Competition and mutualism

In the last days of September I wrote about the strange flowers of a century plant (Agave americana). I’ve been visiting it regularly since then, looking at how the flowering progressed week by week from the bottom of the stalk to the top. While the flowers dropped off the bottom of the stock, parasites found a place on it. White woolly aphids began gathering on it, tended by their guardian ants. This was interesting, since the century plant is a recent import, and neither the aphids, nor the plants would have adapted to each other. So this species of aphids must be generalist feeders, attacking plants opportunistically. And this plant would have no specific defense mechanism against the pests.

This was also obvious from the rate at which the aphid colony grew. In a week and a half I saw them grow into a mass of white covering the remnants of the cymes of fallen flowers. The mutualism of aphids and ants is very interesting, with ants providing protection and cleaning services to these aphids blooms. It seems that the honeydew secreted by aphids often limits their growth, but the ants harvest this excretion, full of sugars and essential amino acids, while fending off aphid predators. They also build retreats for the aphids (I must look for these aphid pens). At the same time, there are predators which seem to have developed many kinds of tricks to fool ants into leaving them alone. How many years have gone into making these webs of life? Aphids seem to have evolved about 280 million years ago, and ants around 100 million years ago. Ants evolved around the same time as flowering plants and moths, and butterflies evolved around 55 million years ago. The mutualistic and antagonistic relationships between them cannot be older of course, but there seem to be no clear indication of how old these relations are. It is such an unsatisfactory state of affairs!

This infestation had bloomed very quickly. Before the flowers had fallen off, the first aphids had begun to climb into the stem of this plant; you can see the first aphids arriving on the stem of this cyme, shepherded by ants. I’d discovered earlier that the flowering of the century plant depletes the leaves of stored moisture. This means that there is a huge flow of sap through the flowering stalk. This is the reason the sap feeding aphids flock to this plant. How did they find the plant? By smell perhaps? That would explain why they arrived only after the first flowers began to drop off; sap must have oozed out of the breaks and attracted these parasites. What a wonderfully complex ecology is revealed in the single flowering of a plant in a garden! The world is full of questions.

Love and death

As I rose, I turned to look at the tall plant that had been at the edge of my vision while I was sitting on the wet log bench in the garden. What was this? A tall stalk with inflorescence symmetrically placed on two sides. I looked down and found it was growing out of a low succulent. It reminded me of an aloe, which spends most of its life as a succulent, but grows a long stem now and then to hold flowers. But this wasn’t an aloe; it turned out to be a century plant (Agave americana).

The bush of succulent leaves was drooping a bit now, but it would never have reached above my hips, so maybe it would have been a meter high. The flower spike, on the other hand, was almost twice my height, so perhaps around 4 meters. From what I read, the plant spends its whole life gathering enough resources to put out this spike with its cyme of yellow flowers. Then, its job done, it dies. In this place I didn’t see any pollinators visiting the flowers.

The native range of the plant seems to be southern USA down to Mexico. It is a desert plant, and is said to take thirty to forty years to bloom. Its succulent leaves store water and nutrients, and when it has enough it spikes into flower. It seems to have been brought to Europe around 1520 CE, and first botanical description was written by Jacob Anton Cortosus of Padua in 1561. It may have come to parts of Asia earlier, perhaps even a thousand years earlier. However, its present use as a garden plant probably spread with European colonialism.

If God exists I hope he has a good excuse

Boris Grushenko (Love and Death, 1975)

I found that water is the crucial ingredient that limits the flowering of a related plant. Although I couldn’t find a similar study for this one, I suppose the fact that it has begun to flower immediately after the monsoon could be an indicator that this one also behaves similarly. It is possible that in many places with abundant light and water, the plant actually has a much shorter life. Its cousin, the Agave deserti, produces one viable seed out of a million, and usually propagates vegetatively. I suppose that must be true of this plant too, since it is a very successful weed.