The Effects Of Light On Plant And Animal Species - IELTS Reading Answers & Explanations
From Cambridge IELTS 05 Academic Reading Test 4 · Part 3 · Questions 27–40
Reading Passage
You should spend about 20 minutes on Questions 27-40, which are based on Reading Passage 3 below.
The effects of light on plant and animal species
Light is important to organisms for two different reasons. Firstly it is used as a cue for the timing of daily and seasonal rhythms in both plants and animals, and secondly it is used to assist growth in plants.
Breeding in most organisms occurs during a part of the year only, and so a reliable cue is needed to trigger breeding behaviour. Day length is an excellent cue, because it provides a perfectly predictable pattern of change within the year. In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount. The seasonal impact of day length on physiological responses is called photoperiodism, and the amount of experimental evidence for this phenomenon is considerable. For example, some species of birds' breeding can be induced even in midwinter simply by increasing day length artificially (Wolfson 1964). Other examples of photoperiodism occur in plants. A short-day plant flowers when the day is less than a certain critical length. A long-day plant flowers after a certain critical day length is exceeded. In both cases the critical day length differs from species to species. Plants which flower after a period of vegetative growth, regardless of photoperiod, are known as day-neutral plants.
Breeding seasons in animals such as birds have evolved to occupy the part of the year in which offspring have the greatest chances of survival. Before the breeding season begins, food reserves must be built up to support the energy cost of reproduction, and to provide for young birds both when they are in the nest and after fledging. Thus many temperate-zone birds use the increasing day lengths in spring as a cue to begin the nesting cycle, because this is a point when adequate food resources will be assured.
The adaptive significance of photoperiodism in plants is also clear. Short-day plants that flower in spring in the temperate zone are adapted to maximising seedling growth during the growing season. Long-day plants are adapted for situations that require fertilization by insects, or a long period of seed ripening. Short-day plants that flower in the autumn in the temperate zone are able to build up food reserves over the growing season and over winter as seeds. Day-neutral plants have an evolutionary advantage when the connection between the favourable period for reproduction and day length is much less certain. For example, desert annuals germinate, flower and seed whenever suitable rainfall occurs, regardless of the day length.
The breeding season of some plants can be delayed to extraordinary lengths. Bamboos are perennial grasses that remain in a vegetative state for many years and then suddenly flower, fruit and die (Evans 1976). Every bamboo of the species Chusquea abietifolia on the island of Jamaica flowered, set seed and died during 1884. The next generation of bamboo flowered and died between 1916 and 1918, which suggests a vegetative cycle of about 31 years. The climatic trigger for this flowering cycle is not yet known, but the adaptive significance is clear. The simultaneous production of masses of bamboo seeds (in some cases lying 12 to 15 centimetres deep on the ground) is more than all the seed-eating animals can cope with at the time, so that some seeds escape being eaten and grow up to form the next generation (Evans 1976).
The second reason light is important to organisms is that it is essential for photosynthesis. This is the process by which plants use energy from the sun to convert carbon from soil or water into organic material for growth. The rate of photosynthesis in a plant can be measured by calculating the rate of its uptake of carbon. There is a wide range of photosynthetic responses of plants to variations in light intensity. Some plants reach maximal photosynthesis at one-quarter full sunlight, and others, like sugarcane, never reach a maximum, but continue to increase photosynthesis rate as light intensity rises.
Plants in general can be divided into two groups: shade-tolerant species and shade-intolerant species. This classification is commonly used in forestry and horticulture. Shade-tolerant plants have lower photosynthetic rates and hence have lower growth rates than those of shade-intolerant species. Plant species become adapted to living in a certain kind of habitat, and in the process evolve a series of characteristics that prevent them from occupying other habitats. Grime (1966) suggests that light may be one of the major components directing these adaptations. For example, eastern hemlock seedlings are shade-tolerant. They can survive in the forest understorey under very low light levels because they have a low photosynthetic rate.
Questions
Questions 27–33 True / False / Not Given
Do the following statements agree with the information given in Reading Passage 3?
Write
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
Questions 34–40 Sentence Completion
Complete the sentences.
Choose NO MORE THAN THREE WORDS from the passage for each answer.
- Day length is a useful cue for breeding in areas where 34 are unpredictable.
- Plants which do not respond to light levels are referred to as 35.
- Birds in temperate climates associate longer days with nesting and the availability of 36.
- Plants that flower when days are long often depend on 37 to help them reproduce.
- Desert annuals respond to 38 as a signal for reproduction.
- There is no limit to the photosynthetic rate in plants such as 39.
- Tolerance to shade is one criterion for the 40 of plants in forestry and horticulture.
