39.0. Introduction (p. 693)
A. Flowering Plants Are Unique
1. Flowering plants (angiosperms) are the most diverse and widespread of all plants; their success is attributable to two characteristics.
a. The structure of the flower allows them to produce seeds within fruits.
b. The flower permits pollination to take place not only by wind, but also by animals.
B. Pollination
1. Flowering plants that rely on animals for pollination have a mutualistic relationship with them.
2. The flower provides nutrients for the pollinator (e.g., a bee, a fly, a beetle, a bird, or even a bat).
(Fig. 39A)
3. In turn, animal pollinators inadvertently carry pollen from one flower to another, allowing pollination to occur.
39.1. Flowering Plants Undergo Alternations of Generations (p. 694)
A. A life cycle is entire sequence of events from fertilization and formation of the zygote to gamete formation once again.
1. In contrast to animals with one type of adult generation, flowering plants exhibit an alternation of generations life cycle that includes the microgametophyte and megagametophyte. (Fig. 39.1) [transp. 206]
2. The sporophyte is the diploid generation in an alternation of generations life cycle.
a. The sporophyte produces haploid spores by meiotic cell division.
b. The spores develop into a haploid gametophyte.
3. The gametophyte generation is the haploid generation in an alternation of generations life cycle.
a. A gametophyte produces haploid gametes by mitotic cell division; gametes fuse to form a diploid zygote.
b. The zygote undergoes mitotic cell division to develop into the sporophyte.
4. In flowering plants, diploid sporophyte is dominant or longer lasting; it is what we commonly recognize.
5. The sporophyte is the generation that contains vascular tissue and has other adaptations suitable to living on land, including the production of flowers. (Fig. 39.1) [transp. 206]
6. A flower produces two types of spores, microspores and megaspores.
a. A microspore is the plant spore that develops into a microgametophyte.
1) The microgametophyte is the pollen grain; it is windblown or carried by an animal to the megagametophyte.
2) When mature, its nonflagellated sperm cells travel by way of a pollen tube to the megagametophyte.
b. The megaspore is the plant spore that develops into a megagametophyte, the embryo sac which remains within the body of the sporophyte plant.
7. The alternation of generations that flowering plants undergo is modified in such a way that a sperm does not require an outside source of water to reach an egg, which nonflowering plants may require.
a. The gametophytes are microscopic and dependent upon the sporophyte.
b. Microgametophytes are transported to a megagametophyte by wind or an animal; water is not needed.
c. The pollen tube carries a sperm to an egg, and again no outside water is needed.
d. Following fertilization, the diploid zygote develops into an embryo protected within a seed enclosed by a fruit.
B. A Flower Is a Sporophyte Structure
1. A flower is the reproductive organ of a flowering plant; develops within a bud.
2. Shoot apical meristem may stop forming leaves to form flowers, or axillary buds become flowers directly.
3. Flower structures are modified leaves attached to a stem tip (receptacle). (Fig. 39.2) [transp. 207]
4. Monocot flower parts are in threes or multiples of three; dicot flower parts are in fours or fives or multiples.
5. Sepals are most leaflike; usually green; this outermost whorl protects the bud as the flower develops within.
6. Petals are leaflike flower parts interior to sepals; coloration accounts for the attractiveness of many flowers.
a. The size, the shape, and the color of a flower are attractive to a specific pollinator.
b. Wind-pollinated flowers often have no petals at all.
7. The pistil is the vaselike structure located at the center of a flower; it contains one or more carpels.
a. A pistil may be simple or compound.
b. A simple pistil contains one carpel; a compound pistil contains multiple carpels, which are often fused.
8. Carpels are the reproductive units of flowers and have three parts..
a. The stigma is an enlarged sticky knob on the end of a style; the stigma serves to receive pollen grains.
b. The style is a slender stalk that connects the stigma with the ovary.
c. The ovary is the enlarged base of a carpel that contains a number of ovules.
9. Grouped about the pistil are stamens, stalked structures that have two parts.
a. The anther is a saclike container within which pollen grains develop.
b. The filament is a slender stalk that supports the anther.
10. Not all flowers have sepals, petals, stamens, and a pistil.
a. Complete flowers are flowers that have sepals, petals, stamens, and a pistil; incomplete flowers do not.
b. Perfect flowers have both stamens and a pistil.
c. Staminate flowers have only stamens.
d. Pistillate flowers have only pistils.
11. If staminate and pistillate flowers are on the same plant, the plant is monoecious; if staminate and pistillate flowers are on different plants, the plant is dioecious.
