Stinky giants and smelly dwarfs
Plants do not usually captivate the attention of the general public. Nature documentaries and news articles alike often overflow with examples of the fastest mammal, the oldest fish, or the deadliest insect—respectively, the cheetah, the greenland shark, and themosquito (indirectly of course). We, as a mobile species, tend to pay more attention to organisms that also move. This is a cognitive bias called plant awareness disparity, which education researcher Kate Parsley has written about cogently. Our animal bias is not our fault, we inherited it in some capacity. We are wired to focus more on other organisms that can move, because they are more likely to pose an immediate threat to us. Every once in a while, though, a plant captivates the attention of the masses. And usually it is one of extremes.
All around the country marketing teams at botanical gardens, arboreta, and nurseries are waiting to share the news about their Titan Arum (Amorphophallus titanum). This one species often brings in more traffic to botanical institutions than any other species in the U.S., and for good reason. This plant is remarkable. Amorphophallus titanum is almost alien in appearance. When theplant has amassed enough sugars—usually after 5–7 years of photosynthesizing—they flower. And flower they do, with a single massive inflorescence roughly 10 feet tall.
These Titan Arum blooms are composed of two parts: a massive fleshy outer leaf-like structure called a spathe and an inner spike named the spadix—similar to a peace lily (Spathiphyllum) in your doctor’s office waiting room but scaled up and ready for Halloween. With such a massive inflorescence, it is one of thelargest blooms in the world. It is not only the visual queue that attracts spectators to this truly marvelous plant, but also the smell—which is of rotting flesh. The genus Amorphophallus has evolved down a route of carrion pollination, meaning they attract flies, beetles, and any other insects that seek out decomposing meat. Consequently, these plants have evolved traits that increase thevisitation rate of their pollinators. The spathe and spadix of Amorphophallus have evolved ripples, grooves, bumps, and discolorations that are strikingly similar to decaying meat, so much so that at first glance you forget it is even a plant.
Amorphophallus paeoniifolius, relative of A. titanum.
This essay is not about the smell and color or pollination syndrome of the Titan Arum. Rather, it is about the juxtaposition between the massive proportions of the inflorescence and theminuscule size of the actual flowers.
What is often thought of as a single flower of Titan Arum is actually the entire inflorescence or a cluster of many flowers. Theindividual flowers are dinky little structures at the base of thespadix. They are so reduced that they generally lack all other traditional floral organs (petals and sepals). They are also unisexual, with pollen-bearing flowers higher up on the spadix and seed-producing flowers lower down. The seed-bearing flowers produce a single tiny fruit and the pollen-bearing flowers produce only 1-5 pollen producing organs—beautifully stripped down to their essential parts.
There is an interesting evolutionary dichotomy here. The flowers have gone down a path of dwarfism or nanism, while the entire inflorescence has gone down an opposing trajectory called gigantism (sometimes giantism). These patterns of size change have been used and abused in evolutionary biology, the most extreme example is Cope’s Rule, which posited that all lineages have a tendency to become larger over time. This “rule” has not stood the test of time, or detailed analysis. Even Stephen Jay Gould had an article on the futility of Cope’s Rule in 1997. While these patterns of increased or decreased size are not universal trends that can be generalized across all life, there are particular lineages which have seemed to follow traverse this landscape . And, in the case of Titan Arum and its relatives, this seems to be the case.
Why, though? Why are the individual flowers so small and theinflorescence so large? Which came first, did the inflorescence become large and then the flowers reduced in size, or vice versa? What is responsible for these changes? My curiosity about these questions started to blossom (I couldn't resist the pun) when Ben and I stumbled upon Amorphophallus paeoniifolius, a much smaller relative of A. titanum, growing in the Kampong during our filming retreat in Miami.
Some of these questions are easier to answer than others. For instance, what is responsible for these large blooms, or put another way, why do Titan arums need such large inflorescences? Theoretical work from Charles Davis and colleagues as well as empirical work from Todd Barkman and colleagues suggest that carrion pollination is the culprit for large blooms. There tends to be a correlation between bloom size and pollination syndrome; Barkman and colleagues found that carrion-pollinated plants tended to have massive flowers or inflorescences. Larger blooms may emit more odor and attract more pollinators, thus increasing the possibility of reproductive success. This may very well be thecase, plants that are pollinated by flies and beetles may be under selection for larger blooms. But, why does the Titan Arum have such minuscule flowers? If there was selection for larger blooms, why did Amorphophallus evolve down a route of gigantism in theinflorescence and not the individual flowers?
This is a classic chicken or the egg question—and as is the case with that, phylogeny or evolutionary relatedness can help provide the answer. In the case of the age-old adage or proverb, theanswer is the egg. Lineages such as turtles, snakes, and lizards all (barring the occasional viviparous lineage) lay eggs; and thus based on reconstructing the state of the common ancestor of chickens and all other vertebrates, the egg came before thechicken. Now what about the tiny flowers and large blooms of A. titanum?
If we look at the evolutionary tree of Titan Arum we see that nearly every lineage related to it has small flowers. This includes other families in the Amorphophallus order (Alismatales) as well as other related orders such as the Acorales, Petrosaviales, and even some Dioscoreales. To find whole lineages that have larger individual flowers we have to traverse several orders of branches outside of the Alistmatales to the Liliales. Thus, the dinky flowers of Titan Arum and the rest of the Araceae were likely ancestrally quite small, and that is what Amorphophallus inherited. The larger blooms evolved later. But why did selection act on bloom size and not flower size? The simple answer may be, it depends.
It could be possible that it is developmentally “easier” for an inflorescence to grow in size than it is for an individual flower to grow in size. However, the glaring exception to this explanation is Rafflesia, the single largest flower in the world which is about thesize of a beach ball (also a carrion imitating plant). It does not necessarily need to be globally more difficult to evolve a larger single flower or “easier” to evolve a larger inflorescence. Rather, what may be more important is the relative likelihood of which evolutionary path a lineage takes—and this may reflect that lineage's evolutionary past.
For instance, Barkman and colleagues found that lineages with ancestry of large individual flowers tended to have higher rates of floral-size evolution, meaning that larger flowers can beget larger flowers. This is likely what occurred in Rafflesia. On the other hand, in Titan Arum and most other members of the family, flowers are small and so rates of floral evolution are likely low. If there is selection for larger blooms (from carrion pollination syndromes), it is more likely that variants with larger overall bloom sizes emerge before variants with larger individual flowers. Once evolution takes the path of larger inflorescence size instead of larger flower size, it is almost a ratcheted approach whereby larger inflorescences beget larger inflorescences, and thealternative evolutionary path of selection acting on individual flower size becomes less and less likely. This finds support in theobservation that large inflorescences are generally composed of tiny flowers, think sunflowers, palms, and figs. Indeed, one of theworld's largest inflorescences—based on a Wikipedia list—is thepalm Corypha umbraculifera, which has tiny flowers (as do just about all other palms).
Where selection acts thus depends on the history of that particular lineage. Ancestrally, is the lineage small flowered with flowers clustered into inflorescences or is the lineage a single-flowered stalk? If the former it may be the case that larger inflorescences are favored by selection for larger inflorescences; if the latter it may be that larger individual flowers are selected for. The Titan Arum highlights how two evolutionary phenomena—nanism and giantism—can co-occur in a single lineage and the importance of contingencies in evolution.
Titan Arum is spectacular to behold, and appreciating how it may have evolved can give you a new perspective upon your next visit. I hope the crowds of spectators visiting Titan Arum around the country continue to grow, mirroring the evolutionary trend of gigantism seen in the plant’s remarkable inflorescence.
June 30, 2025
References
Barkman, Todd J., et al. "Accelerated rates of floral evolution at theupper size limit for flowers." Current Biology 18.19 (2008): 1508-1513.
Chartier, Marion, Marc Gibernau, and Susanne S. Renner. "Theevolution of pollinator–plant interaction types in the Araceae." Evolution 68.5 (2014): 1533-1543.
Gould, Gina C., and Bruce J. Macfadden. "Gigantism, dwarfism, and Cope's rule:“nothing in evolution makes sense without a phylogeny”." Bulletin of the American Museum of Natural History 2004.285 (2004): 219-237.
Hetterscheid, Wilbert, et al. "Botanical background to Amorphophallus." Konjac Glucomannan: Production, Processing, and Functional Applications (2020): 46-50.
Parsley, Kathryn M. "Plant awareness disparity: A case for renaming plant blindness." Plants, people, planet 2.6 (2020): 598-601.
