Tuesday, February 23, 2016

From Weeds to Trees and Back Again

The evolution of the first plants into the modern trees included the incorporation of numerous evolutionary novelties. I'm going to focus on the ones that I think are important for the story I'm trying to tell, though I'll likely get them in the wrong order. It has been many years since I took a course emphasizing this topic and it is likely I also won't be able to find links discussing all the fossils from which this knowledge originally came. In the end, this first section below is really about setting context.



The first novelty was water transporting tissue (tracheids) that let plants grow taller (a few inches) than water diffusing through previous tissues would reach. The size of these early plants was then limited by how thick their stems could grow from their apical meristems (called primary growth). The ability to restart growth from tissue previously laid down by the apical meristems, called secondary growth, allowed plants to grow thicker (and taller, since the thicker stems could support greater height). Eventually the plants were again limited in size by their strength. The evolution of the first true wood (thick cellulose cell walls reinforced with lignin) allowed what we would first recognize as trees to develop.

The trunks of these first trees could only grow to a certain size, however, because of a quirk of their secondary growth. They grew outwards by dividing their outer cells parallel to the outer surface (periclinal division). As the trunk grew, the outer cells got thinner and and wider. Eventually the cells were too thin to divide further. The growth of the trunks slowed to a stop. Eventually, a group of trees developed the ability for its outer cells to divide in half from side to side (anticlinal division). By mixing periclinal and anticlinal divisions, these trees could grow beyond the limited size of their ancestors with only periclinal divisions. They could grow thicker, stronger, and thus taller.

from: www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_07/devo_07.html

At this stage we effectively have modern trees. There are lots of other interesting developments to talk about (vessel cells, seeds, leaves, flowers, etc.), but for some other time.



So, now that I've discussed the context...  what is the main topic I want to discuss?

Modern flowering plants (angiosperms) all count some of those ancient trees as ancestors. They range from the exceedingly tiny (Wolffia borealis), to the stupendously huge (Ficus benghalensis, others). Though most families of flowering plants don't range to such extremes, they generally count trees and small herbaceous plants among their members.

The milkweed family (Asclepiadaceae) is mostly small herbaceous species (such as Cynanchum barbigerum and Asclepias involucrata), but includes at least one moderately sized tree (Calotropis Procera). The dogwood family (Cornaceae) is most woody shrubs and trees (such as Cornus florida and C. kousa), but includes at least one herbaceous weed (C. canadensis).

If a tiny weed is introduced to an island, it can in a reasonably short time (from an evolutionary perspective) evolve into shrubs and large trees. This isn't just a story, an idea that sounds nice. The ecologies of isolated islands are often filled with very closely related plants filling wildly divergent ecological roles. Early on the geologic history of the Hawaiian islands, a tarweed seed managed to find a foothold and grow. It's descendants now fill the island in the form of weeds, vines, shrubs, and trees. On the Galapagos islands, a member of the common cactus genus Opuntia has evolved into a tree (Opuntia megasperma). On the channel islands, a member of the common herbaceous weedy genus Coreopsis has evolved into a tree (Coreopsis gigantea).

Coreopsis gigantea (from plants.usda.gov/core/profile?symbol=COGI)

Numerous other examples can likely be found where a tree has quickly evolved from an herbaceous group (or an herbaceous plant from a tree group) by examining the flora of the various isolated islands around the world.

Though the evolution of a flowering herbaceous plant into a tree or the reverse is interesting (and would make for really neat garden specimens), it doesn't require any dramatic evolutionary changes. Every flowering plant inherited the genetic/developmental toolkit necessary for growing as an herbaceous plant or as a tree from its ancestors among the very first trees. If an organism is missing some trait that its ancestors had, it is likely that the organism still carries most of the genetic tools needed to quickly evolve that trait in the future.


References

Tuesday, February 16, 2016

The Impermanence of Being (a Fossil)

A fossil you find may have existed hidden away underground for millions of years, but they have a very short lifespan once uncovered and exposed to the elements. Rain, snow, wind, animals, plants, and people (not just collectors) all wear away at exposed fossils that are almost invariably fragile. Within a relatively few years of becoming exposed, they crumble away to unrecognizable gravel. Good fossil exposures are transient things.

