- Describe modern nonvascular plants.
- Give an overview of living vascular plants.
- Outline the classification and evolution of seed plants.
- Summarize the adaptations and evolution of flowering plants.
- seed coat
The types of living plants today reflect the evolutionary past of the plant kingdom. From tiny nonvascular mosses to large flowering and fruiting trees, there are modern plants that represent each of the major evolutionary changes that occurred in this important eukaryotic kingdom.
Nonvascular plants are called bryophytes. Despite the dominance of vascular plants today, more than 17,000 species of bryophytes still survive. They include liverworts, hornworts, and mosses.
Characteristics of Nonvascular Plants
Most bryophytes are small. They not only lack vascular tissues; they also lack true leaves, seeds, and flowers. Instead of roots, they have hair-like rhizoids to anchor them to the ground and to absorb water and minerals (see Figure below). Bryophytes occupy niches in moist habitats. Without the adaptations of vascular plants, they are not very efficient at absorbing water.
The rhizoids of a bryophyte (shown in purple) may be so fine that they are just one cell thick.
Bryophytes also depend on moisture to reproduce. Sperm produced by a male gametophyte must swim through a layer of rainwater or dew to reach an egg produced by a female gametophyte. The tiny, diploid sporophyte generation then undergoes meiosis to produce haploid spores. The spores may also need moisture to disperse.
Evolution of Nonvascular Plants
The first nonvascular plants to evolve were the liverworts. The hornworts evolved somewhat later, and mosses apparently evolved last. Of all the bryophytes, mosses are most similar to vascular plants. Presumably, they share the most recent common ancestor with vascular plants.
Diversity of Nonvascular Plants
The three types of modern nonvascular plants are pictured in Figure below.
- Liverworts are tiny nonvascular plants that have leaf-like, lobed, or ribbon-like photosynthetic tissues rather than leaves. Their rhizoids are very fine, they lack stems, and they are generally less than 10 centimeters (4 inches) tall. They often grow in colonies that carpet the ground.
- Hornworts are minute nonvascular plants, similar in size to liverworts. They also have very fine rhizoids and lack stems. Their sporophytes are long and pointed, like tiny horns. They rise several centimeters above the gametophytes of the plant.
- Mosses are larger nonvascular plants that have coarser, multicellular rhizoids that are more like roots. They also have tiny, photosynthetic structures similar to leaves that encircle a central stem-like structure. Mosses grow in dense clumps, which help them retain moisture.
Liverworts, hornworts, and mosses are modern bryophytes. Liverworts are named for the liver-shaped leaves of some species. Hornworts are named for their horn-like sporophytes.
Vascular plants are known as tracheophytes, which literally means “tube plants.” The earliest vascular plants quickly came to dominate terrestrial ecosystems. Why were they so successful? It was mainly because of their tube-like vascular tissues.
The vascular tissues for which these plants are named are specialized to transport fluid. They consist of long, narrow cells arranged end-to-end, forming tubes. There are two different types of vascular tissues, called xylem and phloem. Both are shown in Figure below.
Xylem is vascular tissue that transports water and dissolved minerals from roots to stems and leaves. This type of tissue consists of dead cells that lack end walls between adjacent cells. The side walls are thick and reinforced with lignin, which makes them stiff and water proof.
Phloem is vascular tissue that transports food (sugar dissolved in water) from photosynthetic cells to other parts of the plant for growth or storage. This type of tissue consists of living cells that are separated by end walls with tiny perforations, or holes.
Xylem and phloem are the two types of vascular tissues in vascular plants.
Evolution of Vascular Plants
The first vascular plants evolved about 420 million years ago. They probably evolved from moss-like bryophyte ancestors, but they had a life cycle dominated by the diploid sporophyte generation. As they continued to evolve, early vascular plants became more plant-like in other ways as well.
- Vascular plants evolved true roots made of vascular tissues. Compared with rhizoids, roots can absorb more water and minerals from the soil. They also anchor plants securely in the ground, so plants can grow larger without toppling over.
- Vascular plants evolved stems made of vascular tissues and lignin. Because of lignin, stems are stiff, so plants can grow high above the ground where they can get more light and air. Because of their vascular tissues, stems keep even tall plants supplied with water so they don’t dry out in the air.
- Vascular plants evolved leaves to collect sunlight. At first, leaves were tiny and needle-like, which helped reduce water loss. Later, leaves were much larger and broader, so plants could collect more light.
With their vascular tissues and other adaptations, early vascular plants had the edge over nonvascular plants. The could grow tall and take advantage of sunlight high up in the air. Bryophytes were the photosynthetic pioneers onto land, but early vascular plants were the photosynthetic pioneers into air.
Diversity of Seedless Vascular Plants
Surviving descendants of early vascular plants include clubmosses and ferns. There are 1,200 species of clubmoss and more than 20,000 species of fern. Both types of vascular plants are seedless and reproduce with spores. Two examples are pictured in Figures below and below.
- Clubmosses look like mosses and grow low to the ground. Unlike mosses, they have roots, stems, and leaves, although the leaves are very small.
- Ferns look more like "typical" plants. They have large leaves and may grow very tall. Some even develop into trees.
