to make up a full, new individual! These tissues occur in different parts of the plants in different species. In the pregnant onion, the tissues that can form new individuals are on the outer surface of the living bulb, just inside the sheath of the tunic, which is a large, dry, dead leaf. There, protected in its shelter, the small bulbs can grow and once they are big enough, with substantial internal stores of food, they can detach. Their internal energy stores are enough to allow them to form leaves and roots and grow for a while, still just lying on the soil surface. Finally, their roots will penetrate into the soil and they will then be able to get nutrients and water and grow into big, strong, independent individuals.
Because this kind of reproduction happens without 'sex' – without the exchange of genes – the new individuals that are formed this way are genetically identical to their 'parent'. So in a real sense they are not children but more like identical twin brothers/sisters of the bulb that produces them.
This is quite a valuable strategy of reproduction and survival. Like many plants, pregnant onions in the wild are vulnerable to being eaten or damaged by many animals. But since every big bulb bears many small bulbs that are genetic copies of it, it can 'survive' … the main bulb can be eaten or destroyed, but the activity of the animal that does so will scatter the smaller bulbs and if some of them survive and grow, then the plant will live on.
But pregnant onions can reproduce in the normal plant way as well. They flower in spring and summer, the flowers being clustered at the tip of a long, stout flower stalk that can reach 1.5 m/5' in length. The flowers are delicate, fragrant and white with a green stripe down the midline of each petal. They are pollinated by insects, and thus the genes get exchanged. The flowers are followed by three-chambered capsule fruits that split open to release the thin, papery seeds, which are distributed by the wind.
Although they are called onions, they are actually not close relatives to onions. They are grouped in the hyacinth or sometimes the asparagus family. These family relationships are still being puzzled out by botanists. But the bottom line is that they are not onions and not edible! They do have some medicinal properties though. The genus Ornithogalum has about 120 currently recognized species. Sometimes it is combined with the genus Albuca, which has another 60 or so known species. Albuca is, however, most of the time easily distinguishable even by laymen, since they have very characteristic flowers with the inner three petals not opening fully along with the outer three, instead forming a tube-like structure around the reproductive parts, while in Ornithogalum proper all the petals spread out evenly. Many Ornithogalums have pretty flowers, and some have interesting spiraled leaves. The pregnant onion is one of the largest and most robust of the Ornithogalums. Its natural habitat is forest and bush where it grows on the moldy floor in the shade of trees and shrubs. The map accompanying my illustration shows where in South Africa it can be found. It ranges into tropical Africa as well, and is not endangered.
Growing Pregnant Onions
This must be one of the all-time easiest plants to grow. Just fill a pot with a general potting soil, put the bulb on top, put it in a place with light to mild shade, give it a bit of water once or twice a week – that's it. It multiplies by itself; you can remove the small bulbs and plant them in a similar way in a new pot, or leave them … the plants will ultimately fill the entire pot; the ones around the edges will remain small since they won't have room to grow bigger. The plants can be left completely dry in winter, but in mild climates can continue growing year-round.
In regions that experience only light frost, pregnant onions can be grown in garden beds. But beware! They are so prolific that they can become invasive and 'take over' the garden, in some regions perhaps even escaping into the surrounding wilds and becoming problem weeds. I'm not aware of this having happened anywhere so far, but the potential is there. Still, they are very easy to remove from beds; they can just be lifted up because of the bulbs not being deep and firmly rooted. Pregnant onions are not very aggressive growers. They take a good amount of time to reach their full size. The bulb can reach 10 cm/4" in diameter, and the rather leathery, strap-like leaves can grow to 1 m/40" in length. They fit in well with succulents and can be planted in a shady portion of a rock garden. They do well as house plants also, and can be handsome specimens as well as conversation pieces (but don't bore people who are not fellow plant enthusiasts!) To keep them neat, when grown indoors, water them sparingly, so that their leaves stay reasonably short and firm rather than becoming long and droopy. This is less of a problem outdoors since the leaves will have room to grow long and spread out over the ground. Also, if you wish you can remove the papery tunic, which indoors is not so important for protection, and the bulb itself has an attractive smooth, shiny-green surface. Grown outside in my own town, Polokwane, which receives an average of 500 mm/20" of rain each summer, this plant needs no extra watering and, indeed, no attention whatsoever.
