These pictures show a plant known as the Monkey Tail Cactus, Cleistocactus colademononis. It lives on cliffs in Peru and Bolivia and has become highly adapted to the cliff habitat.
The most notable thing you can see about the Monkey Tail Cactus is that it hangs down. It actually grows downwards. This is odd because most plants exhibit positive phototropism and negative gravitropism, meaning they grow towards light and against gravity. These are fundamental factors for the survival of most plants as they seek to grow upwards towards their only source of energy, the sun.
So think of the evolution of plant growth regulation that was needed to make a plant grow away from the light and following gravity. The genus Cleistocactus are generally tall, thin, upright growing cactuses. However, it must have been a terrible disadvantage for a tall thin cactus to grow upwards on a cliff with only small root space and poor footing. Such a disadvantage in fact, that most early cactus colonizers of cliffs fell over and did not succeed in this new habitat. Over time however a few plants were less inclined to grow upwards and so natural selection was towards plants that grew downwards.
Eventually however, if they kept growing downwards, they would all end up at the bottom of the cliff. So they developed flowers, fruits, and seeds favored by birds, so the birds would carry their seeds upwards and start new generations of the cactus further up the cliff.