Chapter 37

1. The pollen tube is required to deliver the sperm cell from the stigma of the plant to the egg nucleus in the embryo sac, which is located in the ovary at the bottom of the style. Failure of the pollen tube to grow properly and reach the egg nucleus will result in failure of reproduction, including failure of seed and fruit formation.

2. A-39

   In the megagametophyte, one of the original nuclei becomes an egg cell; the remainder form the protective and nutritive embryo sac. In the microgametophyte, the eight original cells from meiosis become four two-celled pollen grains, each containing a sperm cell as well as a tube cell, which forms the pollen tube. The female structure is larger and contains nutrients for the developing embryo. The pollen grains are tiny and numerous, making movement easier and pollination more likely.

3. In triploid cells undergoing meiosis, there cannot be pairing of homologous chromosomes in meiosis I, so meiosis I is abnormal and functional gametes do not form. A fruit is formed from the ovary wall of the flower. Seedless grapes are probably propagated by cuttings (vegetative reproduction).

4. All fruits protect the seed and aid in seed dispersal, but the variety of fruits means they can fulfill these functions in many different ways, depending on locations and environmental factors. Some seeds, such as many tree seeds, fall to the ground and remain near their parent. Others are much smaller and have structures (wings, burs) that enable their dispersal by wind or animals. In this way, they can move long distances, increasing their range and chances of survival.

1. 1. The mutation stabilized the CO protein.

   2. The mutation caused nonfunction of the FD protein.

   3. The mutation increased expression of the FLC protein.

   4. The mutation caused constitutive expression of the CO protein.

2. Long-day plants typically bloom when days are long and nights are short, as in the summer. But in a greenhouse setting, plants can be tricked into responding as though the nights are short by turning on the lights for a short time in the middle of the night (night interruption lighting). This breaks up the long night, so the plants act as though the night is short—that is, they begin to flower.

3. Examples of evidence include:

   • Spinach will not flower if the shoot is masked to imitate short days but will flower if the bud (and not the shoot) is masked.

   • If a photoperiodically induced leaf is immediately removed from a plant, flowering will not occur. But if the leaf remains on the plant for several hours (allowing time for the substance to move to the bud), flowering will occur.

   • In grafting experiments, if only one plant (or plant part) is induced to flower, the other will flower as well.

1. Asexual reproduction is rapid, and it conserves the genetic adaptations of a plant. However, in asexual reproduction genetic homogeneity makes the offspring and population derived from it susceptible to a changing physical or biological environment, since the phenotypes in the population are all the same. Sexual reproduction produces genetic diversity in offspring and the population derived from it, and the phenotypic diversity makes the population adaptive in a changing environment. However, sexual reproduction is a slower process than asexual reproduction.

2. Asexual reproduction can occur without the need for pollination, producing many plants quickly. Many grow by means of stolons or rhizomes, which make them well adapted for poor or eroded soils, such as those on hillsides, or as pioneer plants in unstable soils such as sand dunes. Asexual reproduction is also adaptive in deserts, where it may be difficult for seedlings to become established.

3. Several approaches might be taken, such as a genetic screen for meiotic cells that do not separate chromosomes at anaphase I, or a search for proteins (and then their genes) that bind to SWII protein.

1. 1. The plants were grown in short-night conditions so that flowering would not be induced by having a long night. The only condition that would affect flowering would be the brief light during a long night.

   2. The data show that a 700-nm wavelength of light was effective at inducing flowering whereas a 680-nm wavelength inhibited flowering, indicating that the receptor is phytochrome. The far-red light for 8 min at 700 nm converted phytochrome to its active form, P_r.

2. During the day there is more red than far-red light, and the P_r form of phytochrome gets converted to the P_fr form. At night the P_fr converts back to P_r. This takes time; in the “No light” column, the data show that it took 8.5 hours for P_r to accumulate to sufficient concentration to promote flowering. This was delayed if red light (660 nm) was administered at the beginning of the dark period, as this converted even more P_r to P_fr. However, if far-red light (720 nm) was administered at the start of the dark period, the P_fr that had accumulated during the day got converted to the active form, P_r, which allowed flowering to occur after a shorter period in the dark (7 hours).

1. The plants with their leaves removed did not flower. Therefore the leaf senses photoperiod and must be present for flowering to occur.

2. The receptor is in the leaf.

3. The environmental signal (photoperiod) is received only by the single treated leaf. The signal sets in motion a signaling pathway that ends in the bud at the shoot apex, where flowering is initiated. The data imply that an inducer of flowering is produced in leaves and is able to travel through the plant from the leaf to the shoot apex.

1. The gi mutant inhibits or greatly delays flowering in the pea plant.

2. Yes, the evidence suggests that the gi mutant is unable to produce the floral chemical. When only the gi mutant is present (gi/gi), flowering never occurs. But whenever the WT is present, flowering does occur. This suggests that only the WT contains the floral chemical and that it is able to move in both directions (from stock to scion, or vice versa), causing the signal to reach the apex of the plant and stimulate flowering.

3. Under long-day conditions, the WT gene is expressed. It stimulates production of the FT protein that acts as a floral signal, and the plant flowers after 15−20 vegetative nodes have formed. However, the gigas gene is a null, or inactive, gene; it does not stimulate production of floral signal. In cases where the gigas gene is expressed, the plant remains vegetative; it either does not flower at all or flowers only after a long delay.