- Brazil nuts.
- Soy beans.
- The taxonomy of citrus.
- The taxonomy of stonefruit.
The answer is apparently megafauna, such as giant ground sloths (genus Megatherium), which have now gone extinct. In other words, the avocado is an ‘evolutionary anachronism’ which is no longer able to manage its seed dispersal.
See the discussions on the Wikipedia and Smithsonian websites. Or watch and listen to a video of someone called Connie Barlow singing a song entitled Ghosts of Evolution (which is actually worth watching).
What is the purpose of a fruit?
The purpose of most fruit is to be eaten by an animal who, at a later time and hopefully elsewhere, excretes the seeds that were contained within the fruit and thus helps the plant to disperse.
Bananas are seedless, so what is the purpose of a banana?
A banana has no purpose. Banana plants, like mules, are sterile.
So, why do bananas exist?
Bananas exist only because humans like eating them. X thousand years ago, a mutant banana plant whose bananas were seedless serendipitously happened to come into existence. A human happened to eat its bananas and thought “yum”. This human happened to be an ur-agriculturalist and he/she happened to take suckers from the plant and grow them. Their progeny or equivalent happened to do the same thing, and so on, thus replacing natural dispersal mechanisms via seed with artificial dispersal mechanisms via sucker.
Where does pollination fit in?
Pollination doesn’t fit in. Because bananas are seedless then, by definition, they are not pollinated.
So, what is the purpose of the male flowers on banana plants?
Male banana flowers have no purpose. They are, like emu wings, just a hangover from history.
How does the banana grow if it is not pollinated?
It is true that fruit usually only grow if the seeds that they contain have been pollinated. But this is because natural selection acts against it (it has costs but no benefits) rather than because there is a biological law which prevents it. After all, it is never the fruit itself that is being pollinated. There will have been many mutant banana plants which had undeveloped seedless bananas, but no ur-agriculturalist ever said “yum” and propagated them, so they no longer exist.
Nerdy aside: when the fruits develop without any pollination, this is called parthenocarpy. As well as bananas, it happens with some figs, pineapples and other plants.
Yosefine Deans has written in to point out that, whilst commercial bananas are seedless, wild bananas have seeds and these have to be pollinated by bats or other animals for the fruit to set. See, for example, this article on the Bat Conservation International website. (the picture right is from Wikipedia)
Whilst we’re on the subject, here’s another fun fact: once a banana plant has fruited, it will never fruit again. So you should cut it down to make room for the other suckers. Has anyone got a machete that I could use?
The flower of a brazil nut tree can only be pollinated by an insect strong enough to lift the coiled hood and with tongues long enough to negotiate the complex coiled flower. In practice, this means the female long-tongued orchid bee.
Bees only exist where they can mate. So, the smaller male bees are also required. But they are only attracted by a certain type of orchid. So, the tree needs to be in the presence of that orchid.
The tree’s fruit is actually the size of a coconut and the brazil nuts are seeds rather than nuts (8-24 within a single fruit). So, the fruit needs to be cracked open and this requires a large rodent (i.e. an agouti).
So propagation of brazil nuts requires agoutis, orchids and strong, long-tongued bees.
The flowers of a fig tree are actually inside the figs. Indeed, all that squishiness inside of a fig is basically flowers. So, a fig is not a fruit; rather, it is something called a syconium (“an enlarged, fleshy, hollow receptacle with multiple ovaries on the inside surface”).
So, if the flowers of a fig are inaccessible to normal pollinators, such as bees, how do they get pollinated?
If you look closely at a fig, you will see a small hole in the bottom of it. Tiny wasps (called fig wasps) crawl in through those holes and do the pollination.
What happens to the wasps after they have done the pollination?
They die in situ.
And what happens to the bodies of the dead wasps?
You eat them when you eat a fig. All the black dots in the picture are dead fig wasps.
For more information, watch this video.
Luckily for the squeamish, some varieties of fig do not require pollination. And most of the figs bought in shops will be from these varieties. But, at least according to Louis Glowinski in this book The Complete Book of Fruit Growing in Australia, the best tasting figs are from those varieties which do require pollination.
Nerdy aside: it is only the female fig wasps that do the pollination. The wingless male fig wasps are borne, impregnate their sisters, drill little holes in the skin of the fig for their sisters to escape by, and then die. The winged female fig wasps are borne, get impregnated by their brothers, get covered in pollen as they leave through the holes created by their brothers, enter another fig through its base, lay their eggs, and then die. So, whilst a male fig wasp only sees one fig in its life, a female fig wasp sees two.
One way that plants ‘know’ the length of the day (or, more accurately, the length of the night) is because they produce substances (X) in the dark which degenerate to other substances in the light so the night length is proportional to the maximum daily concentration of X. For many plants, day length is at least as important as temperature and this is one reason why so many plants can be grown over so much of the world. This subject is discussed in Chapter 9 of Science and the Garden: The Scientific Basis of Horticultural Practice.
One of the plants that is most sensitive to light is soy beans. Indeed, if there is any street lighting in the vicinity, they apparently consider it to be daylight and often never flower. So, if you live in urban Melbourne, it seems unlikely that you can ever grow soy beans.
To which Heather Elliot responded: For the past few years I have been growing soybeans, latterly of the Edamame kind, and getting a decent crop. Whilst the garden doesn’t receive bright street lighting, it does, until late evening, normally receive some light filtering in from the two townhouses next door.
The taxonomy of citrus
- The 3 progenitors of most of the other citrus species are citron, mandarin and pomelo.
- The standard lemon is (probably) a cross between a citron and a pomelo.
- The sweet orange is (probably) a cross between a mandarin and a pomelo.
- The Meyer lemon is (probably) a cross between a sweet orange and a standard lemon.
- So, the Meyer lemon is a different species than the standard lemon.
- There are at least 14 different species which are called limes, 8+ species which are called lemons and 8+ species which are called oranges.
- I presume that, in general terms, citrus are called lemons when they have yellow fruit, limes when they have greenish fruit and oranges when they have orange fruit (but why, then, tangerines etc?), but I haven’t actually seen anything which says this.
- All 8 of the Australian limes are original species, not crosses.
- The benchmark citrus fruit for marmalade production is the bitter orange, not the sweet orange.
- In tropical regions, with no winter, all citrus fruits remain green thru to maturity. (posted November 4 2015)
The taxonomy of stonefruit
A few random points:
- Stonefruit refers to the fruit of the prunus genus.
- Unlike citrus, stonefruit species are normal species and don’t naturally hybridise much.
- 7 prunus are commonly eaten: almond, apricot, cherry (2 species), peach and plum (2 species).
- Whilst Sweet Cherries are eaten raw, it is usually a different species – Sour Cherries – that are used in cooking.
- Surprisingly, the European and Japanese plums are different species (with the former being a polyploid and the latter a diploid).
- Nectarines are ‘just’ varieties of peach where the skin is smooth.
- A plumcot is a plum crossed with an apricot. Pluots are 25% apricot and 75% plum. Apriums are 75% apricot and 25% plum.
But the really interesting species is the almond. The wild almond is both bitter and poisonous (it contains cyanide). So, how come prehistoric humans decided to domesticate it? How did they know that if they spent years breeding the cyanide out of it then it would taste yummy? How many people died before the modern day almond came into being? Was there a series of Cleopatra equivalents who tested poisonous almonds on slaves and some survived and reported back that almonds were potentially edible if only the bitterness were removed?