12. It is not strictly correct to call the pistil the female part of a flower, or the stamen the male part; these organs do not produce gametes, but megaspores and microspores that mature to produce eggs and sperm.
C. The Gametophytes Are Separate
1. The ovary in a carpel contains one or more ovules. (Fig. 39.3a) [transp. 208]
2. Ovules contain central mass of parenchyma cells almost covered by integument with an opening, the micropyle.
3. One parenchyma cell enlarges to become a megasporocyte.
a. The megasporocyte undergoes meiotic cell division to produce four haploid megaspores.
b. Three megaspores disintegrate, leaving one functional megaspore; nucleus divides mitotically into eight nuclei.
c. When cell walls form around the nuclei later, there are seven cells, one of which is binucleate.
d. The megagametophyte (embryo sac) consists of these seven cells: one egg cell, two synergid cells, one central cell with two polar nuclei, and three antipodal cells.
4. An anther has four pollen sacs; each contains many microsporocytes. (Fig. 39.3a) [transp. 208]
a. Microsporocytes are microspore mother cells.
b. Microsporocytes undergo meiotic cell division to produce four haploid microspores. (Fig. 39.3b) [transp. 208]
c. Each microspore develops into a microgametophyte (pollen grain).
5. Pollen grains are windblown or carried by various kinds of animals to the stigma of a pistil.
D. Pollination Precedes Fertilization
1. Pollination and fertilization are separate events.
2. Pollination is strictly transfer of pollen from the anther to the stigma of the pistil. (Fig. 39.3b) [transp. 208]
a. Pollination is brought about by the wind or with the assistance of a particular animal pollinator (Fig. 39A).
b. Self-pollination is transfer of pollen from the anther to the stigma of the same plant.
c. Cross pollination is transfer of pollen from the anther of one plant to the stigma of another plant; it is evolutionarily advantageous because it promotes genetic recombination resulting in new and varied plants.
3. Fertilization Is Double (Fig. 39.3b) [transp. 208]
a. Fertilization is the fusion of nuclei, as when sperm nucleus and egg nucleus fuse.
b. Following pollination, a pollen grain germinates, forming a pollen tube. (Figs. 39.4b and 39.5) [micro. slides 79-80] [transp. 209]
c. A germinated pollen grain, containing a tube cell and two sperm, is the mature microgametophyte.
d. As a pollen tube grows, it passes between cells of the stigma and style to reach the micropyle of the ovule.
e. The pollen tube grows through the micropyle and releases both sperm cells into the ovule.
f. One sperm nucleus unites with the egg nucleus, forming a 2n zygote, and the other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming the 3n endosperm nucleus.
g. Double fertilization occurs.
4. The zygote divides mitotically to become the embryo, and the endosperm cell divides mitotically to become the endosperm. (Fig. 39.3b) [transp. 208]
a. The embryo, in most plants, is a young sporophyte and the result of the first cell divisions following fertilization until germination.
b. The endosperm is the tissue that will nourish the embryo and seedling as they undergo development.
39.2. The Embryo Develops in Stages (p. 701)
A. Stages in Development of Dicot Embryo ( Fig. 39.6) [transp. 209]
1. After double fertilization, as the ovule is becoming a seed the following occurs:
a. The endosperm nucleus divides to produce a mass of endosperm surrounding the embryo.
b. The single-celled zygote also divides, forming two parts: the embryo and the suspensor, which anchors the embryo and transfers nutrients to it from the sporophyte plant.
B. Embryonic Development
1. After differentiation into embryo and suspensor, one or two cotyledons develop.
a. Cotyledon is a seed leaf providing nutrient molecules for the developing plant before a plant photosynthesizes.
b. Dicot embryos develop two cotyledons, whereas monocot embryos develop only one cotyledon.
c. During the development of a monocot embryo, the cotyledon rarely stores food; rather, it absorbs food molecules from the endosperm and passes them to the embryo.
d. During the development of a dicot embryo, the cotyledons usually store the nutrient molecules that the embryo uses, obtaining those nutrients from the endosperm.
2. The embryo continues to differentiate into three parts. (Fig. 39.6) [transp. 210]
a. The epicotyl is above the cotyledon; it contributes to shoot development.
b. The hypocotyl is below the cotyledon; it contributes to shoot development.
c. The radicle is below the hypocotyl and contributes to root development.
