Study Guide

Plant Biology Themes

  • Evolution

    A plant's "goal" in life is to survive and pass on its genes. Here we will look at different ways in which plants have evolved adaptations to their environment to enhance their survival. We’ll look at cacti and their arsenal of spines as well as the story behind the toxins in poisonous plants so you understand just why Poison Ivy’s kiss is so deadly for Batman.

    Biology teachers like to quote the famous but funny-named biologist Theodosius Dobzhansky, who said, "Nothing in biology makes sense except in light of evolution." What he meant was that even though sometimes living things exhibit crazy behavior, like birds performing ridiculous mating dances, cacti growing tons of prickly thorns, or poison dart frogs producing absurdly toxic chemicals, all have an evolutionary explanation. In some way, each of these things help their organisms survive and reproduce. What does this mean for plants? We know plants can take on many different shapes and sizes, and some are spiky and others are poisonous or make delicious fruit. These characteristics always have some evolutionary context.

    If you ever venture into a desert, you will notice that the plants have certain characteristics in common: their stems are fat and succulent, there aren’t a lot of broad green leaves, and they are prickly and hurt if you bump into them. If you’re thinking that deserts only have sand and palm trees, think again! Deserts are full of plant life and some, such as the Sonoran Desert, can be quite green and lush. So what’s the deal? Why do desert plants look so different and why do they want to hurt us?

    The plants don’t really want to hurt us. They are just defending themselves, just as you would if someone came up and tried to take a bite out of you. Imagine a desert with a bunch of plants with leaves that look like this: (insert leaf photo here) and a bunch of plants that look like this: (insert spine photo here). You’re hungry, you’re thirsty, and like a good little herbivore you’re going to eat some leaves to satisfy these needs. Which leaves are you going to eat first? Right. You’re going to devour the tame, non-pointy leaves with such devotion that those plants have little chance of surviving and reproducing. 

    Replay this situation thousands of times and you’ll see how we ended up with deserts full of spiny plants. It’s an example of survival of the fittest (thank you, Mr. Darwin). In this case the plants that are "fitter" are the ones that are spiny because they don’t get eaten. Pointy spines also lose water more slowly than flat leaves, giving their plants another advantage over the typical leaf shape found in other habitats. The succulent nature of plants in arid environments has also been selected for, evolutionarily speaking: plants that can retain water by storing it in their stems, such as a barrel cactus, survive dry spells much better than plants that don’t store water this way. In the photos below, the cactus is happy because it stored water in its stems, unlike the rose, which doesn’t store water in its stem. Never mind that the rose in question was probably cut off of the main plant, and now has no roots.

    A similar problem has played out in other habitats, too. Leaves are green and juicy, and as Kermit the Frog told us, it isn’t easy being green. Or juicy. Plants from many habitats have evolved a way of dealing with herbivores: poison! Certain plants make toxic chemicals to discourage animals from eating them. These chemical defenses are called secondary compounds because they serve a purpose that is secondary to growth and reproduction. 

    It should be no surprise that poison hemlock is poisonous, but did you know that apple seeds can also poison you if you eat too many? That’s right, they have cyanide in them. One or two won’t hurt, but don’t eat a whole cup of them. Tomato leaves, raw shells of cashews, and rhubarb leaves, and wild potatoes all contain serious amounts of toxins that can harm or kill humans.

    So what’s the deal with all these crazy poisonous plants? How humans domesticated them and managed to overcome all the toxins is another question, but we can understand why they are poisonous by thinking about evolution. Plants that make toxic secondary compounds are trying to protect themselves from damage. If individual caterpillars of Species X die every time they eat the leaves of Plant 1, those caterpillars don’t get to reproduce and pass on their genes. Meanwhile, the individuals of Species X that eat non-toxic Plant 2 live, reproduce, and pass on the genes that make them find Plant 2 so tasty. In Plant 1’s ideal world, this is perfect. Plant 1 kills off the caterpillars that eat it and is left to live in peace. Plant 2 might also die out because it keeps getting eaten before it can reproduce.

    In reality, Plant 1 doesn’t kill off all the caterpillars that try to eat it. Some caterpillars are resistant to the plant’s toxin and keep eating. Uh oh. This is bad news for Plant 1 because the caterpillars that can tolerate its toxin will probably pass this trait on to their offspring, and their offspring will be able to eat the plant too. As caterpillars wage war on Plant 1 and the rest of its species, the individual plants that have the strongest, most toxic poison have the best chance of surviving. Over many generations, the plants develop more potent toxic defenses and the caterpillars develop resistance to higher levels of the toxin. This process is called coevolution, and is why many plants (and many prey animals) make highly toxic substances. 

  • Levels of Organization

    From cell to leaf to plant to population, species and ecosystem, plants have a lot of different levels of organization, like other organisms. The great redwood forests exist only on the western coast of the United States, in California and Oregon. They live in places with coastal fog, and the redwood trees are the tallest living things. A redwood forest is a whole ecosystem, but it is made up of many individual trees, which are made up of cells.
    Starting with cells, what makes plants unique? Plant cells have rigid cell walls, and they have special machinery to allow them to convert sunlight to food:

    chloroplasts. The chloroplasts are organelles that supply energy for the cell and the plant, by converting using solar energy to make sugars. It would be pretty easy to tell a plant cell from an animal cell, assuming you have a microscope to look through.
    Of course cells don’t exist in a vacuum—cells make up different tissues and plant structures that we have already discussed in this unit: shoot and root systems, including stems, leaves, flowers, fruits, and roots. When light hits a redwood leaf, chloroplasts in the palisade parenchyma start converting the light into energy the plant can use. But not all leaves are created equal; leaves at the top of a tree look and act differently than their counterparts closer to the ground.

    If you walk around a redwood forest, you’ll notice it is pretty dark on the ground. The extremely tall trees with all their branches don’t let a lot of light reach down to the forest floor, though some plants cope with the low light and grow there anyway. Because of the difference in light, leaves on top of a redwood tree are much thinner and smaller. At the top of the tree, these leaves get plenty of light but also have to deal with wind, which wicks water away from the leaves. Smaller leaves have less surface area, which means less water can evaporate from them. Further down the tree, leaves are wider. These leaves grow in shade, so they have a large surface area that maximizes the amount of light they get. These leaves also aren’t as exposed to wind since they are protected by the trees around them. The bigger leaves growing in the shade are actually more efficient at photosynthesis than sun leaves are; the shade leaves have to be, since they don’t get much sun.

    The vascular tissue (phloem in this case) carries the products of photosynthesis (photosynthates) to other areas of the plant. In the meantime, xylem carries water up from the roots to the shoot system. However, redwoods are extremely tall; can the vascular tissue really carry water all the way up the tree trunk? Redwoods actually have another way of dealing with their thirst needs. They don’t just rely on water in the ground to reach the tops of their trunks. They have found another solution: fog! Redwood trees drink in the fog that surrounds their canopies as another water source, and circumvent the problem of getting water all the way up to the top of the tree.

    Redwoods and support whole ecosystems by creating habitat for a variety of animal life, scavengers, herbivores and predators. The holes left when branches fall off a tree are perfect nesting spots for these animals. In Redwood National Park in California, a whole bunch of threatened or endangered animals make redwood forests their home, including bald eagles, marbled murrelets, northern spotted owls, Coho salmon, Chinook salmon, and Steelhead trout. That’s right, fish can live in redwood forests—as long as a stream runs through.

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