A Shout Out to Western Skunk Cabbage

Photo by Martin Bravenboer licensed under CC BY-ND 2.0.

We all have our biases and one of my biggest botanical bias is that I often think of plants from eastern North America before my mind heads further west. I can’t really fault myself for it because so many of my early plant experiences occurred east of the Mississippi. I want to remedy this a bit today by drawing your attention to a wonderful aroid who frequently gets overshadowed by its eastern cousin.

I am of course talking about western skunk cabbage (Lysichiton americanus). This incredibly beautiful plant enjoys a distribution that ranges from southern Alaska to central California and west into Wyoming and Montana. Like its eastern cousin, western skunk cabbage was awarded its common name thanks to the pungent odor it produces. Its blooming period ranges from March into May depending on where they are growing and the inflorescence is truly something to write home about.

The spadix of western skunk cabbage complete with a tiny rove beetle pollinator. Photo by Walter Siegmund lincensed under CC BY-SA 3.0

Emerging from the base of the plant is a bright yellow structure called a spathe. The spathe envelopes the actual flowering parts, a phallic-looking structure covered in flowers called a spadix. The spadix emits various volatile compounds that function as pollinator attractants. However, whereas many would suggest flies are the preferred pollinator, research indicates that a tiny species of rove beetle called Pelecomalium testaceum takes up the bulk of pollination duties for western skunk cabbage throughout much of its range.

The volatile compounds aren’t there to trick the beetles into thinking they are getting some sort of reward. The plant does actually reward the rove beetles with pollen to eat and relatively safe place to mate. We call these types of signals “honest signals” as they act as an honest calling card that signifies rewards are to be had.

A closer look at a Pelecomalium rove beetle. Not sure which species. Photo by Judy Gallagher licensed under CC BY 2.0

Unfortunately, the beauty of western skunk cabbage has seen it enter into novelty garden collections in other temperate regions of the world. In northern Europe, western skunk cabbage has escaped the confines of the garden and is now considered an invasive species in wetlands of that region. Take care to choose you garden plants wisely. Always plant native plants when the option presents itself.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

How Aroids Turn Up the Heat

Photo by Jörg Hempel licensed under CC BY-SA 2.0

A subset of plants have evolved the ability to produce heat, a fact that may come as a surprise to many reading this. The undisputed champions of botanical thermogenesis are the aroids (Araceae). Exactly why they do so is still the subject of scientific debate but the means by which heat is produced is absolutely fascinating.

The heat producing organ of an aroid is called the spadix. Technically speaking, a spadix is a spike of minute flowers closely arranged around a fleshy axis. All aroid inflorescences have one and they come in a wide variety of shapes, colors, and textures. To produce heat, the spadix is hooked up to a massive underground energy reserve largely in the form of carbohydrates or sugars. The process of turning these sugars into heat is rather complex and surprisingly animal-like.

Cross section of a typical aroid inflorescence with half of the protective spathe removed. The spadix is situated in the middle with a rings of protective hairs (top), male flowers (middle), and female flowers (bottom). Photo by Kristian Peters -- Fabelfroh licensed under CC BY-SA 3.0

It all starts with a compound we are rather familiar with - salicylic acid - as it is the main ingredient in Aspirin. In aroids, however, salicylic acid acts as a hormone whose job it is to initiate both the heating process as well as the production of floral scents. It signals the mitochondria packed inside a ring of sterile flowers located at the base of the spadix to change their metabolic pathway.

In lieu of their normal metabolic pathway, which ends in the production of ATP, the mitochondria switch over to a pathway called the "Alternative Oxidase Metabolic Pathway." When this happens, the mitochondria start burning sugars using oxygen as a fuel source. This form of respiration produces heat.

Thermal imaging of the inflorescence of Arum maculatum.

As you can imagine, this can be a costly process for plants to undergo. A lot of energy is consumed as the inflorescence heats up. Nonetheless, some aroids can maintain this costly level of respiration intermittently for weeks on end. Take the charismatic skunk cabbage (Symplocarpus foetidus) for example. Its spadix can reach temperatures of upwards of 45 °F (7 °C) on and and off for as long as two weeks. Even more incredible, the plant is able to do this despite freezing ambient temperatures, literally melting its way through layers of snow.

For some aroids, however, carbohydrates just don't cut it. Species like the Brazilian Philodendron bipinnatifidum produce a staggering amount of floral heat and to do so requires a different fuel source - fat. Fats are not a common component of plant metabolisms. Plants simply have less energy requirements than most animals. Still, this wonderful aroid has converged on a fat-burning metabolic pathway that puts many animals to shame. 

The inflorescence of Philodendron bipinnatifidum can reach temps as high as 115 °F (46 °C). Photo by Tekwani licensed under CC BY-SA 3.0

P. bipinnatifidum stores lots of fat in sterile male flowers that are situated between the fertile male and female flowers near the base of the spadix. As soon as the protective spathe opens, the spadix bursts into metabolic action. As the sun starts to set and P. bipinnatifidum's scarab beetle pollinators begin to wake up, heat production starts to hit a crescendo. For about 20 to 40 minutes, the inflorescence of P. bipinnatifidum reaches temperatures as high as 95 °F (35 °C) with one record breaker maxing out at 115 °F (46 °C)! Amazingly, this process is repeated again the following night.

It goes without saying that burning fat at a rate fast enough to reach such temperatures requires a lot of oxygen. Amazingly, for the two nights it is in bloom, the P. bipinnatifidum inflorescence consumes oxygen at a rate comparable to that of a flying hummingbird, which are some of the most metabolically active animals on Earth.

The world's largest inflorescence belongs to the titan arum (Amorphophallus titanum) and it too produces heat. Photo by Fbianh licensed under CC0 1.0

Again, why these plants go through the effort of heating their reproductive structures is still a bit of a mystery. For most, heat likely plays a role in helping to volatilize floral scents. Anyone that has spent time around blooming aroids knows that this plant family produces a wide range of odors from sweet and spicy to downright offensive. By warming these compounds, the plant may be helping to lure in pollinators from a greater distance away. It is also thought that the heat may be an attractant in and of itself. This is especially true for temperate species like the aforementioned skunk cabbage, which frequently bloom during colder months of the year. Likely both play a role to one degree or another throughout the aroid family.

What we can say is that the process of plant thermogenesis is absolutely fascinating and well worth deeper investigation. We still have much to learn about this charismatic group of plants.

LEARN MORE ABOUT AROID POLLINATION HERE



Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8]