Clubmosses like these are often confused with mosses.
There’s no confusing ferns with mosses. Why do these ferns look more plant-like?
Seed plants are called spermatophytes. The evolution of seeds by vascular plants was a very big deal. In fact, it was arguably as important as the evolution of vascular tissues. Seeds solved the problem of releasing offspring into a dry world. Once seeds evolved, vascular seed plants and their descendants diversified to fill terrestrial niches everywhere. Today, vascular seed plants dominate Earth.
Parts of a Seed
As shown in Figure below, a seed consists of at least three basic parts: the embryo, seed coat, and stored food.
- The embryo develops from a fertilized egg. While still inside the seed, the embryo forms its first leaf (cotyledon) and starts to develop a stem (hypocotyl) and root (radicle).
- The tough seed coat protects the embryo and keeps it from drying out until conditions are favorable for germination.
- The stored food in a seed is called endosperm. It nourishes the embryo until it can start making food on its own.
A typical plant seed, like this avocado seed, contains an embryo, seed coat, and endosperm. How does each part contribute to the successful development of the new plant?
Many seeds have additional structures that help them disperse. Some examples are shown in Figure below. Structures may help them travel in the wind or stick to animals. Dispersal of seeds away from parent plants helps reduce competition with the parents and increases the chance of offspring surviving.
Dandelion seeds have tiny “parachutes.” Maple seeds have “wings” that act like little gliders. Burdock seeds are covered with tiny hooks that cling to animal fur.
Classification of Seed Plants
The two major divisions of seed plants are the gymnosperms (seeds in cones) and angiosperms (seeds in ovaries of flowers). Figure below shows how the seeds of gymnosperms and angiosperms differ. Do you see the main difference between the two seeds? The angiosperm seed is surrounded by an ovary.
In gymnosperms, a seed develops on the scale of a cone. Only an angiosperm seed develops inside an ovary.
There are only about 1,000 living species of gymnosperms, whereas there are hundreds of thousands of living species of angiosperms. Because angiosperms are so numerous and diverse, they are described separately below. Living gymnosperms are typically classified in the divisions described in Table below. Most modern gymnosperms are trees with woody trunks. The majority are conifers such as pine trees.
There is only one living species (Ginkgo biloba); some living trees are more than 2000 years old; they originated in Asia but now are cultivated all over the world; they have been used for medicines for thousands of years.
There are more than 700 living species; most are trees such as pines with needle-like leaves; they are often the dominant plants in their habitats; they are valuable to humans for paper and timber.
There are about 300 living species; they are typically trees with stout trunks and fern-like leaves; they live only in tropical and subtropical climates; they have large, brightly-colored seed cones to attract animal pollinators.
There are fewer than 100 living species; most are woody vines with evergreen leaves; they live mainly in tropical climates; they are the least well known gymnosperms but the most similar to angiosperms.
Evolution of Seed Plants
The earliest seed plants probably evolved close to 300 million years ago. They were similar to modern ginkgoes and reproduced with pollen and seeds in cones. Early seed plants quickly came to dominate forests during the Mesozoic Era, or Age of the Dinosaurs, about 250 to 65 million years ago.
As seed plants continued to evolve, Earth’s overall climate became drier, so early seed plants evolved adaptations to help them live with low levels of water. Some also evolved adaptations to cold. They had woody trunks and needle-like, evergreen leaves covered with a thick coating of waxy cuticle to reduce water loss. Some of the trees were huge, like today’s giant sequoia, a modern conifer (see Figure below).
The person standing at the foot of this giant sequoia show just how enormous the tree is. Some early seed plants also grew very large.
Eventually, some gymnosperms started to evolve angiosperm-like traits. For example, cycad ancestors were the first plants to use insects as pollinators. They also used birds and monkeys to disperse their brightly colored seeds. Of modern gymnosperms, Gnetae probably share the most recent common ancestor with angiosperms. Among other similarities, Gnetae produce nectar, a sweet, sugary liquid that attracts insect pollinators. Most modern flowering plants also produce nectar.
Angiosperms, or flowering seed plants, form seeds in ovaries. As the seeds develop, the ovaries may develop into fruits. Flowers attract pollinators, and fruits encourage animals to disperse the seeds.
Parts of a Flower
A flower consists of male and female reproductive structures. The main parts of a flower are shown in Figure below. They include the stamen, pistil, petals, and sepals.
- The stamen is the male reproductive structure of a flower. It consists of a stalk-like filament that ends in an anther. The anther contains pollen sacs, in which meiosis occurs and pollen grains form. The filament raises the anther up high so its pollen will be more likely to blow in the wind or be picked up by an animal pollinator.
- The pistil is the female reproductive structure of a flower. It consists of a stigma, style, and ovary. The stigma is raised and sticky to help it catch pollen. The style supports the stigma and connects it to the ovary, which contains the egg. Petals attract pollinators to the flower. Petals are often brightly colored so pollinators will notice them.
Sepals protect the developing flower while it is still a bud. Sepals are usually green, which camouflages the bud from possible consumers.
A flower includes both male and female reproductive structures.