39.3. The Seeds Are Enclosed by Fruit (p. 702)
A. Seeds and Fruits
1. As the zygote develops into an embryo, the integuments of the ovule harden and become a seed coat.
2. The ovule matures into the seed, containing sporophyte embryo plus stored food, enclosed in a protective seed coat.
3. The ovary, and sometimes other floral parts, develops into a fruit, a mature ovary that usually contain seeds.
4. As a fruit develops from an ovary, the ovary wall thickens to become the pericarp.
C. Fruits Are Varied (Fig. 39.7) (Table 39.1)
1. A simple fruit develops from an individual ovary, either simple or compound.
2. A fleshy fruit has a fleshy pericarp (e.g., peach, plum, olive, grape, tomato, apple, and pear).
3. A dry fruit pericarp is dry (e.g., milkweed, pea, bean, lentil, poppy, sunflower, acorn, rice, and barley).
4. A compound fruit develops from a group of individual ovaries.
a. Aggregate fruit develops from ovaries from single flower (e.g., blackberry), while an aggregate fruit where each ovary becomes a one-seeded fruit is called an achene (e.g., strawberry).
b. A multiple fruit develops from ovaries that are from separate flowers fused together
(e.g., pineapple).
D. How Seeds Disperse and Germinate
1. Hooks and spines of clover, bur, and cocklebur attach to fur of animals for dispersal.
2. Birds and mammals often eat fruits, including the seeds, and defecate them some distance from the plant.
3. Squirrels and other animals gather seeds and fruits and bury them some distance away.
4. Woolly hairs, plumes, and wings are adaptations for dispersal by wind.
5. Fruit of a coconut palm floats hundreds of kilometers; some plant seeds have trapped air or inflated sacs.
6. The touch-me-not plant has seed pods that swell as they mature and burst, hurling their ripe seeds.
E. Germination of Seeds
1. Some seeds do not germinate until they have been dormant for a period of time.
a. For seeds, dormancy is the time during which no growth occurs, even though conditions may be favorable.
b. In temperate zones, seeds often have to be exposed to cold weather before dormancy is broken.
c. In deserts, germination does not occur until there is adequate moisture, which helps to ensure that seeds do not germinate until the most favorable growing season has arrived.
2. Germination is complex; it takes place if there is sufficient water, warmth, and oxygen to sustain growth.
a. Regulation of germination involves both growth inhibitors and growth simulators.
1) Fleshy fruits contain inhibitors so that germination does not occur until the seeds are removed and washed.
2) Growth stimulators are present in the seeds of some temperate zone woody plants.
b. Mechanical action may also be required to facilitate germination.
1) Water, bacterial action, and even fire can act on the seed coat, allowing it to become permeable to water.
2) Water uptake causes the seed coat to burst.
3. Germination in Dicots (Fig. 39.8) [transp. 211]
a. Prior to germination, the embryo consists of the following:
1) two seed leaves (cotyledons) that supply nutrients to embryo and seedling shrivel and disappear eventually;
2) a plumule---a rudimentary plant that consists of an epicotyl bearing young leaves;
3) the hypocotyl, which becomes the stem; and
4) the radicle, which develops into the roots.
b. As a dicot seedling emerges, the shoot is hook-shaped to protect delicate plumule.
c. As a seed germinates in darkness, it etiolates; the stem increases in length and leaves remain small.
d. Phytochrome pigment sensitive to red and far-red light, regulates response and induces normal growth in light.
4. Germination in Monocots (Fig. 39.9) [transp. 212]
a. The endosperm of a monocot seed is the food-storage tissue, and the cotyledon does not have a storage role.
b. The monocot "seed" is actually the fruit; the outer covering is the pericarp.
c. Prior to germination, embryo consists of one cotyledon, a plumule, and a radicle.
d. The plumule and radicle are enclosed in protective sheaths, the coleoptile and coleorhiza, respectively.
e. The plumule and the radicle burst through these coverings when germination occurs.
39.4. Plants Can Reproduce Asexually ( p. 708)
A. Means of Asexual Propagation
1. Plants contain nondifferentiated meristem tissue and routinely reproduce asexually by vegetative propagation.
2. In asexual reproduction, offspring arise from a single parent and inherit the genome of that parent only.
3. Vegetative propagation utilizes the meristematic tissue of a parent plant.
a. The nodes of stolons will produce strawberry plants. (Fig. 39.10)
b. Violet plants grow from nodes of rhizomes.
c. Each eye of a potato plant tuber is a bud that will produce a new plant.
d. Sweet potatoes can be propagated from modified roots.
e. Many trees can be started from small "suckers."
