"What is the purpose of the giant sequoia tree? The purpose of the giant sequoia tree is to provide shade for the tiny titmouse." --Edward Abbey

I. Plants -- Recommended supplementary Web site: Introduction to the Plantae

1. General notes on plant structure
   1. Multicellular, with several differentiated cell types (unlike algae, which are protists).
   2. Almost all plants are photosynthetic and contain chlorophyll. (Exceptions: a few saprophytic or parasitic plants, such as dodder and Indian pipe)
   3. Cell walls made of cellulose. When this is mature, it prevents a plant cell from expanding in size.
   4. Growth occurs at the tips of roots and shoots, in special zones of immature cells without thick cell walls. These are called meristems.
   5. True plants also have a waxy layer, or cuticle, surrounding the outer layer of cells.
   6. Most land plants have several specialized organs:
      1. Roots -- absorb water and certain nutrients; may be used to store nutrients (as in beets)
      2. Stems -- usually support; may photosynthesize. Underground horizontal stems are often mistaken for roots; they're called rhizomes (crabgrass, irises, potatoes)
      3. Leaves -- usually the main site of photosynthesis. Modified leaves include vine tendrils, cactus spines.
2. Two broad groupings of plants:
   1. Bryophytes include mosses, plus some less-familiar plants such as liverworts and hornworts. These lack the water-conducting cells that form vascular tissue.
   2. Tracheophytes, or vascular plants, include both plants that reproduce using seeds, and plants that do not produce seeds. (More will follow on plant reproduction.)
      1. Non-seed plants include the ferns and horsetails.
      2. Seed plants include the sago palms, ginkgos, conifers, and flowering plants.
   3. All of these plants do have vascular tissue, which comes in two general types:
      1. Xylem -- transports water and minerals from roots to rest of plant. In its mature form, xylem is made up of dead cells. (In a tree, xylem makes up the wood.)
      2. Phloem -- transports sugars (as sap) made by photosynthesis from leaves to rest of plant. Mature phloem is still alive. (In a tree, phloem makes up most of the inner bark.)
   4. Because bryophytes can't transport water, they usually live in very moist habitats, and they never grow very tall. There are mosses that live in deserts, however -- they are able to dry out completely, and then revive when water is available.
   5. Woody trees have xylem on the inside, phloem on the outside. . .
      1. . . . and in between the two, a thin ring of actively dividing tissue called the cambium.
      2. The cambium lays down new xylem (on its inside) and new phloem (on its outside).
      3. Since the cambium is active in spring and summer, and shuts down in winter, its action produces growth rings of xylem, virtually always one ring per year.
      4. Thus you can count the growth rings of a tree and determine its age.
         
         Cross section of a young woody stem. Click on it to view an enlargement.