Bot 305; Summer 2004                                                  Name:  KEY

Test #1

Give succinct (40 words or less), precise answers in the space provided after each question.Unless otherwise noted in the margin each section of each question is worth 4 points.

1.       (a) When scientists get an insight that explains a complicated phenomenon, they feel a sense of excitement and pride. According to the article Strong Inference, what is a pitfall that this feeling can lead to, and how can a scientist avoid this pitfall?

Ans.: It could lead one to favor their single explanation and not consider other, equally plausible hypotheses, which could bias the design of experiments. Scientists can avoid this pitfall by proposing alternative plausible explanations of the phenomenon observed.

(b) Following the rules of Strong Inference is, according to John Platt, “like climbing a tree”. Explain.

Ans.: In  a tree climb, at each branch of a tree, only one of two paths up the tree is followed. Following the rules of Strong Inference, after each round of experiments one of two plausible explanations is eliminated.

2.       (a) Name an early pioneer in research on plant electrical activity, and indicate why his work was featured in The Secret Life of Plants.

Ans.: Bose was an early pioneer. His work was featured because, although initially rejected by specialists, it was ultimately verified. This suggests that apparently outlandish claims (e.g., those in The Secret Life of Plants) can sometimes turn out to be true.

(b) When scientists visited Backster and asked him to demonstrate how plants could respond electrically to threats or to witnessing the injury or death of an animal, often the plants would not respond. What was Backster’s explanation of these failures?

Ans.: Backster proposed that the unresponsive plants had fainted out of fear or distrust of the inquiring scientist.

(c) Following the Rules of Strong Inference, propose an alternative to Backster’s  explanation described in your answer to 2(b).

Ans.: The instances of apparent electrical response of plants to threats, etc. were examples of random electrical noise.

(d) It should be possible to carry out an experiment that followed the third rule of Strong Inference to resolve whether your explanation or Backster’s was more likely to be correct. Describe two features of such an experiment that would help assure it was following the third rule of Strong Inference.

Ans.: To carry out the experiment so as to get a credible result (= 3^rd rule), it would be important to have many replicates of the tests and to carry out the experiments under conditions that minimized stray electrical noise.

3. (a) When pollen is made, the parent packages some molecules into the exine layer of the pollen. In some cases these molecules can determine whether the pollen germinates or not. Explain.

Ans.: In plants that have sporophytic self-incompatibility, the male self-recognition factor is put in the pollen by the parent plant, and on self stigma it can react with the female self-recognition factor to induce the blockage of pollen germination.

(b)      When pollen grains germinate on a Petri dish, the pollen tubes that emerge grow out in random directions, but when pollen germinates on a stigma it grows unerringly toward the egg. What is guiding the directional growth of the tubes growing out from stigma-germinated pollen?

Ans.: A chemical gradient is guiding the pollen tube. Typically the highest concentration of the chemical is at the entrance to the egg.

4. (a) How many sperm cells are carried in the pollen tube, and what are their functions?

Ans.: Two sperm cells are in the pollen tube. One fertilizes the egg and one fuses with two polar nuclei in the central cell to form a triploid nucleus of a cell that will divide and become the endosperm.

(b) What is dormancy, and why is water loss a feature of this stage of development?

Ans.: Seed dormancy is a period after embryo maturation that is characterized by very low water content and very low rates of metabolism of the embryo and endosperm.

(c) What is the correlation between seed size and the tendency of seeds to need light as a signal to induce germination? What is the adaptive advantage of this correlation?

Ans.: Smaller seeds are more likely to require light for germination. They have smaller food reserves and it helps their survival if they do not germinate until they are in light and can make their own food by photosynthesis.

(d) What chemical change occurs in many desert seeds during a rainstorm that helps promote their germination?

Ans.: Rain water washes out a hormone inhibitor of germination.

5.  (a) In programmed cell death, enzymes need to be produced. Give two examples of these enzymes and state what they do that help promote the programmed cell death.

Ans.: RNase, DNase, proteinase, lipase. They all digest macromolecules needed for cell structure and life.

(b) The programmed cell death of peripheral root cap cells loosens their connection with the root cap. What benefit does this confer on the root?

Ans.: These cells can serve as buffers against the friction of growing through soil, and are easily sloughed off without harming the more critical cells of the root cap.

(c) What benefit is conferred on the plant by the programmed cell death of leaf cells that are invaded by a pathogen (virus or bacteria)?

Ans.: The death of these cells isolates the invading pathogen in the dead cell so that it cannot move into and infect the other cells of the plant.

6. (a) What structural feature of plant cells allows them to be interconnected by cytoplasmic continuity?

Ans.: plasmodesmata

(b) In what organelle would you find the colorful pigments that characterize flower petal cells?

Ans.: In the vacuole.

(c) What is the main function of chloroplasts?

Ans.: To carry out photosynthesis.

7.  (a) Indicate one principal way that xylem and phloem cells are structurally different.

Ans.: Xylem cells are totally dead and have no cytoplasm or plasma membrane. Phloem cells are alive and have both cytoplasm and plasma membrane.

(b) What is the main product transported by (i) xylem and (ii) phloem?

Ans: (i) water; (ii) sugar

(c) It is said that the flow of material in the phloem is from source to sink. Give an example of a ”sink” tissue that uses more of the phloem transport material than it makes.

Ans.: A fruit or a root or the young growing tip of the shoot of a plant.

8. (a) The bending of a leaf hair may or may not trigger the closing of the Venus fly trap. Explain.

Ans.: If only one hair is bent, this will not trigger the closing; if two hairs are bent within a short time, this will trigger the closing of the trap.

(b) The closing of the Venus fly trap can be controlled by the movement of salt. Explain.

Ans.: To close the trap, the central tissue between the two lobes of the trap must pump salt out. Water follows the salt and the tissue shrinks, allowing the trap to close.

(c)  Contrast how long a Venus fly trap would stay closed if it closed on a small rock compared to if it closed on a fly. What mechanism is the basis of this difference?

Ans.: Traps stay closed while absorbing amino acids from whatever it trapped. So traps closed on rocks do not stay closed long but traps closed on flies stay closed as long as it’s absorbing amino acids from them.

(d) The touch-induced bending of the gland-bearing stalks of Sundew is directional. In what direction do they bend after being touched, and what is the mechanism that explains the directional bending?

Ans.: The stalks bend in the direction of the touch stimulus. The mechanism is that cells on the touched side lose water (shrink), and cells on the opposite side swell, forcing the stalk to bend toward the stimulus.

9. (a) The touch stimulus both stimulates and inhibits the growth of bean stems. Explain.

Ans.: It stimulates the radial growth of the stem and inhibits its elongation growth.

(b) The growth response of plants to the touch stimulus is called “thigmomorphogenesis”. What is the survival benefit of this response of plants to touch?

Ans.:This growth response allows plants that are shaken repeatedly by wind to take on a shorter, stockier form that allows them to resist better the damaging effects of the wind.