CHAPTER 16
REVIEW QUESTIONS
16.1
b.
Mitosis: before haploid or diploid, after the same as the original number. Meiosis: before
diploid, after haploid.
Mitosis growth, tissue repair; meiosis reproduction. (In algae and plants that exhibit
alternation of generations meiosis is used in asexual formation of spores and mitosis in
formation of gametes in sexual reproduction.)
16.2
a.
b.
c.
d.
e.
f.
g.
mitosis
meiosis
mitosis (asexual) or meiosis (sexual)
mitosis
mitosis
mitosis
mitosis
16.3
In both divisions, daughter cells containing the same types of chromosomes as the original cell
are formed and replication of the chromosomes (forming chromatids joined at the centromere)
occurs before the process begins. In mitosis there is a single nuclear division, the chromatid
pairs join joins the spindle fibre at the centromere which results in the separation of the
chromatids into daughter cells. The two daughter cells therefore have the same number of
chromosomes as the original cell. Thus mitosis can occur in either haploid or diploid cells. In
meiosis, tetrads of homologous chromatids link with the spindle fibre. Two divisions are
required – one to separate the tetrads and the next to separate the chromatids. Thus four
daughter cells are formed, each with half the number of chromosomes. Only cells with pairs of
chromosomes, therefore, can undergo meiosis.
16.4
In plants haploid spores are produced, during asexual reproduction, from meiosis of special
cells of the diploid sporophyte generation. These spores germinate and grow by mitosis into a
haploid, sexually reproducing gametophyte generation. In the reproductive tissues of these
plants, gametes are formed by mitosis. Fertilisation, producing a zygote, restores the diploid
condition. The zygote grows by mitotic cell division to form the next sporophyte generation.
16.5
a.
16.6
a.
b.
c.
d.
e.
The male gametophyte is the pollen grain (enclosing the male sex cell nucleus) which is
dispersed to the female gametophyte (the ovule embedded within the sporophyte ovarian
tissues) by wind or other pollinating agent. Thus the plant does not depend on an external
source of water for transfer of the male gamete to the female gamete.
Flowering plants are no longer tied to a moist environment by reproductive requirements,
and so can exploit a greater range of terrestrial habitats.
Decreases the randomness of wind pollination and thus increases the chance of pollen of
the correct species encountering the stigma.
Attracts fruit eating animals which will aid in the dispersal of seeds.
Aids in the dispersal of the seeds and so reduces competition for resources between
germinating seeds and the parent plant.
16.7
The specialised diploid ‘mother cell’ within the embryo sac of the ovule undergoes meiosis to
form four haploid spore cells. Three of these degenerate whilst the remaining one undergoes
rapid maturation. The nucleus then undergoes three successive mitotic divisions resulting in
eight haploid nuclei, four at each end of the embryo sac. One nucleus from each end migrates
to the centre of the embryo sac to form polar nuclei. The remaining six nuclei each become
separated by cell walls. One of the cells at the micropylar end develops into the functional egg
cell.
16.8
Each species of flower has pollen grains of a particular size, shape or chemical constitution.
The stigma of each species will only stimulate pollen tube growth if it has the correct features.
Successful pollination, resulting in the growth of a pollen tube, requires compatibility between
pollen features and stigma features.
16.9
In most species the function of the sperm is to locate the egg cell and inject its nucleus into it.
The growth of the zygote, therefore, is dependent upon the food reserves within the egg cell.
Thus the sperm are relatively short lived, requiring only enough energy to reach the egg. The
greater the number of sperm, the greater the likelihood that one will encounter an egg cell. The
production of one functional cell in the female, with most of the cytoplasm (and thus energy
reserves) ensures greater success for the developing embryo.
16.10
Feature
Petals
Nectary
Pollen grains
16.11
Wind-pollinated
Absent or reduced
Absent
Light, dry; often winged
Insect-pollinated
Brightly coloured
Present
Sticky/rough to adhere to
insect
16.12
a.
b.
c.
d.
e.
8
8
24
16
16
16.13
a.
b.
Fruit.
The pod is a dehiscent fruit. The pod dries out and this creates a mechanical force which
explosively opens the pod and forcibly ejects the seeds.
Any two answers from:
Wind dispersal, e.g. poppy seeds, dandelion.
Water dispersal, e.g. black bean, coconut.
Animal dispersal, e.g. cobbler’s pegs, mulberry.
