A Plant's Life · PlantLab

A bean plant

This is a common bean growing in a root box: the front of the box is glass and the roots grow against it, so you can see the plant above and below ground.

Flowering plants have six organs. Roots, stems and leaves absorb water and minerals, transport materials and make organic matter; they are the vegetative organs. Flowers, fruits and seeds produce the next generation; they are the reproductive organs.

Click an organ on the right and the camera moves to it. Switch the growth stage to see the same plant at different times.

Conditions for seed germination

Each of the four Petri dishes holds 12 seeds on filter paper. Dish 1 gets no water (dry); dish 2 is moistened; dish 3 is filled until the seeds are under water (no air); dish 4 is moistened and put in a refrigerator (cold).

Dish 2 is the control; each of the other three changes just one condition. This is controlling variables.

Seeds germinate only with enough water, a suitable temperature and plenty of air, and the seed itself needs a complete, living embryo with stored food. Play the timeline and watch which seeds swell first and then put out a radicle.

Emergence and etiolation

When a bean seed germinates, the radicle breaks through the seed coat first and grows down into the main root. Then the hypocotyl lengthens, bends into a hook that pushes up through the soil and pulls the two cotyledons out of the ground. The hook keeps the delicate shoot tip behind it, away from scraping soil grains.

The left root box is in the light; the right one is covered by a dark box. In the light the hook soon straightens and the cotyledons open and turn green. In the dark the stem grows long, thin and yellow and the hook never straightens: this is etiolation. Chlorophyll needs light to form.

Once the food stored in its cotyledons runs out, the seedling in the dark falls over and dies. Try sowing the seed very deep.

Root growth and gravitropism

From the very tip upward, a root tip has a root cap (a protective cap), the meristematic zone (small cells that keep dividing), the elongation zone (cells grow longer quickly; the root grows fastest here) and the maturation zone (outer cells grow into countless root hairs, which absorb most of the water and minerals).

A root grows because meristem cells divide, which adds cells, and elongation-zone cells lengthen, which makes them bigger.

On the chosen day the root box is turned. A few hours later the root tip curves downward again: this is root gravitropism. Only the part just behind the tip that is still lengthening bends; older parts keep their shape.

Light intensity and photosynthesis

After dark, only one grow light is on in the greenhouse. The beaker under it holds hornwort, a submerged water plant. The oxygen released by photosynthesis bubbles out of the cut stem; count the bubbles per minute to compare how fast photosynthesis runs.

Move the lamp closer and the light gets stronger and the bubbles come faster, but beyond a certain light intensity they stop increasing. That intensity is the light saturation point. When the lamp is too far away, photosynthesis makes less organic matter than respiration uses, so the net photosynthesis rate is negative. The intensity at which the two are exactly equal is the light compensation point.

Now raise the CO₂ concentration or change the temperature: the raw materials (CO₂, water), the condition (light) and enzyme activity (temperature) all affect the rate.

Green leaves make starch in light

Work through the steps at the bottom one by one.

Dark treatment: keep the plant in the dark for a day and a night so the starch already in the leaves is carried away and used up. Shading: cover part of one leaf on both sides with black paper; this is the control. After several hours of light, pick that leaf. Bleaching: put it in a small beaker of alcohol and heat it in a water bath. The chlorophyll dissolves in the alcohol and the leaf turns pale yellow (alcohol is flammable, so never heat it directly). Rinse the leaf and add iodine solution.

Result: the part that got light turns blue and the shaded part does not. Starch is a product of photosynthesis, and light is essential for photosynthesis.

Transpiration

Each bean plant is covered with a clear plastic bag tied around the base of the stem. The left plant keeps its leaves; the right one has had all its leaves removed. A few hours later the inside of the left bag is covered with droplets while the right one stays almost dry: water is lost mainly through the leaves.

Water leaves the plant as water vapour through stomata in the leaf surface. This is transpiration. Each stoma is surrounded by a pair of guard cells; it opens in the day and closes at night, and also closes when the plant is short of water.

Heat, dry air, wind and strong light all speed up transpiration. Stop watering and the soil dries out until the leaves wilt; water again and they soon stand up. Transpiration pulls water and minerals up through the plant and also cools the leaves.

Phototropism and auxin

Five maize coleoptiles are lit from one side. ① Intact: bends toward the light. ② Tip cut off: neither grows nor bends. ③ Tip covered with an opaque foil cap: grows straight. ④ Foil sleeve below the tip: still bends toward the light. So the tip senses the light, and the part below the tip does the bending (Darwin).

⑤ Tip cut off, and an agar block that had tips standing on it is placed on one side of the cut. Even without light, the coleoptile bends away from that side. Something made in the tip moves down and promotes growth; it was later identified as auxin (Went).

One-sided light makes more auxin collect on the shaded side, so the shaded side grows faster and the shoot bends toward the light.

Flowering and fruiting

The bean flower is papilionaceous (butterfly-shaped): a large standard petal on top, two wing petals at the sides and two lower petals joined into a coiled keel that wraps the stamens and the pistil. Pollen lands on the flower's own stigma before the flower fully opens, so beans are self-pollinating.

After fertilization the petals fall. The ovary develops into the fruit (the pod), the ovules inside it develop into seeds, and the fertilized egg becomes the embryo. The pod first lengthens, then swells section by section as the seeds fill, and finally turns yellow, dries and ripens.

When daytime temperatures are too high (above 32 °C), pollen loses its vigour, many flowers drop and fewer pods set.

Free growing

Start from one seed and grow your own bean plant. Every condition changes how it looks: in weak light the stem is long and thin and the leaves turn yellow; cold slows growth and too much heat makes flowers drop; watering too rarely causes wilting; more CO₂ speeds up photosynthesis; lack of nitrogen gives yellow leaves and a small plant; one-sided light bends it toward the light.

The curves on the right track height, leaf area and dry mass. After you change a condition, the whole growth period is recalculated with the new settings.