Essential Concepts: Characteristics of Plants
Obj 1 Describe common characteristics of all plants
Plants are multicellular photosynthetic (with a few parasitic exceptions) eukaryotic organisms. Multicellular means they are made of many specialized cells, usually organized into tissues with specific functions. Photosynthetic refers to their ability to convert the sun’s solar energy into the chemical energy contained in the bonds of sugars. Eukaryotic refers to a type of complex cell that has nuclei and other organelles. Plants also have a unique tissue that animals lack: meristem, which is perpetually juvenile tissue that enable plants to grow indeterminately.
Plants and other photosynthesizing organisms (cyanobacteria, algae, and a few other protozoa), are the only means of capturing the sun’s energy and making it available to all other life forms, including humans. In addition, as a by-product of photosynthesis, free oxygen has become a major component of the atmosphere. Plants could survive without animals (although many flowering plants would have difficulty moving pollen around), but animals certainly could NOT survive without plants.
Unlike animals, plants exhibit an alternation of generations in which the haploid gametophyte generation produces gametes and the diploid sporophyte generation produces spores.
Gametophyte (1N)
mitosis mitosis
spores (1N) gametes (eggs and sperm 1N)
meiosis fertilization
Sporophyte (2N)
Obj 2Describe the anatomy, life cycle, and ecological importance of bryophytes.
Bryophyes include mosses, liverworts and hornworts, all of which grow only in habitats where water is available because water is necessary for their sperm to swim to eggs. These are relatively simple plants that lack a vascular system and thus must be small enough that water can be absorbed by their cells from their environment. The “plantlike” generation that we usually think of when we think of moss is the gametophyte. This is the dominant generation that undergoes photosynthesis, has root-like rhizoids, and a short axis (“stem”) with thin leaf-like structures (not true leaves).
The gametophyte produces gametes (eggs and sperm). The sperm swim through water to reach the eggs at the top of the moss. The diploid zygote becomes the sporophyte, which is dependent on the gametophyte and grows on top of it (in moss, it’s wiry with a pod on top that releases spores). Spores are the means by which moss (and ferns) spread to new areas.
moss: liverwort: hornwort:
Because bryophytes don’t have vascular roots they can grow on moist rocks and are often pioneer organisms in new habitats. They aid in the breaking down of rocks and the formation of soil, which enables other plants to eventually move into the habitat.
Sphagnum (“peat moss”), which grows in swampy peat bogs, can hold 20 times it’s weight in water and is thus useful as a soil amendment and for lining planters. In some places, like Ireland and Scotland, peat is also used for fuel.
Obj 3Describe the anatomy, life cycle, and importance of ferns and fern allies.
Ferns and fern allies (club mosses, ground pines, and horse-tails) have true vascular systems with roots, stems and leaves in the dominant sporophyte. The gametophyte is much reduced and is often not noticed. Like bryophytes, flagellated sperm still must swim through water to reach eggs, thus this group of plants also requires water for sexual reproduction. Many ferns and fern allies also reproduce asexually by means of rhizomes (underground stems) or runners (stems that spread over the ground) that root and form new plants.
Fern sporophyte Fern gametophyte Club Moss Horsetail
During the Paleozoic era, ferns (including tree ferns) and giant horsetails dominated forests. Today, only one genus (with about 40 species) of horsetails remains, Equisetum, with only small representatives. Thereare over 10,000 species of ferns (including tree ferns in tropical areas).
The leaves of ferns are called fronds. Often sporangia (organs that produce spores) can be found on the underside of fronds, but in some species separate non-green fertile fronds are produced separate from the green vegetative fronds.
Ferns are important to the floral industry and are important parts of a gardener’s repertoire.
Obj 4Describe the anatomy, life cycle, and importance of gymnosperms.
Gymnosperms, include familiar needle- and cone-bearing plants (conifers) like pines and spruces as well as broadleaved ginkgos and palm-like cycads. Gymnosperms have a great advantage over bryophytes and ferns in that they reproduce with seeds and cover large areas of the Earth’s surface. The development of pollen frees gymnosperms from relying on water as a vehicle for transporting sperm. Gymnosperms are wind pollinated.
The life cycle of gymnosperms involves two types of cones, the seed cone and the pollen cone. In the seed cone, megasporangia (mega = big) form megaspores by meiosis. One of the four produced by meiosis develops into a female gametophyte within the cone. This gametophyte will produce the egg within the cone. Meanwhile, small microspores (micro = small) are produced in pollen cones and develop into male gametophytes within pollen grains which are released by the pollen cone. When the pollen lands near a female gametophyte in a seed cone, a pollen tube develops to deliver the sperm to the egg. Fertilization takes place when egg and sperm fuse and a multicellular seed develops, with a sporophyte embryo, stored food from the megaspore, and a protective seed coat. Seeds are much better adapted for dispersal than are the spores of more primitive land plants, which are single haploid cells, and represent a major evolutionary invention.
CycadGinkgo
Conifers are not only important components of modern forests, where their seeds provide food for wildlife, but are common horticultural specimens as well. In addition, conifer wood is used in construction and in the paper making process. Cycads (in frost-free areas) and ginkgos are also important plants in the landscaping business.
Some other economically important products derived from conifers include turpentine and gin (made by redistilling spirits with juniper “berries” for flavoring).
Obj 5Describe the anatomy, life cycle, and importance of angiosperms.
Angiosperms, or flowering plants, are by far the most successful plants, with over six times the number of species of all other plant groups combined. And they are amazingly diverse, ranging from tiny duckweeds to cacti, and with habitats that include deserts, mountains, polar regions, lakes, forests and grasslands. We as humans are totally dependent on angiosperms: virtually all of our food is derived from flowering plants or animals that eat flowering plants. Corn, wheat, rice, and all fruit and grains are from flowering plants, as are cotton and linen. Most of our medicines, dyes, drugs, teas and spices come from angiosperms. Although gymnosperms are important sources of lumber, when wood of particular beauty or strength is desired, angiosperm trees, like walnut, oak, teak and mahogany fit the bill.
Angiosperm reproduction involves flowers. The female part of a flower, the carpel contains an ovary which encloses one or more ovules where, like in gymnosperms, the tiny female gametophyte develops from a megaspore. The male part of a flower, the stamen, contains microspores, which develop into pollen. Unlike gymnosperms, after fertilization the ovule matures and becomes the seed coat while the ovary becomes the fruit surrounding the seed. The word angiosperm literally means “hidden seed,” referring to the fact the seed is hidden within a fruit. (The term gymnosperm means “naked seed” because the seed is not surrounded by fruit – a mature ovary.) Like gymnosperms, angiosperm reproduction involves megaspores (which develop into female gametophytes) and microspores (which develop into pollen).
Angiosperms success may be attributed to their co-evolution with animals. Rather than relying on water or wind for reproduction, many angiosperms entice animals to do their work. Insect and other pollinators move pollen efficiently from flower to flower and birds and mammals eat their fruit and scatter seeds.