Because of the temporary nature and physically limited size of fossil sites, many fossil-hounds develop a habit of being somewhat vague when describing where they've found a really nice specimen (at least until they have gotten to know you well). If you widely spread the news about some interesting site you found, you're more likely to find the site completely picked over the next time you visit. These days, I can imagine fossils being quickly stripped from a site for sale online. I don't have a problem with someone selling fossils, but I would definitely despair at finding an interesting site emptied between one visit and the next. Much of the value of an interesting fossil in the context. (The geologic era, what species were found with it, etc.) This information can easily be lost if a site is picked over with too much haste.

The former fossil site, now a movie theater.
I recently checked in on a site where I once found numerous wonderful fossils. The site was directly behind my high-school in San Antonio, Tx. Thanks to GoogleMaps, I now know it to be the parking lot for a movie theater. I have no problem with telling the wider world exactly where the site is located. There is no further damage over-exposure can do to it.

Previously, there had been a wide, flat hilltop covered with multi-pound specimens of Exogyra ponderosa (a reef-forming oyster), along with the numerous shells from several smaller relatives. The fossiliferous layer was the very top of the hill, so one could walk along and easily visualize how the ecosystem was organized back in the Cretaceous era when this hilltop was the floor of a shallow sea. Though all the animals had been extinct for 60 million years, the fossils were comprehensive enough to clearly be a well-populated oyster reef. The site was impressive. I expect ecological studies could have been done there. The nearby area of undeveloped land probably contains fossils, since much of Cretaceous limestone does, but the oyster reef did not extend into that area. The reef no longer exists and only the few fossils remaining from the site in the hands of collectors like myself (and the biology teacher who pointed me towards the site) are evidence for it having ever existed.

A ~6in long Exogyra ponderosa from the oyster reef.


I always meant to spend more time exploring the area, but the classes and drama of high-school always seemed to get in the way. When I graduated and moved on to college in Austin-Tx, my parents moved out of state. I no longer had any connection to the neighborhood. It wasn't until years later, when I too had moved out of state, that I got my first car and with it the freedom to go wandering around looking for fossils and the like.

Now I have other fossil sites to visit and keep quiet about. (For example, where I found another mollusk... the-biologist-is-in.blogspot.com/2014/05/a-gastropods-lesson.html).


References:

Tuesday, February 9, 2016

"Chauncy"

The University of Minnesota just had the rare event of a blooming specimen of Amorphophallus titanum. The flower structure grows to nearly 6 feet tall, then opens to reveal a scent that befits one of its common names, the "Corpse Flower". I was able to stop by for a visit on 08-Feb-2016, a day after the bloom had first fully opened. From discussion around campus, it seems the scent produced by the flower was so overpowering for the first day that nobody was willing to go near it.

The structure isn't actually a single botanical flower. Instead it is a contains hundreds of relatively tiny and nondescript flowers along the base of the large spire, hidden inside the prominent shroud-like bract. The greenhouse staff cut a small window through the bract, so that visitors could see the inner structure (as well as get a very close sniff, for the brave).

The university staff had named the plant "Chauncy" and had posted some paper for people write their descriptions of the flower's scent. One description that caught my eye and well fit my experience of the flower's scent was, "A dead racoon 3 days old". The scent wasn't over-powering by the time I visited, but it did linger in my nose for about half an hour after I left the greenhouse.

In a neighboring pot to "Chauncy" was another specimen of A. titanum that isn't blooming this year. This plant was about 12 feet tall and appeared as a small tree with a few branches and luxuriant leaves. I use the word "appeared" because the entire above-ground structure of this plant is botanically a single leaf. It is somewhat like the leaf of a tomato plant, with stem-like structures, branches, and leaflets. Only in this case, the single leaf grows to tower overhead. After some time, the leaf will die and fall, only to be replaced by a new and larger leaf (if the plant is happily growing). The persistent body of the plant is a large tuber, securely hidden underground.



References: