Download Chapter 24 All plants have a life cycle in which the diploid

Chapter 24
All plants have a life cycle in which the diploid sporophyte alternates with a haploid gametophyte generation. The
gametophyte produces the sperm and eggs, which join to form the sporophyte generation.
In seed plants, what we recognize as the plant is the sporophyte generation. The gametophytes are hidden within. In
gymnosperms, they are within the cones, and in angiosperms they are within the flowers.
Reproduction in Gymnosperms:
Gymnosperms produce two types of cones1. Pollen Cones- Produce the male gametophytes, called pollen grains.
2. Seed Cones- Produce the female gametophytes. Near the base of each scale are two ovules in which the gametophyte
develops.
Gymnosperm Life Cycle: Mature sporophyte (tree) produces pollen cones (male pollen) and seed cones (female ovule). If
a pollen grain fertilizes the egg, the zygote forms and is nourished by the female cone. It eventually develops into a new
tree.
Structure of Flowers:
Sepal- Green, like a leaf. Enclose the bud to protect developing flower.
Petal- Just inside the sepal. Often brightly colored to attract pollinators.
Stamen- Male part made of filament and anther (produces pollen)
Carpel- Female part made of ovary (where eggs are produced), style (stalk), and stigma (where pollen lands)
Pollination: After gametophytes have developed, pollination takes place. Gymnosperms and some angiosperms are by
wind. Most angiosperms are by animals.
After fertilization, the embryo begins to grow within the seed. As angiosperm seeds mature, the ovary walls thicken to form
a fruit that encloses the developing seeds. A fruit is a ripened ovary containing seeds. Fruit is ANY seed enclosed within a
wall. Apples, grapes, peas, corn, rice.
Why does natural selection favor fruits? Because it helps in seed dispersal. Fruits are a food source for animals. The
seeds are covered with a tough coating to protect it from digestive enzymes. The seeds are then carried to areas where
they don't have to compete with the parent plants, and sprout in the animal's feces.
Seeds dispersed by wind and water are generally lightweight. Maple seeds are encased in wing like structures that allow
them to glide away from the parent. Coconuts float in the sea for many weeks, allowing it to reach remote islands.
Some seeds sprout rapidly (like beans). Others have a period of dormancy during which the embryo is alive, but not
growing. The length depends on the plant species. Environmental factors such as temperature (even fire) and moisture
end dormancy and allow germination (sprouting) to begin.
Plants can also reproduce asexually by vegetative reproduction. This is the production of new plants from horizontal
stems, plantlets, and underground roots.
-Horizontal stems- Strawberries send out long trailing stems called stolons that produce roots when they touch the ground.
Once established, it can be broken off forming an independent plant.
-Plantlets- Plants produced on the ends of long stems. These can break off and grow into new plants. Spiderplants.
-Underground roots- Long underground stems can send up shoots that will grow new plants. Bamboo forests are often the
descendent of a single bamboo plant!
Plant Propagation: horticulturists use cuttings, graftings, or budding to make many identical copies of a plant.
-Cuttings: Cut a length of stem that has meristematic tissue (growing tissue) and plant it. Can use plant hormones to
facilitate root growth.
-Grafting and Budding- Best in dormant, woody plants that don't produce strong root system or seedless plants. Cut a
piece off the plant called a scion. It is then placed onto a plant that does have a strong root system. If the scion is a piece
of stem, it's called grafting. If a bud is used, it's called budding. Is only successful if the vascular tissue of the scion and
stock are firmly connected.
Chapter 25
Patterns of Plant Growth: Plants grow in response to light, moisture, temperature, and gravity. Unlike animals, growth
occurs throughout a plant's lifetime. Chemicals direct, control, and regulate plant growth.
A hormone is a substance produced in one part of an organism that affects a different part (target cell). A target cell must
have the proper receptor to be affected by a hormone. Different target cells may have different receptors for the same
hormone, causing different reactions (hormone A may stimulate growth in stems but inhibit it in roots.)
Phototropism is the tendency of a plant to grow toward a source of light. Darwin discovered that when the tip of a plant
was covered with an opaque cap, the plant won't grow toward the light. This proved that something in the tip was guiding
phototrophism. Later, the hormone auxin was found to be produced in the apical meristem and stimulate cell elongation.
Gravitropism is the response of a plant to the force of gravity. Auxins build up on the underside of roots and stems. In the
stems, auxins stimulate cell elongation, causing the trunk to grow upright. In the roots, auxins are the opposite- they inhibit
cell elongation, causing the roots to grow downward due to gravity.
Cytokinins are plant hormones that are produced in growing roots and developing seeds and fruit. They stimulate cell
division and the growth of lateral buds, and cause dormant seeds to sprout.
Gibberellins are growth promoting substances. They produce dramatic increases in size, particularly in stems and fruits.
In response to auxins, fruit tissues release small amounts of the hormone ethylene that stimulates fruit to ripen.
The responses of plants to external stimuli are called tropisms. Gravitropism (gravity). Phototropism (light). Thigmotropism
(touch). A vining plant that has tendrils that wrap around any object they encounter is an example of thigmotropism.
Rapid responses are responses that do not involve growth and are very fast. For example, when a fly triggers the sensory
cells on the inside of a flytrap's leaf, electrical signals are sent from cell to cell. A combination of osmotic pressure and cell
wall expansion causes the leaf to snap shut.
Photoperiodism is responsible for the timing of seasonal activities such as flowering and growth, and is dependent on the
number of daylight/dark hours a plant receives. Phytochrome is the pigment that absorbs red light and controls this cycle.
Phytochrome also regulates changes to prepare a plant for winter dormancy- the period during which growth and activity
decrease or stop. By turning off photosynthetic pathways, transporting materials from leaves to roots, and sealing off
leaves from the plant, the plant is protected in winter.
Plant Adaptations
-Aquatic Plants: Most grow in stagnant, oxygen depleted mud. Have tissues with large airfilled spaces to take in O2.
Mangroves have air roots. Cypress have "knees". Seeds can float on water and delay germination for long periods.
-Desert plants: Called xerophytes. Must tolerate strong winds, heat, sandy soil, little water. Adaptations include extensive
roots to absorb water, small leaves to reduce water loss, and thick stems to store water. Seeds can remain dormant for
years. When it does rain, some plants must quickly mature, flower, and set seed within weeks.
-Carnivorous Plants- Live in areas where nutrients are poor, so they have adapted ways to trap and digest insects.
-Epiphytes are plants that are not rooted in soil, but grow on the bodies of other plants. Generally in moist areas
(rainforest, swamp). Spanish moss is an example.
-Chemical defenses: Many plants defend themselves against insect attack by making compounds that have powerful
effects on animals. Lethal poison and hormones that inhibit growth and reproduction. Many of these are used in medicines
(aspirin, codeine, etc...)