Bean seedlings on a windowsill: can light really slow growth?
A learner investigates whether light intensity affects the growth of bean seedlings. Twelve seedlings from the same packet are planted in identical pots of the same soil and watered 20 cm³ every second day. Six pots go on a windowsill; six go inside a dark cupboard. Height is measured every day for ten days with a 30 cm ruler held beside each stem. Results are uneven: one windowsill seedling reads 4.2 cm on day 5 and 3.9 cm on day 6. The cupboard seedlings are tall but pale, and on day 7 a second learner takes over the cupboard measurements. The learner concludes that light slows down growth. A classmate disputes this.
1. Identify two measurement problems in this investigation and explain how each could produce a misleading comparison between the two groups.
Read the worked answer
The 4.2 cm → 3.9 cm change is the giveaway: a stem cannot shrink, so at least one reading is wrong by about 0.3 cm.
Sources of error:
- The ruler is held beside the stem rather than fixed, so the zero end may not sit at soil level and the ruler may lean.
- The reader's eye is not level with the top of the stem, giving a parallax error of a few millimetres.
- A different learner measures the cupboard pots from day 7, and two people judge the top of a floppy stem differently.
Each of these shifts a reading by roughly the amount a seedling grows in a day, so a difference between the windowsill group and the cupboard group can be created by the method rather than by light. Fixing a ruler to a cane in each pot, marking the soil level, having one person read at eye level to the nearest millimetre, and taking two readings per stem and averaging would remove most of this.
2. The learner concludes that light slows down growth. Evaluate this conclusion.
Read the worked answer
The conclusion should be rejected as stated. The cupboard seedlings are taller but pale, and height is only one dimension of growth.
In darkness a stem elongates rapidly because cells lengthen as the plant in effect searches for light, and it stays pale because chlorophyll synthesis requires light. Tall and pale is therefore a sign of a struggling plant, not a fast-growing one.
A fair measure of growth is dry mass, leaf area or chlorophyll content. A cupboard seedling would almost certainly have a lower dry mass, because it cannot photosynthesise and is living on food stored in the seed. The windowsill plants are shorter, greener and sturdier.
To test the claim properly, the learner should dry both sets at about 80 °C to constant mass and compare mean dry masses. Only if the cupboard mean were higher would the conclusion stand.
3. Suggest one improvement to the design and explain the specific way it would make the comparison more trustworthy.
Read the worked answer
The most valuable improvement is to measure a quantity that cannot be faked by stem elongation - final dry mass - because that directly tests whether a plant added living material rather than merely stretched.
A second improvement is to control position on the windowsill: a pot at the back of the shelf receives much less light than one at the front, which is a hidden second light variable inside the 'light' treatment. Rotating and swapping the pots daily evens this out.
A third is to use more seedlings per group, for example ten rather than six, so that one unusual plant cannot decide the mean, and to water by mass rather than by volume, since a pot in the cupboard loses water more slowly and may become waterlogged. Together these changes make the two groups differ in light and in nothing else.