Course Icon
Natural Science - Year I

Unit 13: Biological Classification, Plants

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

SO Icon

Science Web Assignment for Unit 13


This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 13:
Classifying Plants

For Class

Lecture Outline

Classification of Plants

If you open up a biology textbook, you will discover a particular method for classifying plants presented in that text. But if you go hunting for systems out on the web, you will discover that the modern classification of plants causes a lot of debate, and for precisely the same reasons that Theophrastus and Dioscorides came up with different systems. Different taxonomists have different purposes for classifying plants, and some species wind up in different kingdoms (Plant, Protist, or Fungi) depending on the importance the taxonomist assigns to particular characteristics.

We are going to use a general approach which is quite common, but be prepared for differences if you go look at classification schemes in other texts. First, we agree that plants are multicellular organisms which use chlorophyll during some part of their life cycle to produce their own food. Fungi are multicellular organisms which get their food by digesting organic materials from outside sources, usually dead organisms.

As with any classification system, we have a few exceptions: there are some saprophytic organisms (that means they are like fungi, using the plant materials of other organisms as a source of nutrition) which bear seeds, so they are classified as plants even though they have no chlorophyll and do no photosynthesis. The Indian pipe or coralroot plants (found on the trails around Mt. Rainier) fall into this category.

Like Theophrastus, we consider the parts of the plants, their reproductive methods, and their life cycles in order to classify them into similar groups. Plants can reproduce in several ways, asexually by budding, by sending up shoots from root systems, tubers, bulbs, or corms (note to all you gardeners: it's time to plant tulips for next spring), and sexually (mix-and-matching chromosomes) using different forms of spores or seeds. The seeds are the result of joining two "half-cells" called gametes, which in plants are pollen and ova or eggs. Reproductive cells that are self-contained are called spores. In many plants, the form of the plant changes during the life cycle, going from a generation which produces sperm and eggs ( the gametophyte stage) to a generation which produces spores (sporophyte stage).

One of the differences between fungi and plants is that fungi produce only spores, not seeds. Both spores and seeds contain the genetic instructions for a new plant, and both can survive in dry conditions for a long time (in some cases, over 1000 years). But seeds contain a mix of the parents' genetic code, while spores contain only one set of genes. Seeds also contain food, and in many species, the embryo of the plant actually develops inside the seed before it is even released from the parent.

Fungi

Fungi

Fungi used to be classified as plants, since they have some apparent similar structures, such as stems and roots. But closer inspection of these structures show that fungi "root" systems are radically different from the root systems of true plants, and that fungi don't use photosynthesis to create their own food. Instead, fungi secrete digestive chemicals into decaying organic matter, then absorb the partially-broken down molecules through their root-like hyphae structures. These structures can cover many acres, making some individual mushroom "complexes" the largest known single organisms.

Please read the overview of fungi characteristics at the University of California Berkeley site on morphology of living things.

Read the summary page, then click on Life History and Ecology, Systematics, and More On Morphology buttons to answer the following questions and those on your worksheet.

    As you read, consider the following points:
  • What are gills, sporangium, and hyphae?
  • How do fungi get their nutrition?
  • What are mycorrhizae? How do they help plants get nutrition from the soil?
  • What are the major divisions of fungi? How do they differ?

Non-vascular plants: Bryophytes

Bryophytes

The simplest plants are non-vascular plants, which just means they have no internal tube system for transporting water and nutrients. This limitation keeps these plants small; and since their sperm are motile (self-propelled swimming cells), they need to be in or around water to reproduce. Bryophytes include mosses, liverworts, and hornworts. The gametophyte is the green leafy part of the plant; periodically, small stalks with brown capsules appear on the plant (usually during a dry spell) and produce spores.

Please read the overview of characteristics of mosses and liverworts

Read the summary page; there are no other subpages.

    As you read, consider the following points:
  • How does the lack of veins to transport water and nutrients affect the size of mosses?
  • What must a moss's habitat have for it to reproduce?
  • Do mosses have true seeds?
  • How do mosses and lichen work together to erode rocks?
  • What are liverworts and hornworts? How do they differ from mosses?

Ferns

SwordFern

The simplest vascular plants have two kinds of tubes: xylem conducts water and minerals from roots to stem and leaves; phloem conducts sugar from the leaves to all other parts of the plant. Because of this transport system, vascular plants can grow larger than non-vascular plants.

Ferns

Whisk ferns, true ferns, club mosses, and horsetails are seedless vascular plants. These plants all have similar life cycles: the sporophyte is the dominant generation of the plant--in fact, it can be completely separate from the gametophyte form, which is very small and hard to find in nature. You'll realize you must be looking at the sporophyte when you check the back of a sword fern leaf and see all the spores on it. Like mosses, the sperm produced by the gametophytes are free swimming and need water to move around and find the eggs, so spore plants are found only in damp climates or where standing water is available part of the year.

Please read about ferns and horsetails.

Read one page on each site.

    As you read, consider the following points:
  • What must the environment contain for ferns and horsetails to survive?
  • How do ferns use spores to reproduce?
  • What changes does the fern plant undergo as it matures from spore to prothalius to primary plant?

Seed plants

Gymnosperms

Seed plants are divided in two major groups: those whose seeds are without coverings (gymnosperms, gymnos = naked), and those which have covered seeds (angiosperms). Angiosperms are the most successful group of plants: there are over 235000 species foundin all parts of the world. The gametophytes of both types seed plants are small and not always present--in gymnosperms, the gametophte is in the cone; in angiosperms, the gametophyte is the flower. The rest of the plant--which is most of what you see when you look at a seed plant-- is the sporophyte generation.

Gymnosperms (including conifers, cycads and ginkgos)

The cycads, ginkgos and Gnetophytes are three small phyla of gymnosperms. Some biologists interpreting fossil evidence think many more species of these existed long ago, but only 100 cycad species, 1 species of ginkgos, and three kinds of Gneophytes are found now in nature.

Nevertheless, a very successful phylum of gymnosperms still exists: the conifers. This phylum includes pines, spruces, hemlocks, junipers--all the plants that produce by cones. They usually have needle-like leaves with thick wax coatings, which helps reduce water loss and protect them during winter from harsh weather conditions. They grow very quickly when water is available, but can survive dry spells and drought conditions.

There are 550 species of conifers found all over the world, and where they are able to grow, they quickly become dominant (take over).

Please read about gymnosperms if you need more information.

Read one page on this site.

    As you read, consider the following points:
  • How do gymnosperms differ from other seed plants?
  • What are the major types of gymnosperms?

[Dr. C spends several hours pulling douglas fir and western cedar shoots out of her yard each fall--30 conifers on one city lot is enough, especially when most of them are over 50 feet high].

Flowering plants (including dicots and monocots)

Wild Rose

Traditionally, flowering plants are divided two groups based on the number of leaves which emerge from the germinated seed.

Monocots have one leaf emerge first from the seed. This group includes grasses, palms, orchids, lilies, and arums. These are non-woody plants with flowers that usually have sets of three petals; the leaves have parallel veins.

Dicots have two "first leaves". Buttercups, roses, birches, legumes, and daisies are all dicots. The diversity of dicots is greater than that of monocots, and includes woody deciduous trees, many shrubs, and herbs. Flowers of dicots are usually in sets of four or five, and the leaves have net-like venation.

Please read about flowering plants.

Read the summary page on this site, then click on the links to roots, stems, leaves, and flowers to examine those structures more closely.

    As you read, consider the following points:
  • Why are angiosperms successful in so many habitats?
  • What are the advantages of seeds over spores?
  • What are stomata and how do they help the plant survive?
  • What is the function of the petal, stigma, style, anther, filament, ovary, sepal, receptical, and pedicel in the flower?

Study/Discussion Questions:

  • How do characteristics of plants determine what conditions they can survive?
  • Which types of plants are most successful (have the most species, or cover the greatest range of habitats)?

Further Study/On Your Own