A taller seedling can contain less dry matter
Visible growth and mass increase answer different questions. A young shoot can lengthen as cells divide and expand while the seed’s total dry mass falls. Water uptake increases fresh mass, reserves move between organs, and respiration uses some stored material. A sound explanation keeps these changes in separate accounts.
The fictional records below describe dark-grown, reserve-dependent seedlings. They are teaching data, not measurements of a named crop or instructions for growing or drying seeds. Start by identifying the complete sample and its units before calculating a change.
What is included in each measurement?
Fresh mass includes water and dry material. Dry mass is the residue after water removal by a stated laboratory measurement method; it includes structural material, remaining reserves and mineral matter. It is a useful biomass proxy with less variation from hydration, but it is not a perfect direct measurement of living tissue or developmental progress.
Every row represents a separate matched cohort initially containing 20 similar seeds from the same lot. Measurements of dry mass are destructive, so the same individuals are not followed through all four dates. At each date, the whole cohort is measured and then partitioned into its storage parts, growing axes and retained coverings. The partitions in that row sum to its total dry mass.
Assume no photosynthetic carbon input, no added organic food, no discarded tissue and negligible leaching or mineral exchange in this simplified model. Water and respiratory gases can enter or leave. “Storage parts” describes reserve-bearing tissue, not a chemical assay of pure starch. In real investigations these assumptions would need checking.
| Day | Whole fresh mass (g) | Whole dry mass (g) | Storage-part dry mass (g) | Axis dry mass (g) | Covering dry mass (g) | Mean axis length (mm) |
|---|---|---|---|---|---|---|
| 0 | 12.0 | 10.0 | 8.8 | 0.6 | 0.6 | 2 |
| 2 | 22.0 | 9.4 | 7.7 | 1.1 | 0.6 | 11 |
| 4 | 31.0 | 8.6 | 6.1 | 1.9 | 0.6 | 28 |
| 6 | 38.0 | 7.7 | 4.3 | 2.8 | 0.6 | 47 |
Graph alternative: fresh mass rises from 12.0 through 22.0 and 31.0 to 38.0 g. Dry mass falls from 10.0 through 9.4 and 8.6 to 7.7 g. The lines join four sampled dates only; they do not show an independently observed value for every intervening day.
Worked analysis 1: separate water from fresh-mass change
In this model, water mass is fresh mass minus dry mass. On day 0 it is 12.0 − 10.0 = 2.0 g. On day 2 it is 22.0 − 9.4 = 12.6 g. Water retained in the cohort therefore increases by 12.6 − 2.0 = 10.6 g.
The fresh-mass increase is only 22.0 − 12.0 = 10.0 g because dry mass falls by 0.6 g over the same interval. The accounting check is 10.6 + (−0.6) = 10.0 g. Saying “the seedling made 10 g of new tissue” would confuse added water with newly accumulated dry material.
Water uptake is important during germination, but the table gives net water retained, not the total water that passed through the cohort. Water entering and later leaving would not all remain in the final mass. No evaporation or transpiration rate can be calculated from these masses alone.
Worked analysis 2: follow reserves into the growing axis
Between days 0 and 4, storage-part dry mass falls by 8.8 − 6.1 = 2.7 g. Axis dry mass rises by 1.9 − 0.6 = 1.3 g. Covering dry mass remains 0.6 g. The whole-cohort change is consequently −2.7 + 1.3 + 0 = −1.4 g, matching 8.6 − 10.0 g.
Stored compounds provide building materials as well as respiratory substrates. Some material becomes part of the developing root and shoot; some is metabolised. Moving material from a cotyledon into an axis does not add the same mass again to the whole cohort. The receiving part can gain while the total loses.
Do not describe the entire 2.7 g storage-part decline as respiration. The axis gain already shows why that is too simple. Nor can this table measure the exact internal transfer flux: material can enter an axis and later be respired there. It records net changes in compartments, not every intermediate movement.
Worked analysis 3: explain dry-mass loss during elongation
By day 6 the mean axis length has increased from 2 to 47 mm, but whole dry mass has fallen from 10.0 to 7.7 g. The loss is 2.3 g, or (2.3 ÷ 10.0) × 100 = 23% of the starting dry mass. There is no contradiction: elongation can involve water-supported cell expansion and redistribution of stored material.
Respiration releases energy for metabolism and exports carbon in carbon dioxide. In this dark-grown model there is no photosynthetic input to offset that export. Seed and seedling respiration is supported by research across 41 plant species; the exact numerical values here remain fictional.
However, 2.3 g dry-mass loss does not imply 2.3 g carbon dioxide released. Carbon dioxide contains oxygen as well as carbon, oxygen can enter from outside, and water participates in metabolism. A gas total requires additional chemical and gas-exchange information. Mass is transferred and transformed, not simply “used up as energy”.
Three practice tasks with answers
Task 1: calculate an interval rate
Calculate the mean whole dry-mass change per day between days 2 and 6. Give both a signed change rate and a loss rate.
Answer: (7.7 − 9.4) ÷ (6 − 2) = −1.7 ÷ 4 = −0.425 g per cohort per day. Equivalently, the average loss is 0.425 g per cohort per day. This is an interval average, not evidence that the rate was constant each day.
Task 2: repair unequal sample sizes
Another fictional record compares 10.0 g dry mass in 20 starting seeds with 11.4 g in 30 later seedlings. A learner concludes that dry mass increased. What is missing?