Answers & Explanations Summary
| # | Answer | Evidence | Explanation |
|---|---|---|---|
| Q27 | TRUE | The seasonal impact of day length on physiological responses is called photoperiodism, and the amount of experimental evidence for this phenomenon is considerable | Excerpt/Passage Explanation: The passage explains that photoperiodism is how light changes affect living things, and it notes that there is a large amount of scientific testing to show this is real. Answer Explanation: The answer is TRUE because the passage says scientists have found a lot of proof for photoperiodism. Reason For Correctness: The correct answer is TRUE because the text explicitly states that the 'amount of experimental evidence' for photoperiodism is 'considerable'. In English, 'considerable' means a large amount or plenty, and 'experimental evidence' refers to scientific proof. This directly matches the statement in the question. |
| Q28 | TRUE | For example, some species of birds' breeding can be induced even in midwinter simply by increasing day length artificially (Wolfson 1964) | Excerpt/Passage Explanation: The passage explains that some birds can be made to breed even in the cold middle of winter just by making the daylight last longer with human-made lights. Answer Explanation: The answer TRUE means that there are some birds that can be made to start the breeding process at a time of year when they normally would not. Reason For Correctness: The correct answer is TRUE because the passage mentions that researchers found they could make certain birds start breeding even during the middle of winter. This is done by 'increasing day length artificially,' which tricks the birds' bodies into thinking it is spring. The word 'induced' in the text means to 'encourage' or 'cause,' and 'midwinter' represents a time that is 'out of season' for normal breeding. |
| Q29 | NOT GIVEN | The seasonal impact of day length on physiological responses is called photoperiodism, and the amount of experimental evidence for this phenomenon is considerable | Excerpt/Passage Explanation: The passage defines photoperiodism as how day length affects living things, but it does not specify which geographic areas have this phenomenon or if it is restricted in any way. Answer Explanation: The answer is NOT GIVEN because the passage does not provide enough information to know if photoperiodism is limited to specific parts of the world or if it happens everywhere. Reason For Correctness: The correct answer is NOT GIVEN because, while the text mentions photoperiodism and uses the temperate zone as an example of where day length serves as a cue, it never states that the phenomenon is restricted to those specific geographic areas. The passage describes how it works but does not discuss whether there are places on Earth where it does not occur. Since the information about geographic limits is missing, we cannot say the statement is true or false. |
| Q30 | FALSE | Day-neutral plants have an evolutionary advantage when the connection between the favourable period for reproduction and day length is much less certain. For example, desert annuals germinate, flower and seed whenever suitable rainfall occurs, regardless of the day length | Excerpt/Passage Explanation: The passage states that desert annuals are plants that grow and make seeds whenever it rains, and they do not care how long the day is. This makes them 'day-neutral' plants. Answer Explanation: The answer means that the statement is incorrect. Desert annuals do not belong to the group of plants known as long-day plants. Reason For Correctness: The correct answer is FALSE because the text identifies desert annuals as an example of 'day-neutral plants' rather than 'long-day plants.' The passage explains that while long-day plants require a specific amount of daylight to flower, day-neutral plants like desert annuals do so 'regardless of the day length,' reacting instead to factors like rainfall. |
| Q31 | FALSE | Bamboos are perennial grasses that remain in a vegetative state for many years and then suddenly flower, fruit and die (Evans 1976) | Excerpt/Passage Explanation: The passage explains that bamboos live for many years just growing leaves and stems, but as soon as they flower and make seeds, they die. Answer Explanation: The answer is FALSE because the text says bamboos die after they flower, meaning they cannot flower more than once. Reason For Correctness: The correct answer is FALSE because the passage states that bamboos stay in a growing state for many years and then suddenly flower, fruit, and die. If a plant dies right after it flowers, it is impossible for it to flower several times during its life. The word 'die' is the key synonym for the end of a life cycle, showing there is only one flowering event. |
| Q32 | TRUE | The climatic trigger for this flowering cycle is not yet known, but the adaptive significance is clear | Excerpt/Passage Explanation: The passage states that although the reason why the plants flower together is understood, the actual weather event or signal that causes the plants to start flowering remains unknown to scientists. Answer Explanation: The answer confirms that experts do not know what specific signal or cause (cue) makes the Chusquea abietifolia bamboo begin its flowering process. Reason For Correctness: The correct answer is TRUE because the text mentions the bamboo species Chusquea abietifolia and explicitly states that the 'climatic trigger' for its flowering cycle is 'not yet known'. In this context, a 'trigger' is a synonym for 'cue', meaning scientists haven't figured out what starts the cycle yet. |
| Q33 | FALSE | Shade-tolerant plants have lower photosynthetic rates and hence have lower growth rates than those of shade-intolerant species For example, eastern hemlock seedlings are shade-tolerant |
Excerpt/Passage Explanation: The passage explains that plants that can live in the shade do not grow as fast as plants that need lots of sun. It then identifies the eastern hemlock as a type of plant that lives in the shade. Answer Explanation: The answer is FALSE because the statement says the plant grows fast, but the text says it has a low growth rate, which means it grows slowly. Reason For Correctness: The correct answer is FALSE because the passage establishes two connected facts: first, that plants categorized as 'shade-tolerant' grow more slowly than others ('lower growth rates'); and second, that eastern hemlock is a 'shade-tolerant' plant. Because it is shade-tolerant, it grows slowly, making the claim that it is 'fast-growing' incorrect. |
