Biology

Modern Plants

Big Picture

There are four groups of plants that represent major evolutionary steps in the plant kingdom. Nonvascular plants are the first basic plant and have no vascular tissues. Vascular plants soon evolved these tissues, which help transport fluids from different parts of the plant body. Seed plants were revolutionary in plant evolution; they solved the issue of releasing offspring, or seeds, into the dry world. Vascular seed plants now dominate the Earth. Flowering plants evolved petals and scents to help attract pollinators so that seeds can be spread more efficiently. Their ovaries may develop into fruits, which also helps encourage animals to disperse the seeds.

Key Terms

Bryophyte: Another name for a nonvascular plant.

Vascular Tissue: Type of tissue in plants that trans-ports fluids through the plant; includes xylem and phloem.

Xylem: Type of vascular tissue in a plant that transports water and dissolved nutrients from roots to stems and leaves.

Phloem: Type of vascular tissue in a plant that trans-ports food from photosynthetic cells to other parts of the plant.

Tracheophyte: Another name for a vascular plant.

Spermatophyte: Another name for a seed plant.

Angiosperm: A seed plant that buds flowers.

Germination: Early growth and development of a plant embryo in a seed.

Endosperm: The stored food in a seed.

Gymnosperm: Type of seed plant that produces bare seeds in cones.

Ovary: A female reproductive organ.

Nectar: Sweet, sugary liquid produced to attract animal pollinators.

Stamen: The male reproductive structure of a flower.

Pollen: Tiny grain that carry the male reproductive cells of seed plants. Used to transfer sperm to female reproductive structures.

Pistil: The female reproductive structure in a flower.

Petal: Outer parts of flowers that are usually brightly colored.

Sepal: Part of a flower that helps protect it while it is still in bud.

Nonvascular Plants

There are more than 17,000 species of bryophytes today and include liverworts, hornworts, and mosses.

Characteristics

  • lack vascular tissues
  • lack true leaves, seeds, and flowers
  • lack roots and instead have hair-like rhizoids to anchor them to the ground and to absorb water and minerals
  • depend on moisture to reproduce

Diversity

There are three types of modern nonvascular plants:

Liverwort

Liverworts have leaf-like, lobed, or ribbon-like photosynthetic tis-sues rather than leaves. Instead of roots, they have fine rhizoids, and they lack stems. They generally are 10 centimeters (4inches) tall and grow in colonies.

Image Credit: Kristian Peters,
CC-BY-SA 3.0
Hornwort

Hornworts, like liverworts, have fine rhizoids and lack stems. They are several centimeters taller than liv-erworts. Their sporophytes are long and pointed.

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Image Credit: alexlomas,
CC-BY 2.0
MassWort

Mosses have coarse, multicellular rhizoids with tiny leaf-like structures that surround a central stem-like structure. They grow in dense clumps, which help them retain moisture.
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Image Credit: Taina Sohlman,
2010, under license from
Shutterstock.com

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Biology

Modern Plants cont.

Evolution

Liverworts were the first nonvascular plants to evolve. Hornworts shortly followed after liverworts, and mosses were last. Mosses are the most similar to vascular plants, sharing the most recent common ancestors with them.

Vascular Plants

Vascular Tissues

Vascular tissues are what separate vascular plants from nonvascular plants. They are long, narrow cells that arrange to form tubes. In fact, vascular plants are also called tracheophytes, meaning “tube plants.” These tubes transport fluids from and to different parts of the plant body. There are two types of vascular tissues:

  • Xylem: Transports water and dissolved materials from the roots to the stems and leaves. The side walls of the xylem are thick with lignin, which makes them stiff and water proof. They lack walls between adjacent cells.
  • Phloem: Transports food(like sugar dissolved in water)from the photosynthetic cells to other parts of the plant body for storage or growth. They have end walls with tiny holes between the adjacent cells.

Diversity

There are two main vascular plants:

  • Clubmosses: Similar to mosses, clubmosses grow close to the ground. However, they have roots, stems, and leaves (although their leaves are small).
  • Ferns: Ferns are more plant-like. They have large leaves and grow very tall-some develop into trees.
Xylem Vessle

Evolution

Vascular plants evolved around 420 million years ago. They became more plant-like by evolving the following:

  • Roots made of vascular tissues. Roots absorb more water and nutrients than rhizoids do. They also anchor the plants securely to the ground, allowing them to grow taller without falling over.
  • Stems made of vascular tissues and lignin. Lignin helps stiffen the stems, allowing plants to grow taller so that they can get more light and air. Vascular tissues help tall plants stay hydrated so they do not dry out in the air.
  • Leaves to collect sunlight. Leaves were at first needle-like and tiny, which helped reduce water loss. Broader leaves evolved later and helped plants collect even more sunlight.
Phloem vessel

Seed Plants

Spermatophytes, or seed plants, evolved around 300 million years ago.

