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A growth journal

Change and motion · level 2, lesson 6 of 6

A question to start

Plant a bean seed in a pot of soil. Water it. Then look at it every day.

For the first few days, nothing at all seems to happen. Then one morning a little green loop pokes up out of the soil, and after that the plant seems to shoot up almost while you watch. Then, after a couple of weeks, it seems to slow down again.

Does a plant really grow in fits and starts like that, or does it only look that way? The only way to know is to measure, every day, and write the numbers down. That is what scientists do, and in this lesson you will learn how to keep a growth journal of your own and read the story hidden in your numbers.

The idea

What is a growth journal?

A journal is a book where you write something down regularly, like a diary. A growth journal is a diary of measurements: every day, at about the same time, you measure your plant and write down the date and the height.

Why write it down? Because memory is not good at small changes. If your plant grows 2 cm, you will not notice by looking. But if you wrote "9 cm" yesterday and "11 cm" today, the numbers show it clearly.

How to measure fairly

A measurement is only useful if you take it the same way every time. Otherwise, your numbers change because you measured differently, not because the plant grew.

  1. Use a ruler with centimetres (cm). Make sure you start at the 0 mark. On many rulers the 0 is a little way in from the end.
  2. Measure from the same place every day: from the top of the soil, not the rim of the pot or the table.
  3. Measure to the same place every day: to the highest point of the main stem, the growing tip (not the tip of a leaf that is sticking up).
  4. Hold the ruler straight up, not leaning.
  5. Measure at about the same time each day, for example after breakfast.
  6. Write it down straight away, with the day.
0510 9 cm soil

In the picture the ruler stands on the soil with its 0 at the soil, and the top of the stem lines up with 9 cm.

One child's journal

Here is the journal a child might keep for a bean planted on day 1. (Your own bean will be different: every plant is, and that is part of the fun.)

Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Height (cm) 0 0 0 0 1 2 4 6 9 12 15 17 19 20 21
Growth since yesterday (cm) 0 0 0 1 1 2 2 3 3 3 2 2 1 1

The third row is the change row from the last two lessons: how much the plant grew each day.

Drawing the journal

A line chart turns the numbers into a picture. Days go along the bottom; height goes up the side.

0510 1520 1357 9111315 waiting fastest day cm

Telling the story

Now we can read the story, in four parts. Each part matches a kind of change from this level.

  1. Waiting (days 1 to 4). The height is 0 every day: a flat line, no change. But the plant was not doing nothing! Under the soil, the seed was soaking up water, swelling, and sending a root down. We just could not see it. A flat line means nothing changed in what we measured.
  2. Slow start (days 5 to 8). The shoot comes up. It grows 1 cm a day, then 2 cm a day. The growth is speeding up: the line starts to bend upwards.
  3. Fastest growth (days 8 to 11). The plant grows 3 cm every day, the biggest daily growth in the journal. This is the steepest part of the line (orange). For these days the growth is steady: the same 3 cm each day, so this piece of the line is straight.
  4. Slowing down (days 11 to 15). The daily growth drops to 2 cm, then 1 cm. The plant is still growing, but slowing down: the line bends over and gets flatter.

Put together, the line looks like a stretched-out letter S: flat, then rising steeply, then flattening again. Many things in nature grow in this S shape: a sunflower over a summer, a puppy growing into a dog, even a child growing into a grown-up (with a fast spurt in between).

Checking the journal

A careful scientist checks the numbers. Use the rule from lesson 1: small changes add up to the big change.

  • Add the growth row: 0 + 0 + 0 + 1 + 1 + 2 + 2 + 3 + 3 + 3 + 2 + 2 + 1 + 1 = 21 cm.
  • From day 1 (0 cm) to day 15 (21 cm) the change is 21 cm. ✓

They match, so the change row was worked out correctly.

A fair test: sun or dark?

Once you can keep a journal, you can do an experiment. Plant two beans in the same soil, in the same kind of pot, on the same day, with the same water. Put one on a sunny windowsill and one in a dark cupboard. Keep a journal for each.

This is a fair test: everything is kept the same except one thing (the light), so any difference must come from that one thing. It is the same idea as a fair race in lesson 2.

