Tuesday, September 13, 2011

The agave's big bang

One of the condominiums near my home has a bit of fancy horticulture at the entrance. Lined up along the road is a row of big agave plants, and lately four of them have been at various stages of flowering. If you haven't seen an agave plant (also called "century-plants") flowering, it's a sight to behold: the succulent leaves are thick and waxy, forming a rosette close to the ground, and from the middle emerges a spike-shaped stalked reaching taller than a person, from which the flowers will eventually unfurl.

Porto Covo January 2011-4a
Agave plants flowering in Portugal (Wikimedia)

The agave plants (see the Agavaceae webpage for more photographs) are also known as century plants because of a misconception that they only flower once a century. It is true however that it can take up to several years or even decades before an individual plant flowers. Once it does so, it apparently withers away and dies. For this reason, probably, in some of the plants that I saw, the flower stalks were cut away soon afterwards.

Why would any organism only flower once in its lifetime? Doesn't natural selection reward those who have more offspring? Isn't having only one crop of offspring akin to putting all your eggs in one basket?

This phenomenon has many names. Colloquially it's called "big bang reproduction", because organisms that do this tend to produce a huge number of offspring (after all, if you only have this one chance...). Zoologists tend to use the term semelparity (adjective "semelparous"), while the botanists have used monocarpy ("monocarpous") or hapaxanthy ("hapaxanthous"). The antonyms are similarly polysyllabic: iteroparity, polycarpy, and pleonanthy respectively.

Trying to find out more about monocarpy in plants, I found an article in the Kew Bulletin of 1980 (vol. 35, no. 2, pp. 235-245) by NW Simmonds that helped to clear up some of the terminology. It turns out that the agave is, strictly speaking, not monocarpous after all!

Agave plants typically have multiple stems. After a flowering event, the stem bearing the inflorescence dies, but the rest can survive. Therefore, it's not the entire plant that dies! The same situation applies to palms that produce suckers, which are new stems that emerge from the roots or base of a plant. The sago palm Metroxylon, for instance, does precisely this. It's the plant that we get the starchy food sago from. The stem which flowers and fruits will die, but the rest of them live on. The banana is yet another suckering food plant that behaves in this manner.

COLLECTIE TROPENMUSEUM Aanplant van sagopalm (Metroxylon familie Palmae Principes) te Buitenzorg West-Java TMnr 10011499
A stand of sago palms (Metroxylon sagu) in Java, photographed in 1904 (Wikimedia)

So which plants are truly semelparous/monocarpous/hapaxanthous? Which ones actually flower and fruit exactly once and then die? By definition this will include the annuals and biennials of seasonal climates. There are also the plants which have a single growing axis which eventually terminate in an inflorescence. Most plants grow vertically at their tips, with cell division taking place in a zone called the apical meristem, for those in the know. Some plants reach the end of their lives when the apex, instead of producing yet more stem and leaves, switches gears to produce flowers instead. Once these flowers bloom and fruit, there's no more apical meristem to grow further, and so the plant dies. Some palms, like the talipot palm Corypha have such a lifestyle, as do the false bananas Ensete. Finally, there are also those plants which don't have such a limit to their growth–their flowers don't sprout from the apical meristem–and yet they somehow just die after flowering and fruiting anyway, such as with many species of bamboo.

This field of study has a connection to Singapore, too. Much of the early research and theoretical speculation on plant flowering cycles (phenology) was done by RE Holttum and EJH Corner. They were botanists at the Singapore Botanic Gardens (Holttum was one of the Directors, and Corner was his Assistant Director), including the period of the Japanese Occupation.

What explains the phenomenon of monocarpy? The existing explanations draw heavily from a field called life history theory, which studies the trade-offs between different life-history traits, such as lifespan, the number of offspring, and the timing of reproduction. These are considered within the framework of natural selection: which combination of traits will maximize the number of viable offspring produced by a certain species under its conditions of existence?

Theoretically, it appears that there is a trade-off between allocating resources for survival vs. for reproduction. Put another way, the choice is between reproducing right now, or holding off to reproduce later. If the chances of survival for an individual are low, then it makes sense to pump energy into reproduction (i.e. developing the gonads quickly and reaching sexual maturity earlier) as soon as possible. If an individual has higher chances of survival, then it might produce more offspring over its expected lifespan by producing fewer offspring each time, but spread out over a long lifespan. The contrast between these two modes of reproduction has been called r vs. K selection. These terms are taken from the equations of population dynamics. r refers to the maximum growth rate of a population: species which live under r-selective conditions mature early and reproduce quickly. K is the carrying capacity of an environment. K-selected species have more stable populations, and tend to invest more in longer lifespans and have fewer offspring each time.

But how does this distinction help us understand the phenomenon of long-lived plants that reproduce only once, but with loads of offspring? I.e. the big-bangers.

Ensete ventricosum 002
A species of false banana, Ensete ventricosum, which flowers once then dies (Wikimedia)

We then have to introduce another consideration: the amount of resources invested per offspring. If more investment per offspring (e.g. food stored in the seed for future germination, diverted to making fleshy fruit to aid dispersal) then by corralling resources for as long as possible, before pumping everything into the production of flowers, fruit and seeds, they might do better than if they had to split resources between provisioning offspring and keeping themselves alive for the next round of reproduction.

There is also an ecological argument, first formulated by Dan Janzen. Having plenty of offspring each time, especially when this is coordinated between entire populations of the same species, is a means of overwhelming potential predators. If a plant flowers and fruits constantly at a low rate throughout the year, it can conceivably support a stable population of fruit- or seed-eaters that end up destroying much of the potential offspring, reducing the plant's fitness. If, however, it waits for ages and ages before fruiting, then potential predators are starved and their populations are kept low. When they finally do bloom, there is an excess of riches, and the hitherto starved fruit- or seed-eaters are stuffed, but their populations have been depressed for so long that they don't do much damage because there are few of them, and enough seeds escape to propagate the species. By producing so many seeds at such widely-spaced intervals, seed predators have to alternate between long periods of starvation with sudden gluts - that's no way to raise a stable family. The same argument applies to diseases and potential parasites, too. It is also one of the explanations for the phenomenon of mast-fruiting in the Old World tropics, where hundreds of unrelated tree species flower and fruit together at irregular intervals, possibly triggered by some sort of climatic event. The forest is boring and green for months and years on end, until suddenly there is a windfall: fruits and seeds everywhere.

There are several explanations, and quite possibly there is no single "answer" - each of these probably has some part to play, each is "true" to some extent. That's one problem with theory in biology: it's always an incomplete model of reality, and one can never hope to have a full knowledge of all the possible factors. As I look at the agaves and wait for them to come to full bloom, though, I'm happy to think that at least some sense can be made out of their curious suicidal tendencies, and that these mysteries are not beyond all hope of understanding!

[For the next two weeks I'll be traveling so updates will be much more infrequent.]

No comments: