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