| Q34 | temperatures | In the temperate zone in spring, temperatures fluctuate greatly from day to day, but day length increases steadily by a predictable amount | Excerpt/Passage Explanation: The passage explains that in some regions, the daily heat or cold can change a lot and is not reliable, but the amount of daylight grows in a way that is easy to predict. Answer Explanation: The answer refers to how hot or cold the weather is. Reason For Correctness: The correct answer is 'temperatures' because the passage contrasts the changing nature of heat and cold with the steady pattern of daylight. It explains that in certain areas, heat levels change a lot and are hard to guess ('fluctuate greatly'), while the length of the day follows a very 'predictable' schedule. This makes day length a much more reliable signal for animals and plants to begin their reproduction processes compared to weather changes. |
| Q35 | day-neutral / day-neutral plants | Plants which flower after a period of vegetative growth, regardless of photoperiod, are known as day-neutral plants | Excerpt/Passage Explanation: The passage explains that plants that flower on their own after growing, without needing a specific amount of light in a day, are called "day-neutral plants." Answer Explanation: The answer "day-neutral" or "day-neutral plants" refers to a specific group of plants that grow and make flowers based on their own internal schedule rather than the amount of daylight they receive. Reason For Correctness: The correct answer is found in the passage where it discusses different types of plants. It explains that some plants flower simply after they have grown for a certain time, no matter what the day length is. The term "regardless of photoperiod" means that these plants do not react to or depend on light levels (the length of the day) to start flowering, which matches the description in the question. |
| Q36 | food / food resources / adequate food / adequate food resources | Thus many temperate-zone birds use the increasing day lengths in spring as a cue to begin the nesting cycle, because this is a point when adequate food resources will be assured | Excerpt/Passage Explanation: The passage explains that many birds living in temperate areas use the longer days of spring as a sign to start building nests and laying eggs. They do this because spring is a time when they can be sure there will be plenty of food available. Answer Explanation: The answer means the things that birds eat to live and grow, such as plants or small animals. Reason For Correctness: The correct answer is supported by the passage, which explains why birds in temperate zones start their nesting cycle when spring days get longer. It says they do this because that specific time of year guarantees there will be enough for them and their babies to eat. The passage uses words like 'adequate food resources' and 'assured' to show that the birds wait for a time when food is certain to be available. |
| Q37 | insects / fertilization by insects / fertilisation by insects | Long-day plants are adapted for situations that require fertilization by insects, or a long period of seed ripening | Excerpt/Passage Explanation: The passage explains that plants that flower during long days are prepared for times when they must use insects to help them make seeds or when they need a lot of time for their seeds to grow. Answer Explanation: The answer refers to the role that insects play in helping certain plants create new seeds. Reason For Correctness: The correct answer is 'insects' or 'fertilization by insects' because the passage states that long-day plants (those that bloom when days are longer) are specifically suited for conditions where they need insects to help them reproduce. In this context, 'fertilization' means the process of making seeds, which is the way plants reproduce. |
| Q38 | rainfall / suitable rainfall | For example, desert annuals germinate, flower and seed whenever suitable rainfall occurs, regardless of the day length | Excerpt/Passage Explanation: The passage explains that desert plants carry out their reproduction (flowering and seeding) when it rains, and they do this without paying attention to how long the days are. Answer Explanation: The answer refers to the water that falls from the sky during wet weather, which acts as a signal for specific plants to grow. Reason For Correctness: The correct answer is derived from the section discussing day-neutral plants. While many organisms use day length as a signal for breeding, desert annuals are different. They do not rely on light; instead, they begin their reproductive processes—such as flowering and making seeds—specifically when there is 'suitable rainfall'. |
| Q39 | sugarcane | Some plants reach maximal photosynthesis at one-quarter full sunlight, and others, like sugarcane, never reach a maximum, but continue to increase photosynthesis rate as light intensity rises | Excerpt/Passage Explanation: The passage explains that while some plants stop increasing their food-making speed after a certain amount of sun, sugarcane keeps going faster and faster as the sun shines brighter. Answer Explanation: The answer is a specific type of plant that keeps making food faster as it receives more sunlight, without reaching a limit. Reason For Correctness: The correct answer is sugarcane because the text states that while many plants reach a maximum rate of photosynthesis (the process of making food from light), sugarcane is an exception that never reaches a maximum. Instead, it continues to increase its photosynthesis rate as the light intensity rises. The text identifies sugarcane as a specific example of a plant that shows no limit in this process. |
| Q40 | classification | Plants in general can be divided into two groups: shade-tolerant species and shade-intolerant species. This classification is commonly used in forestry and horticulture | Excerpt/Passage Explanation: The passage explains that plants are split into two categories based on shade, and it uses the word classification to describe this grouping method. Answer Explanation: The answer "classification" means the act of putting things into different groups or categories. Reason For Correctness: The correct answer is classification because the text describes how plants are sorted into two groups (shade-tolerant and shade-intolerant) based on how they react to shade. The passage says this system of grouping, or "classification," is frequently used in the fields of forestry and horticulture (plant science). |