Many children are surprised by what happens. The bean in the dark often grows taller and faster at first! But it is pale yellow and thin and floppy, with tiny leaves. It is stretching to find light. The bean in the sun is shorter, but green and strong. So "grew more" is not the same as "grew better". Numbers tell part of the story; your eyes and your notes tell the rest.

Worked examples

Example 1. Fastest day. In the journal above, between which days did the plant grow the most?

  1. Look at the change row for the biggest number: 3 cm.
  2. It appears three times: day 8 to 9, day 9 to 10, and day 10 to 11.
  3. The fastest growth was from day 8 to day 11, 3 cm each day.

Example 2. Filling a gap. A child forgot to measure on day 6 of a different journal. Day 5 was 4 cm and day 7 was 8 cm. What is a good guess for day 6?

  1. From day 5 to day 7, the plant grew 8 − 4 = 4 cm in two days.
  2. If it grew about the same each day, that is 2 cm a day.
  3. A good guess for day 6 is 4 + 2 = 6 cm. (This is like joining the dots on a line chart in lesson 3.)

Example 3. A suspicious number. A journal says: day 1: 5 cm, day 2: 7 cm, day 3: 6 cm, day 4: 9 cm. What looks wrong?

  1. Changes: up 2, down 1, up 3.
  2. A healthy plant does not shrink overnight. A "down 1" probably means the measurement on day 3 (or day 2) was made differently: from the pot rim, or to a different leaf, or the ruler was tilted.
  3. The careful thing to do is to write a note: "day 3 may be measured wrong", and measure extra carefully the next day.

Example 4. Comparing two plants. Bean A: 3 cm on day 5 and 15 cm on day 10. Bean B: 6 cm on day 5 and 14 cm on day 10. Which grew faster between day 5 and day 10?

  1. Bean A grew 15 − 3 = 12 cm in those 5 days.
  2. Bean B grew 14 − 6 = 8 cm in the same 5 days.
  3. Same time, more growth: Bean A grew faster, even though it started shorter. (A fair comparison: same days.)

Try it

Use this journal for questions 1 to 5.

Day 1 2 3 4 5 6 7 8
Height (cm) 2 3 5 8 12 15 17 18
  1. Write the change row (growth each day).
  2. On which day-to-day step did the plant grow the most?
  3. When was the growth speeding up? When was it slowing down?
  4. How much did it grow from day 1 to day 8? Check by adding the change row.
  5. If the growth keeps slowing in the same way, what is a good guess for day 9?
  6. Why should you measure from the soil, and not from the table?
  7. Why should you measure at about the same time every day?
  8. In a fair test of sun against dark, why must both beans get the same water?
Answers
  1. 2 to 3: 1. 3 to 5: 2. 5 to 8: 3. 8 to 12: 4. 12 to 15: 3. 15 to 17: 2. 17 to 18: 1.
  2. Day 4 to day 5: 4 cm.
  3. Speeding up from day 1 to day 5 (growth 1, 2, 3, 4). Slowing down from day 5 to day 8 (growth 3, 2, 1).
  4. 18 − 2 = 16 cm. Check: 1 + 2 + 3 + 4 + 3 + 2 + 1 = 16. ✓
  5. The growth went 3, 2, 1, so the next might be 0 or a tiny bit: about 18 cm, perhaps a little more.
  6. The pot and the table do not grow. If you measure from the table, you add the height of the pot, and if you measure from different places on different days, your numbers would change even if the plant did not.
  7. A plant keeps growing all through the day. If you measured in the morning one day and the evening the next, one "day" of growth would really be longer than the other, and the change row would not be fair.
  8. If one bean got more water, we would not know whether a difference came from the light or the water. In a fair test only one thing is different.

Common mistake

Measuring a different way on different days. From the soil one day and from the pot rim the next; to the stem one day and to a leaf tip the next. Then the numbers show changes that did not happen. Decide once how you will measure, write it at the front of your journal, and stick to it.

Telling the story from the heights alone. "It got taller every day" is true, but it misses the best part. The change row tells you when it was waiting, speeding up, growing fastest and slowing down.

Where it leads