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The Evolution of Vascular Plants

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  • 0:01 Evolution of Vascular Plants
  • 0:25 Evolution from Algae
  • 1:25 Early Evolution
  • 3:50 Vascular Tissue
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will cover the basics of the earliest evolution of vascular plants, as well as why and how they came to be. Terms such as stomata, rhizoids, xylem, and phloem are defined.

Evolution of Vascular Plants

There's a theory that land animals evolved from marine animals. Maybe some fish had a mutation that allowed them to walk on land, maybe not. We don't really know for sure. We're also not fully sure how terrestrial plants evolved but the theory goes that they, like land animals, evolved from aquatic organisms. This lesson goes over the basics of how vascular plants evolved early on.

Vascular Plants Evolved from Algae

The two core parts of a land dwelling plant are its root system, which acquires water and nutrients from the soil, and its shoot system, which acquires CO2 and light from the air. But the algal ancestors from which land plants evolved were very different than the terrestrial plants of today. They got all of their resources directly from the water they were in, and there was no real need to worry about transporting nutrients from one part of the organism to another because every cell was close to the nutrients it needed.

Just like you'd expect that the earliest animals on land looked pretty similar to the animals they evolved from in the water, the earliest plants were nonvascular like the algae they evolved from. By nonvascular, I mean they were organisms that did not have conducting tissue through which water and nutrients were carried, much like blood is carried through our body in our vascular tissue, the blood vessels.

Early Evolution

What set the earliest terrestrial plants apart from their algal ancestors was that, although they were nonvascular, they had photosynthetic shoots growing above the water in which they lived. The key thing about these shoots is they had waxy cuticles and few stomata, which are small openings that are involved in gas exchange and water loss. This means these plants didn't have to worry about excessive water loss.

These early plants also had some anchoring and absorbing functions, like the roots of modern land plants, thanks to rhizoids, hair-like outgrowths that anchor a plant in place. As you can imagine, the land plants tried to outcompete one another for resources over time and had to evolve newer and better ways of taking advantage of their environment. What did they compete for? Well, everything from light, to water, to nutrients like minerals.

Let's take light as an example. What kinds of plants would be favored to absorb as much light as possible?

A. Short plants

B. Tall plants with no appendages

C. Tall plants with large, broad, and flat leaves

I think you'd agree the answer is C. Short plants would be in the shadows of the taller plants, and plants without leaves would have a smaller surface area for the absorption of light. Thus a taller plant with larger leaves would have an advantage in collecting light.

As always, root systems are important as well. As plants became taller and had bigger and bigger appendages, their surface area increased. This meant more water would be able to evaporate from them and that of course meant they would need more water to survive. At the same time, larger shoots needed better anchors much like a large and tall building needs a larger and deeper foundation.

So how were these problems overcome? Roots. Multicellular branching roots evolved to anchor the plant in place and allow for the absorption of more water and more nutrients from the soil.

But as the plants evolved to become taller, the distances between the leaves and the roots increased. This posed another problem. How to transport the nutrients absorbed by the roots to the rest of the plant, and how to transport the products of photosynthesis around the plant?

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