It reduces competition for resources between parent plants and germinating seeds.
c.
d.
16.14
Underground stems, e.g. rhizomes of couch grass.
Above-ground stems, e.g. strawberry runners.
Modified roots, e.g. root tubers of the dahlia.
Modified leaves, e.g. onion bulb.
16.15
All three are methods of artificial vegetative propagation of plants. In layering, a shoot, with a
cut made between two nodes, is pegged to the ground so that the cut is in contact with the soil.
This induces root formation at the cut, after which the shoot can be removed from the parent
plant.
Both budding and grafting involve the growth of a part of a specific variety of the species onto
a hardy stock plant to ensure that the resulting plant has specific characteristics.
Budding involves cutting a section from the bark of the stock plant stem and inserting and
binding a bud removed from a plant with the desired features. Growth of this bud, by removal
of stock plant buds, results in the main aerial part of the plant.
Most of the stem of the stock plant is removed and incised to expose the cambium, and a
complementary woody twig from the desired plant, similarly cut to expose the cambium, is
bound in place on the stock plant stem in grafting.
16.16
Both vegetative and sexual reproduction in flowering plants ensures continuity of the species
through the production of a new generation. Both involve cell division, growth and
development. Although vegetative reproduction is a simpler process involving one parent and
is thus more energy efficient (no production of gametophytes and specialised structures to aid
pollination and dispersal of seeds) than sexual reproduction, capable of producing a large
number of offspring in a short period of time, unlike sexual reproduction there is no genetic
variability in the offspring (which could be a disadvantage in a changing environment or in
lack of resistance to specific pathogens) or dispersal of offspring (which could lead to
overcrowding and competition for resources).
16.17
Growth is the increase in dry mass of an organism or cell whereas development refers to the
changes in the organism as it matures.
16.18
Cell differentiation is the process whereby a generalised cell becomes specialised for a
specific function.
16.19
The intensity, quality and duration of light has a direct effect on production of chlorophyll,
photosynthesis, photoperiodism and phototropism in plants. Seasonal changes have a
significant effect on plant growth. Similarly water availability, nutrient supply, temperature
and amount of oxygen influence plant growth. Internal factors controlled by genes or
controlling gene expression, which may be species or individual specific, can also influence
the growth of the organism.
16.20
A deficiency disease is a disease resulting from lack of, or inadequate supply of, an essential
nutrient. Any one example from Table 16.4.
16.21
Water, oxygen and a particular temperature and illumination which is species specific.
16.22
In both types of germination there is activation of the seed embryo, involving uptake of water
and oxygen and the respiration of the food reserves to provide the nutrients and energy
required for embryo growth and development. Hypogeal germination involves rapid
elongation of the plumule causing it to push out of the ground and leave the cotyledons in the
soil. In epigeal germination there is rapid elongation of the top of the radicle causing both the
plumule and cotyledons to be thrust out of the ground.
16.23
Primary growth is achieved by rapid mitotic cell division in special generalised tissues, called
meristems, particularly at the apices of the stem and root and at the bases of nodes and leaves
in monocots (intercalary). After production, the new cells go through a phase of elongation
and then development, forming the appropriate tissues of the plant stem and root.
16.24
Secondary growth occurs in woody plants and palms, and results in an increase in stem and
root diameter. It occurs in two types of lateral cambium – vascular cambium, forming new
xylem and phloem, and in cork cambium that produces cork, the protective outer layer of
woody plants. As the vascular cambium undergoes division it produces new xylem cells on its
inner surface and new phloem cells on its outer surface. As the new xylem matures it takes
over the function of the existing vessels which then become filled with plant wastes and lignin
to form the hard supporting heartwood. The old phloem cells and cork are sloughed off as
bark, to be replaced by new tissues.
16.25
Both primary and secondary growth increase the dry mass of the plant and involve division of
undifferentiated cells, elongation and development. Primary growth occurs in all plants but
secondary growth only occurs in woody plants and palms. Primary growth results in an
increase in the height of the plant whereas secondary growth achieves an increase in the girth
of the main stem and root of the plant.
16.26
a.
b.
c.
A: bark – composed of cork, epidermis and phloem
B: wood – composed of functional and non functional xylem
C: sapwood – functional xylem
D and E: heartwood of two consecutive growth seasons.
Three years old.
In the second year (D) the growth ring is wider and the xylem vessels larger than in the
other two years, suggesting that environmental conditions (correct temperature and
availability of water containing dissolved mineral salts) were conducive to a high growth
rate.