Flowers and Pollinators
Many flowers have bright colors, strong scents, and sweet nectar to attract animal pollinators. They may attract insects, birds, mammals, and even reptiles. While visiting a flower, a pollinator picks up pollen from the anthers. When the pollinator visits the next flower, some of the pollen brushes off on the stigma. This allows cross-pollination, which increases genetic diversity.
See The beauty of pollination at http://www.youtube.com/watch?v=xHkq1edcbk4 for an amazing look at this process.
Other Characteristics of Flowering Plants
Although flowers and their components are the major innovations of angiosperms, they are not the only ones. Angiosperms also have more efficient vascular tissues. In addition, in many flowering plants, the ovaries ripen into fruits. Fruits are often brightly colored, so animals are likely to see and eat them and disperse their seeds (see Figure below).
Brightly colored fruits attract animals that may disperse their seeds. It’s hard to miss the bright red apples on these trees.
Evolution of Flowering Plants
Flowering plants are thought to have evolved at least 200 million years ago from gymnosperms like Gnetae. The earliest known fossils of flowering plants are about 125 million years old. The fossil flowers have male and female reproductive organs but no petals or sepals.
Scientists think that the earliest flowers attracted insects and other animals, which spread pollen from flower to flower. This greatly increased the efficiency of fertilization over wind-spread pollen, which might or might not actually land on another flower. To take better advantage of this “animal labor,” plants evolved traits such as brightly colored petals to attract pollinators. In exchange for pollination, flowers gave the pollinators nectar.
Giving free nectar to any animal that happened to come along was not an efficient use of resources. Much of the pollen might be carried to flowers of different species and therefore wasted. As a result, many plants evolved ways to “hide” their nectar from all but very specific pollinators, which would be more likely to visit only flowers of the same species. For their part, animal pollinators co-evolved traits that allowed them to get to the hidden nectar. Two examples of this type of co-evolution are shown in Figure below.
The bat is active at night, so bright night-blooming flowers attract it. The hummingbird hawk-moth has a long needle-like mouthpart (called a proboscis) to reach nectar at the bottom of the tube-shaped flowers. In each case, the flowering plant and its pollinator co-evolved to become better suited for their roles in the symbiotic relationship.
Some of the most recent angiosperms to evolve are grasses. Humans started domesticating grasses such as wheat about 10,000 years ago. Why grasses? They have many large, edible seeds that contain a lot of nutritious stored food. They are also relatively easy to harvest. Since then, humans have helped shaped the evolution of grasses, as illustrated by the example in Figure below. Grasses supply most of the food consumed by people worldwide. What other grass seeds do you eat?
The plant on the left, called teosinte, is the ancestor of modern, domesticated corn, shown on the right. An intermediate stage is pictured in the middle. How were humans able to change the plant so dramatically?
Classification of Flowering Plants
There are more than a quarter million species of flowering plants, and they show tremendous diversity. Nonetheless, almost all flowering plants fall into one of three major groups: monocots, eudicots, or magnolids. The three groups differ in several ways. For example, monocot embryos form just one cotyledon, whereas eudicot and magnolid embryos form two cotyledons. The arrangement of their vascular tissues is also different. Examples of the three groups of flowering plants are given in Table below.
- Nonvascular plants are called bryophytes. They include liverworts, hornworts, and mosses. They lack roots, stems, and leaves. They are low-growing, reproduce with spores, and need a moist habitat.
- Vascular plants are known as tracheophytes. Vascular tissues include xylem and phloem. They allow plants to grow tall in the air without drying out. Vascular plants also have roots, stems, and leaves.
- Most vascular plants are seed plants, or spermatophytes. They reproduce with seeds and pollen. Some modern seed plants are gymnosperms that produce seeds in cones.
- Most modern seed plants are angiosperms that produce seeds in the ovaries of flowers. Ovaries may develop into fruits. Flowers attract pollinators, and fruits are eaten by animals, which help disperse the seeds.
Lesson Review Questions
1. Describe nonvascular plants.
2. Identify the parts of a seed and the role of each part.
3. Name and describe the division of gymnosperms.
4. Describe the male and female reproductive structures of flowers and their functions.
5. State how fruits help flowering plants reproduce.
6. Charles Darwin predicted the existence of a moth with a very long “tongue” after he discovered a species of night-blooming flowers with extremely long, tube-shaped blooms. About 50 years after Darwin died, such a moth was discovered. Apply lesson concepts to explain the basis for Darwin’s prediction.
7. Compare and contrast xylem and phloem.
8. How did vascular tissues and lignin allow vascular plants to be “photosynthetic pioneers into air”?
9. Explain how flowering plants and their animal pollinators co-evolved.
Points to Consider
In this chapter, you read about the evolution and classification of plants. In the next chapter, you can read more about the special cells, tissues, and organs of plants that make them such important and successful organisms.
- How do you think plant cells differ from the cells of other eukaryotes, such as animals? What unique structures do plant cells contain?
- Besides vascular tissues, what other types of tissues do you think plants might have?
For Table above, from top to bottom,
For Table above, from top to bottom,
For Table above, from left to right, top to bottom,