4. Stem cuttings have long been used to propagate a wide array of plants.
5. The discovery that auxin will cause roots to develop has expanded the ability to use stem cuttings.
B. Plants Propagate in Tissue Culture
1. In 1902, German botanist Gottleib Haberlandt suggested entire plants could be produced from tissues.
2. Tissue culture is the process of growing tissue artificially in a liquid culture medium. (Fig. 39.11)
3. Haberlandt stated plant cells were totipotent; each cell has the full genetic potential of the organism.
4. Only in 1958 did Cornell botanist F. C. Steward grow a complete carrot plant from a tiny piece of phloem.
5. When cultured cells are provided with sugars, minerals, vitamins, and cytokinin, the undifferentiated cells divide and initially form a callus, an aggregation of undifferentiated cells.
6. The callus then differentiates into shoot and roots and develops into complete plants.
7. Hybridization is the crossing of different varieties of plants or even species.
a. This is routinely done to produce plants with desirable traits.
b. Hybridization, followed by vegetative propagation, generates many plants with same characteristics.
8. Micropropagation is a commercial method of producing thousands, even millions, of identical seedlings, through tissue culture, in a limited amount of space.
9. Meristem culture is a form of micropropagation in which many new shoots will develop from a single shoot apex culture in a medium with the correct proportions of auxin and cytokinin.
a. Since the shoots are genetically identical, the adult plants that develop from them are clonal plants.
b. Clonal plants have the same genome, and therefore, display the same traits.
c. Meristem culture generates meristem that is virus-free; therefore the plants produced are also virus-free.
d. Flower meristem culture is a type of meristem culture in which propagation is accomplished by using the somatic embryos that form at the top of a callus; done for certain vegetables and ornamentals.
1) Somatic embryos are asexually produced by meristem culture by the millions in large tanks (bioreactors).
2) Embryos are encapsulated in a protective hydrated gel ("artificial seeds") and can be shipped anywhere.
10. Anther culture is micropropagation with mature anthers cultured in a medium of vitamins and growth regulators.
a. Haploid tube cells within a pollen grain divide mitotically, producing proembryos made of as many as 40 cells.
b. Finally, the pollen grains rupture, releasing haploid embryos.
1) The researcher can then generate a haploid plant.
2) Alternatively, chemical agents can be added that encourage chromosomal doubling, resulting in plants that are not only diploid but also homozygous for all their alleles.
c. This technique produces homozygous recessive plants that would take generations with hybridized plants.
11. Cell suspension culture is a form of micropropagation in which rapidly growing calluses are cut into small pieces and shaken in a liquid nutrient medium.
a. Single cells or small clumps break off and form a suspension of cells that all produce the same chemicals as the entire plant.
b. This technique is a more efficient way of producing chemicals used in drugs, cosmetics, and agricultural applications than farming plants simply for the purpose of acquiring the chemicals they produce.
12. A protoplast is a "naked" cell from which the cell wall has been removed.
a. Enzymes can be used to digest the cell walls of a small piece of plant tissue, usually mesophyll from a leaf, resulting in the production of protoplasts. (Fig. 39.12) [micro. slide 81]
b. The protoplasts regenerate a new cell wall and begin to divide.
c. These clumps of cells can be manipulated to produce somatic embryos or entire plants.
d. Plants generated from somatic embryos vary somewhat because of mutations that arise during the production process; these somatoclonal variations are another way to produce new plants with desirable characteristics.
C. Genetic Engineering of Plants
1. Genetic engineering alters the genes of organisms so that they have new and different traits.
2. Transgenic plants carry one or more foreign genes that have been introduced into their cells.
(Figs. 39.13 and 39.14)
3. Protoplasts in particular lend themselves to direct genetic engineering in tissue culture.
a. Using high voltage electric pulses, pores are created in the plasma membrane of protoplasts so that genetically recombined DNA can be introduced into the cells.
b. When genes for production of firefly enzyme luciferinase were inserted into tobacco protoplasts, adult plants glowed when sprayed with the substrate luciferin.
c. A desired gene is inserted into a plasmid of the bacterium Agrobacterium; the bacterium, which normally infects plant cells, can be used to deliver the recombinant DNA to the target cells. (Fig. 39.14b).
d. Theodore Klein of Cornell University developed the particle gun to bombard plant cells in culture with DNA coated metal particles; later, adult plants are generated.
4. Crops have been engineered to resist frost, fungal and viral infections, insect predation, and herbicides.
5. Future crops could have higher protein content and require less water and fertilizer.