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PostPosted: Sun Jun 30, 2013 3:56 pm 
Broken Crown Panelist
Broken Crown Panelist

Joined: Sun Oct 21, 2012 1:59 pm
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Many times we learn about the biology of plants or animals, and the features they have inherited from their ancestors: an evolutionary approach. At the same time, we know that these features need to function in order to promote the health and viability of the species. We must remember that there is beautiful choreography between the inheritance of traits through evolution, and the function of these features in the environment in which this organisms survive.

So to when we teach botany, we tell students that plants have several organs: roots, stems, leaves, flowers, fruits, and seeds. We sometimes teach about these parts in a form/function approach, other times we discuss the evolution of these structures. Today we will discuss form, function, and the evolution of plant parts to get a holistic view. We began this conversation last month with a discussion of the importance of roots and their function for colonizing new areas. This month, I wanted to focus on the shoot system, and especially stems.

Above: modern stems (from Wiki Commons)

The stem shoot system is the most ancestral part of all land plants, dating back to when we start to find fossils 450 million years ago. It is hypothesized that roots and leaves, and many accessory reproductive parts of a plants evolved originally from stems. In fact, the early evolution of plants was a game of modification on a basic design of stems. The earliest land plants were very small, moss-sized, and would have looked like a bunch of small green sticks dotting a Mars-like landscape (see below).

Above: Early land plants (from Wiki Commons)

These tiny plants had features that made them able to adapt to the land environment that the green algae (their ancestors) did not possess. For example, these plants had to have a method to prevent drying out, unlike their aquatic cousins. They need to have a special coating, which we now call cuticle, to prevent desiccation. They also had to secure themselves to the landscape, which wasn't as important in an aquatic environment. As discussed last month, early plants did not have true roots, but hair-like rhizoids which only held them to the substrate. These plants were still dependent on a moist or saturated environment to absorb water and minerals. This absorption would have occurred by osmosis through the “skin” of a horizontally growing stem. These plants would have been flattened, green stems that appeared leaf-like, similar to modern liverworts. Early upright stems would have been a very efficient method to get reproductive structures high enough to disperse spores on the wind. Life would have been good for early plants…

Above: Modern liverwort (from Wiki Commons)

Eventually, plants would start to compete for space and new adaptations would have arisen, given some plants advantages over others. The stems that plants used to disperse spores, were a gateway to grow taller than the surrounding plants, and natural selection co-opted these shoots to create a bigger plant. Plants began to grow taller and even evolved the ability to branch or bifurcate, which was not found in the liverworts. While this was happening externally, the plants were adapting a method to move water from the ground level up to these growing tips. Plants needed a piping system to move water, as they grew taller and taller. Eventually plants become as tall as humans, and bushy as shrubs, and in a short (geological) time become tree-sized. Of course, plants are evolving sophisticated methods for collecting light (i.e. leaves), moving water (i.e. anatomy) and complex methods for reproducing (e.g. seeds), but the overall complexity we would have seen on this landscape would have come from a dazzling evolution of the plant stem shoot system. Even today, we tend to describe plants based upon the size and behavior of their stems (e.g. trees, shrubs, vines, etc.). Early plant evolution is a game of evolving stems.

Above: First “tree” on Earth, Eospermatopteris (Stein et al. 2012)

So if we landed on planet that had the potential for life, and especially photosynthetic life, would we expect to see a similar evolution of these organisms? This is obviously a huge question in evolutionary biology, and I don’t have any concrete answers. We would expect to find similar evolutionary principles on this world as we see on Earth, but they would be affected by the abiotic environment in which they live. If organisms have evolved a method to capture light to make stored energy, then we would expect them to start as extremely small: bacteria-sized. Over billions of years, new mutations would yield larger forms, which would be plagued with the same problems of early algae and land plants. As you get larger, you need to evolve new specialized systems to handle transport of essential materials. Those groups that evolve these systems will become larger, others will remain small and find new niches. It’s not unreasonable to think this world could follow a similar trajectory to the Earth’s evolution, but it would all depend on how life began on this world, and the effect of the non-living environment on the evolution of life. It will be an amazing day for evolutionary biology if/when we find another world that evolved life…

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