The very short answer is that their genes tell them to. But...
Actually Nature doesn't like straight things very much. Even carrots don't grow straight given half a chance (you'll soon know the truth of this if you have a home garden).
Look at this picture of banana fruit growing on a banana tree:
Note that the bananas don't just grow bent, they grow curving UPwards.
I can't find any really definitive answers on this one, but I'm guessing from how the bananas can be seen to grow...
If they grew straight, they'd be at much higher risk of any type of physical damage before ripening. By curving in tight bunches, the fruit
A) protect each other
B) help each other ripen -- they emit a gas that helps them ripen. And that gas gets more concentrated in a small area (rather than getting blown away) because the bunch is tightly bound.
C) are less likely to fall off early. They put less stress on their join to the stalk by bending.
D) ripen more evenly -- at the top (near the gas concentration) and at the bottom (heat from sun). Hence they are better off bending up rather than down.
So this way of growing (up and bending) probably confers various advantages, certainly is not disadvantageous... That's the reason most genes happen the way they do.
Actually Nature doesn't like straight things very much. Even carrots don't grow straight given half a chance (you'll soon know the truth of this if you have a home garden).
Look at this picture of banana fruit growing on a banana tree:
Note that the bananas don't just grow bent, they grow curving UPwards.
I can't find any really definitive answers on this one, but I'm guessing from how the bananas can be seen to grow...
If they grew straight, they'd be at much higher risk of any type of physical damage before ripening. By curving in tight bunches, the fruit
A) protect each other
B) help each other ripen -- they emit a gas that helps them ripen. And that gas gets more concentrated in a small area (rather than getting blown away) because the bunch is tightly bound.
C) are less likely to fall off early. They put less stress on their join to the stalk by bending.
D) ripen more evenly -- at the top (near the gas concentration) and at the bottom (heat from sun). Hence they are better off bending up rather than down.
So this way of growing (up and bending) probably confers various advantages, certainly is not disadvantageous... That's the reason most genes happen the way they do.
