Aloe or Agave?

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Convergent evolution is the process by which unrelated organisms evolve similar traits in response to similar environmental constraints. One amazing example of convergent evolution has occurred among the Aloe and Agave. These two distinct lineages are separated both in space and time and yet they often look so similar that it can be hard for the average person to tell them apart. With that in mind, lets consider the similarities and differences between these two lineages.

To start, Aloe and Agave hail from two completely different spots on the botanical family tree. Each also has its own unique geographic origin. Agave is a New World genus with species ranging in their distribution from tropical South America north into arid portions of North America. Genetic analysis places the genus Agave in the family Asparagaceae.

Agave americana  in bloom

Agave americana in bloom

Aloe, on the other hand, enjoys an Old World distribution, from Africa and Madagascar to the Arabian Peninsula as well as many islands scattered throughout the Indian Ocean. Taxonomically speaking, Aloe has undergone more than a few revisions through time, however, recent genetic work suggests that the Aloe belong to the family Asphodelaceae.

Experts believe that the lineages that gave rise to these two distinct genera branched off from a common ancestor some 93 million years ago. Despite all of that intervening time and space, the rigors of their arid habitats have managed to shape these plants in strikingly similar ways. Morphologically speaking, there is a lot of superficial similarity between Aloe and Agave.

Aloe hereroensis in situ

Aloe hereroensis in situ

Both groups exhibit water-storing, succulent leaves arranged in rosettes. These leaves are often adorned with spines or other protrusions aimed at deterring herbivores. Both groups also utilize CAM photosynthesis for their energy needs. When it comes time to flower, both groups frequently produce brightly colored, tubular flowers arranged at the tip of long stalks.

It is worth noting that the harsh environments that have shaped these two plant lineages also seems to have induced a backup plan for reproduction. Both Aloe and Agave produce tiny offshoots called "pups." These pups gain nourishment from the parent plant until they are large enough to fend for themselves. All pups are clones but if the parent plant had what it takes to survive in that spot, there is a good chance that its cloned offspring will as well. That way, even if sexual reproduction fails, these cloned progeny will get another shot.

Despite all of this convergence, these two lineages nonetheless exhibit vastly different developmental pathways and thus there are plenty of differences separating the two. For starters, slice into the leaves of each type and you will quickly find one major morphological difference. As many already know, Aloe leaves are largely filled with a gooey pulp and not much else. Aloe leaves function as water storage organs. Agave also store plenty of liquid in their leaves, however, they also produce numerous long strands of fiber that provide much more structural integrity.

Cross section of an Aloe leaf showing gelatinous pulp.

Cross section of an Aloe leaf showing gelatinous pulp.

Agave leaf showing fibrous interior.

Agave leaf showing fibrous interior.

Aloe and Agave each have evolved their own reproductive strategies as well. Aloe are perennial bloomers. Under the right conditions, many Aloe species will produce a profusion of flower stalks year after year. The stalks emerge from between the leaves and are largely pollinated by birds and insects in their native habitats. Agave, on the other hand, are monocarpic meaning they invest all of their energy into one single bloom. The Agave flowering stalk emerges from the center of the rosette and are pollinated by myriad insects, birds, and even bats. After flowering is complete, the main Agave plant dies.

Aloe flowers

Aloe flowers

Agave flowers

Agave flowers

Convergent evolution will never cease to amaze me. Despite millions of years and hundreds of miles separating these two lineages, Aloe and Agave have nonetheless been shaped in similar ways by similar environmental conditions.

Photo Credits: Wikimedia Commons

Further Reading: [1]

CAM Photosynthesis

 

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I was in a lecture the other day and I heard something that made the plant nut inside of me chuckle. The professor was trying to make the point that C3 photosynthesis is the most common photosynthetic pathway on the planet. To do this he said "it is the vanilla pathway." In this context, he was using vanilla as an adjective meaning "plain or ordinary." Of course, this was all very facetious, however, I thought it interesting and funny how, if taken literally, that statement was just plain wrong. 

I have written before about the reproductive ecology of Vanilla orchids (http://bit.ly/1LcC857). They are anything but vanilla the adjective. The other part of the statement that was wrong (again, if taken literally) is that C3 is the photosynthetic pathway of the vanilla orchid. In reality, vanillas are CAM photosynthesizers.

Last week I wrote about the C4 pathway and how it has helped plants in hot, dry places, but the CAM pathway is yet another adaptation to such climates. The interesting thing about CAM photosynthesis is that it separates out the different reactions in the photosynthetic pathway on a temporal basis. 

CAM is short for Crassulacean acid metabolism. It was first described in succulents in the family Crassulaceae. Hence the name. Similar to the C4 pathway, CO2 is taken into the leaves of the plant and stored as an organic acid. This is where the process differs. For starters, having acid hanging around inside your leaves is not necessarily a good thing. CAM plants deal with this by storing it in large vacuoles. That is one reason for the succulent appearance of many CAM species. 

Because these plants so often grow in hot, dry climates, they need to minimize water loss. Water evaporates from holes in the leaves called stomata so to avoid this, these holes must be closed. However, closing the stomata means not letting in any CO2 either. Whereas C4 plants get around this by only opening their stomata during the cooler hours of the day, CAM plants forgo opening their stomata entirely when the sun is up. 

Instead, CAM plants open their stomata at night when the vapor pressure is minimal. This ensures that water loss is also minimal.  Like camels storing water for lean times, CAM plants store CO2 as organic acid to use when the sun rises the next day. In this way, CAM plants can close their stomata all the while the hot sun is baking the surrounding landscape yet still undergo ample photosynthesis for survival. 

Not all orchids do this. In fact, some can switch photosynthetic pathways in different tissues. However, there are many other CAM plants out there including some very familiar species like pineapples, cycads, peperomias, and cacti. If you're like me and prone to talking to your plants, it is probably best to talk to your CAM plants after the sun has set. Not only does it confuse neighbors and friends, it provides them with CO2 when they are actively absorbing it. 

Photo Credit: C T Johansson (Wikimedia Commons)

Further Viewing: https://www.khanacademy.org/science/biology/cellular-molecular-biology/photosynthesis/v/cam-plants