Parts of a Seed

There are three basic parts of a seed:

  • Embryo: Develops from a fertilized egg. Inside the seed, an embryo forms its first leaf(cotyledon)and starts to develop a stem(hypocotyl) and root(radicle).
  • Seed coat: Protects the embryo until it is ready for germination.
  • Endosperm: Nourishes the embryo until it can start making food of its own.

Seeds may have additional features that help them disperse, such as tiny hooks that cling on animals or wings that help them travel in the wind.

Spermatophytes
Image credit: Mariana Ruiz Villarreal (LadyofHats), Public Domain

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Biology

Modern Plants cont.

Seed Plants (cont.)

Classification

There are two major seed plants:

  • Gymnosperm: A plant here the seed develops on the scale of a cone. Male cones contain pollen, and female cones contain eggs. There are only around 1,000 species of gymnosperms, and a majority of modern gymnosperms are conifer trees.(Pine trees are a type of conifer trees.)Divisions of gymnosperm include: Ginkgoes, Conifers, Cycads, and Gnetae.
  • Angiosperm: A plant where the seed develops inside an ovary. Angiosperms are much more diverse than gymnosperms, and hundreds of thousands of species are classified as angiosperms.
Gymnosperm & Angiosperm
Image credit: Tameeria, Public Domain

Evolution

Seed plants came to dominate Earth's forests during the Mesozoic Age(the Age of Dinosaurs), 250 to 65 million years ago. Seed plants adapted to the dry climate of Earth, such as evolving woody trunks and needles with a waxy coating to reduce water loss. Eventually, some gymnosperms began to form angiosperm-like traits. Gnetae, a division of modern gymnosperms, share common ancestors with angiosperms. Gnetae also produce nectar, a sweet, sugary liquid that attracts insect pollinators.

Flowering Plants

Parts of a Flower

A flower has both female and male reproductive structures. Here are the main parts of a flower:

  • Stamen: It has a stalk-like body(the filament)that ends with an anther, which produces pollen. The filament raises the anther so that the pollen has a higher chance of being spread by wind or animal.
  • Pistil: The female reproductive structure in a flower. The pistil consists of the stigma, style, and ovary. The stigma is raised and sticky so that it can catch the pollen grains for reproduction. The style connects the stigma to the ovary, which contains the egg. Petals are attached to the flower to attract pollinators with their noticeable colors.
  • Sepals: Sepals protect the flower when it is developing and has not yet bloomed. Sepals are usually green to camouflage the flower from predators.

Characteristics

Flowers often are colorful with strong scents and a sweet nectar to attract pollinators. While visiting a flower, the pollinator (these can be birds, mammals, insects, even reptiles) picks up pollen from the anthers and carries it on to the next flower, where some of the pollen brushes off on the stigma. The result of pollination is often cross-breeding, which increases genetic diversity.

Angiosperms have evolved features that are even more effective than flowers. Some flowering plants have ovaries that ripen into brightly colored and scented fruits. This helps to attract animals that will eat the flesh and disperse the seeds

Classification

Despite the quarter of a million flower species today, there are three main groups of flowering plants:

  • Monocots: examples include grasses and orchids
  • Eudicots: examples include daisies and peas
  • Magnoliids: examples include magnolias and avocados

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Biology

Modern Plants cont.

Flowering Plants (Cont.)

Parts of a Flower (Cont.)

Parts of a flower
Figure: Parts of a flower
Image credit: CK-12 Foundation,
CC-BY-NC-SA 3.0

Evolution

Although the earliest fossils found of flowers are 125 million years old, scientists believed they started to evolve around 200 million years ago from gymnosperms. Angiosperms evolved colorful petals and nectar to attract insects and animals that could carry pollen from flower to flower, which was more efficient than depending on the wind to spread pollen. Flowers developed a way to "hide" their nectar from all except specific pollinators. This way the pollinators are more likely to visit flowers of the same species, increasing chances of pollination.

More recent angiosperms evolved to become grasses.
Grasses have many large, edible seeds that contain nutrition. Humans started to domesticate grasses around 10,000 years ago.

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Notes

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