16.27
Unicellular organism: receptor molecules on the surface of the membrane detect changes in
the environment that are significant to the organism. This may cause changes in chemicals
inside the cell (communication) which result in chemical changes which are seen as the
response.
Angiosperms have specialised areas which can detect specific environmental stimuli, e.g.
direction and intensity of light. Hormones are released at these sites and travel by diffusion to
the appropriate cells (communication) which respond by changing the rate of a chemical
reaction specific to that hormone. Changes in cell chemical composition act as a stimulus for
intracellular responses.
A multicellular animal uses both hormonal and nervous communication. Detection of an
environmental change chemically activates sense organs which transmit their information to
appropriate cells for chemical response, either via hormones travelling in the blood stream or
via electro-chemical changes along a series of nerve tracts.
16.28
Hormones are chemicals which regulate body functions in multicellular plants and animals.
16.29
Hormones may be made in one part of the body and travel to the target site in either a
transport system or by diffusion.
16.30
Tropic movements in plants are directional growth movements under the control of hormones.
Tactic movements are made by the whole organism or free part of an organism (e.g. gamete)
towards or away from variations in a directional stimulus.
Nastic movements are made by part of a fixed plant in response to a non-directional stimulus.
16.31
Auxins promote root growth at low concentrations, whereas cytokinin is either inactive or
inhibits primary cell growth but promotes lateral root growth. Auxin promotes apical
dominance and cytokinin is antagonistic to auxin with respect to apical dominance. Whilst
cytokinin is not involved in abscission, this process is inhibited by auxin.
16.32
Many examples, eg.: ethene gas is used to ripen green fruit in storage containers, application
of auxin to unfertilised ovaries in the production of seedless fruit and use of auxin sprays to
supplement pollination.
16.33
The plant is showing a phototrophic response to light. On the side of the plant exposed to the
unilateral light auxins either migrate to the shaded side or are not produced (riboflavins photoxidise the enzymes used in the production of auxin). The presence of auxins on the shaded
side stimulates cell elongation just below the tip and thus the plant bends towards the light.
16.34
The statoliths accumulate in the lower part of the tissues of both the root and shoot tips. This
causes an accumulation of auxin in those cells. These tissues respond differently to
concentrations of auxin. In the root, high concentrations inhibit growth whereas they stimulate
cell elongation in the shoot. In the normal situation, cell root growth occurs just above the tip
where there is not an accumulation of auxin whereas in the shoot it occurs in the tip.
16.35
At a particular stage the levels of auxin levels in the leaf decrease and the levels of ABA
increase. This stimulates the plant to withdraw chlorophyll from the leaf, to decrease protein
production and begin breakdown of cell membranes. At the same time, a layer of tissue
formed from the breakdown of cells at the base of the leaf, the abscission layer, is produced
which seals the rest of the plant from the external environment as the dead leaf breaks from
the plant.
16.36
Vernalisation is a process whereby bud-break of a deciduous plant, seed germination or
flowering is inhibited unless the plant has undergone a period of cold. Since these plants
originate in cold-winter areas, vernalisation ensures that the plant does not begin growth and
production of flowers at an inappropriate time which would ultimately lead to its demise.
16.37
a.
b.
16.38
Photoperiodism is thought to be associated with response by the plant to infra-radiation (580–
730 nm wavelength). Most plants respond to the shorter red wavelengths (580–660) whilst the
far-red light wavelengths (700–730) can inhibit germination and growth. Plants absorb red
light using the blue pigment phytochrome. This pigment is in two inter-convertible forms,
either absorbing the optimum 660 or 730 wavelengths. Absorption of one wavelength rapidly
converts it to the other form. Sunlight contains more red than far-red. The P660 is thus
converted to P730 during the day and back to P660 at night. P730 stimulates hormone production
that activates enzymes, some of which stimulate cell growth and others that inhibit it.
In short-day plants P730 inhibits flowering whilst in long day plants P 730 stimulates flowering.
The short-day plants require sufficiently long periods of darkness to convert P 730 to P660 and
the long-day plants require short nights to keep most of the phytochrome in the P 730 form.
The tip of the coleoptile is the area which detects light.
Petunias are long-day plants which require at least ten hours of daylight before they are
stimulated to flower. Chrysanthemums, on the other hand, are short-day plants, requiring
at least 12 hours of uninterrupted darkness to flower.