Answer: cohort totals are not directly comparable because the seed counts differ. Mean masses are 10.0 ÷ 20 = 0.50 g and 11.4 ÷ 30 = 0.38 g per individual. The later mean is lower. Even that comparison needs matched starting sizes, sampling conditions and complete tissue collection before it can support a biological explanation.
Task 3: complete the day-6 account
Find water mass and the fraction of total dry mass in the axis on day 6.
Answer: water mass = 38.0 − 7.7 = 30.3 g. Axis fraction = 2.8 ÷ 7.7 ≈ 0.364, or 36.4%. This percentage describes allocation of remaining dry matter; it is not the percentage of reserve originally transferred.
Read growth curves and controlled germination evidence
Growth can be measured as an increase in length, area, volume, mass or cell number, but those measures answer different questions. Development concerns changes in form and function. The retained seedling mass lesson shows why fresh mass and whole-cohort dry mass need separate accounts. The following records add longer-term growth and the conditions for germination.
| Time / weeks | Mean height / cm |
|---|---|
| 0 | 2 |
| 2 | 3 |
| 4 | 8 |
| 6 | 20 |
| 8 | 36 |
| 10 | 45 |
| 12 | 47 |
Worked case 1: compare interval growth rates
From weeks 0–2 the mean rate is (3−2)/2 = 0.5 cm/week. Weeks 6–8 give (36−20)/2 = 8 cm/week. Weeks 10–12 give (47−45)/2 = 1 cm/week. The initial slow increase, rapid middle rise and later slowing form an approximately sigmoid pattern. Rate is represented by gradient, not by the greatest height. The tallest point at week 12 does not have the largest growth rate.
The slowing might reflect developmental limits or limiting resources, but height alone cannot identify which. A separate controlled comparison is needed. The graph does not show mortality, total biomass or exact cell-division rate. In organisms with moulting, an external size measure can increase in steps; not every growth record has a smooth sigmoid shape.
Worked case 2: isolate germination conditions
In a fictional teacher-supplied record, each treatment uses 20 matched viable, nondormant seeds of a species for which light is not required for germination. Germination means visible radicle emergence by day 7. These are data to interpret, not an apparatus or culturing procedure.
| Treatment | Water available | Oxygen available | Temperature | Germinated / 20 |
|---|---|---|---|---|
| A | Yes | Yes | 25°C | 18 |
| B | No | Yes | 25°C | 0 |
| C | Yes | No | 25°C | 0 |
| D | Yes | Yes | 5°C | 2 |
| E | Yes | Yes | 25°C, dark | 17 |
A versus B isolates water availability; A versus C isolates oxygen; A versus D tests temperature under the stipulated matched conditions. Water activates hydrated metabolism and expansion; oxygen supports aerobic respiration; a suitable temperature permits enzyme-controlled processes at a sufficient rate. A’s percentage is 18/20 × 100 = 90%, D’s is 10%. A and E differ by one seed, so this small record does not support a strong light effect. It also does not show that all species germinate in darkness.
“No emergence by day 7” is not identical to “dead”. Low temperature may delay germination; dormancy and viability would require their own evidence in a real sample. Seed reserves can support early growth before effective photosynthesis. Later growth also depends on light, mineral supply, water, temperature and other conditions; germination and sustained growth are not the same endpoint.
A limiting-factor interpretation
A separate model gives 6 cm mean growth in two weeks under low light, and 11 cm under increased light with water, nutrients, temperature and starting size matched. Greater light is consistent with greater growth under those conditions. It does not show that limitless light causes limitless growth: a different resource or damage threshold may then limit the response. Adding fertiliser to the brighter group only would confound the comparison.
Independent practice with answers
- Task: Use the growth graph for weeks 4–6 and 8–10. Answer: (20−8)/2 = 6 cm/week; (45−36)/2 = 4.5 cm/week. The earlier interval is faster.
- Task: A new seed set has 21 germinated out of 30. Answer: 70%, with the day and criterion required for comparison.
- Task: A dry, cold group is compared with a wet, warm group. Can either factor be isolated? Answer: No; both vary. Change one while matching the other.
- Task: Does a 47 cm final height prove 47 cm of growth during this record? Answer: No. Starting height was 2 cm, so the observed increase is 45 cm.
Background: seed development and germination. The numerical records are original, not measurements of a named species.
Frequently asked questions
Why can fresh mass be misleading?
Hydration can change it greatly without a matching increase in dry material. State whether the question concerns water, biomass or size.
Does falling dry mass mean that no growth occurred?
No. The axis can develop and increase in length and dry mass while the entire seed-plus-seedling sample loses dry material.
Are storage-part mass and reserve mass identical?
No. Storage tissue also contains structural and other material. This table does not measure the mass of a particular reserve chemical.
Why use separate matched cohorts?
Dry-mass measurement destroys the sample for continued growth. Matching cohorts makes comparisons meaningful without pretending to remeasure the same living seedlings.
Would illuminated seedlings show the same pattern?
Not necessarily. Once photosynthesis contributes carbon, the balance between input and respiratory loss can differ from this dark-grown model.
Can the graph prove the cause of every loss?
No. The explanation depends on the stated model assumptions. Real results would require checks for lost tissue, leaching, sampling variation and other exchanges.
What should a good graph include?
Label day on the horizontal axis, mass and its units on the vertical axis, and distinguish both series with labels and symbols. Keep the exact data available.
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