Nature Elective

The Four Major Groups of Plants
First posted June 13, 2004* Last updated October 19, 2005

Mosses, Ferns, Conifers, and Flowering Plants

There are 280,000 plants on Earth
but we can simplify this diversity into these four groups

Land plants evolved about 500 million years ago. They faced a problem that did not exist for aquatic plants: they needed to live in two different worlds. They needed to be part of the soil world, to get water, nutrients, and stability, but they also needed to be in the air, to get sunlight and carbon dioxide. Land plants solved this problem by developing roots as well as stems and leaves, and a system of vessels (xylem and phloem) to connect them. All four of the land plant groups have these features (except mosses do not have vessels). Their differences are seen in whether or not they have seeds or flowers, and in aspects of these features.

All land plants have another characteristic, called alternation of generations. We will not go into the details of this, other than to state the two generations are called the gametophyte (produces gametes) and the sporophyte (produces spores) generations. We need to mention these because the four plant groups each have unique alternation of generations.

Mosses The first group of land plants are the mosses and their allies, the liverworts and hornworts. Together, they are called the bryophytes. They are land plants, but do not have seeds or flowers. The gametophyte generation, that is, the generation that is the larger, more easily seen, is the one that produces gametes, not the one that produces spores. The sporophyte generation is a little plant that grows on or just under the soil and is rarely seen. Mosses reproduce with spores. If you look closely, you can sometimes see a little bulb on a thin stalk, sticking up from the moss. This structure is called a seta, and it is the sporophyte generation. The stalk is called a foot, and the bulb at the end is called a capsule. It contains the sporangium, which is the structure that produces the spores. Mosses lack vessels, so they are restricted to smaller sizes and more moist environments than other land plants. Mosses, then, are land plants without seeds or flowers, with a dominant gametophyte generation. The photograph at right was taken along a small creek that flows into Austin Creek, just above camp.

 

Ferns Ferns, horsetails, and their allies make up the second group of land plants, the pteridophytes. These plants all have vascular systems, made up of xylem (flow of water and nutrients from roots to leaves) and phloem (flow of sugars and other metabolic products from leaves to roots). Mosses do not have vascular systems. Ferns, however, do not have flowers.	This fern is found throughout Caz, under the shade of the redwoods.	This horsetail is found along Ausin Creek.
Ferns reproduce by means of spores. You can often see them if you turn over a mature leaf. You will see rows of little dots. Each dot is a sori, which contains the sporangia, which is the structure that produces thousands of spores. Each spore grows into a very small plant, called a gametophyte, and is the gametophyte generation. The gametophyte produces gametes (eggs and sperm), which unite to produce the fern plant. The fern plant that we know of, that has leaves, stems, and roots, produces spores, so it is the sporophyte generation.

Conifers The gymnosperms ("naked seeds") make up the third group of land plants. They produce seeds, not spores, that are contained within a cone. Seeds are a great evolutionary development, since, unlike spores, they are multicellular and contain nutrition for the new, developing plant, all within a protective coat. The largest group of gymnosperms are the conifers ("cone bearer"), which include the redwoods and Dougles-firs which are the dominant plants of Caz.
Conifers produce pollen cones, in which develop microsporangia, which undergo meiosis, producing pollen grains, which are immature male gametophytes. The pollen is blown by the wind onto female cones. This is a fairly inefficient process, so the conifers must produce a very large amount of pollen to ensure the female cones get fertilized. If you are in a conifer forest in the spring, you will find that your tent and car become covered in pollen grains. When a female cone gets fertilized with pollen, it produces seeds. The seeds are not contained within an ovary, so are considered "naked". The tall plant we recognize as a redwood is the sporophyte generation.

Flowering Plants

The final group of land plants are the angiosperms, also known as the flowering plants. The evolution of the flower represents the high point of plant evolution. The flower attracts many animals which assist in pollination, making the process of pollination more efficient and less random than in the gymnosperms. The seed develops in an ovary, which becomes a fruit. The fruit serves to help seed dispersal, since animals eat the seeds, which generally pass unharmed through the animal's intestinal tract. Birds and mammals may deposit the seeds, along with a little bit of fertilizer, a long way from the original plant.

The tanbark oak is also a flowering plant, but its flowers are small and inconspicuous. The "acorn" that is produced is the seed.

The SIU webpage on moss.

Part of this section is based on the excellent book Biology, by Campbell and Reece. It is a college text for beginning biology students. I cannot recommend it highly enough. If it has been a few years (!) since you have had biology, you will not believe how much things have changed. Not only is there a lot more known about biology, the presentation of the material is vastly improved. From text that is closer to literature than dry explanation, to color illustrations and color photographs that are so widely used, I doubt that in its entire 1240 pages, there are any without at least three or more color graphics of some sort. And that does not even begin to explore the enclosed CD or associated online material. I will eat my hat if you don't love the book. I bought mine online, at Amazon.com.

Acknowledgements

The section on land plants is based on the excellent book Biology, by Campbell and Reece. It is a college text for beginning biology students. I cannot recommend it highly enough. If it has been a few years (!) since you have had biology, you will not believe how much things have changed. Not only is there a lot more known about biology, the presentation of the material is vastly improved. From text that is closer to literature than dry explanation, to color illustrations and color photographs that are so widely used, I doubt that in its entire 1240 pages, there are any without at least three or more color graphics of some sort. And that does not even begin to explore the enclosed CD or associated online material. I will eat my hat if you don't love the book. I bought mine online, at Amazon.com.

*This webpage was created while in Budapest, Hungary, June 13, 2004.