Leave a bean seedling on a windowsill for two days and it will lean toward the glass. The plant has no eyes and no nerves, yet it sensed where the light was and changed its shape to reach it. Plants respond to their surroundings by changing how they grow, and they coordinate those responses with chemical messengers called plant hormones (also called growth regulators). This topic looks at what a plant needs to grow, how hormones steer that growth, and the two ways plants make new plants: sexually through flowers and seeds, and asexually from pieces of the parent.
Factors that affect plant growth
| Factor | What the plant uses it for | What happens if it is lacking or extreme |
|---|---|---|
| Light | Energy for photosynthesis. Intensity (brightness), quality (colour: chlorophyll absorbs red and blue light best and reflects green) and duration (hours per day) all matter. | Too little light slows photosynthesis; seedlings grown in dim light become tall, thin and pale, which is called etiolation. |
| Water | A raw material for photosynthesis, the medium for transport, and turgor pressure that holds non-woody tissue firm. | Wilting, closed stomata and slowed photosynthesis. Waterlogged soil is also harmful, because roots run short of oxygen. |
| Nutrients | Nitrogen (N) for proteins, chlorophyll and DNA; phosphorus (P) for ATP, DNA and cell membranes; potassium (K) for enzyme activity and guard cell function; magnesium (Mg) at the centre of each chlorophyll molecule. | Nitrogen deficiency typically yellows the older leaves first. A shortage of any essential nutrient limits growth even if everything else is ideal. |
| Temperature | Sets the rate of enzyme-controlled reactions such as photosynthesis and respiration. | Too cold: reactions slow. Too hot: enzymes denature and water loss increases. |
| Soil pH | Affects which nutrients dissolve and are available to roots. | Many crops grow best in slightly acidic to neutral soil, around pH 6 to 7, while some plants, such as blueberries, need more acidic soil. |
Plant hormones
| Hormone | Mainly made in | Main effects |
|---|---|---|
| Auxins | Shoot tips and young leaves | Stimulate cell elongation in stems; control phototropism and gravitropism; keep side buds dormant (apical dominance); stimulate roots to form on cuttings |
| Gibberellins | Young leaves, developing seeds, roots | Stimulate stem elongation between nodes; trigger seed germination; increase fruit size (sprayed on some seedless grapes) |
| Cytokinins | Mainly roots | Stimulate cell division; encourage side buds to grow; delay leaf ageing |
| Ethylene | Ripening fruit and ageing tissues (it is a gas) | Speeds fruit ripening; promotes leaf and fruit drop |
| Abscisic acid | Leaves and seeds | Closes stomata during drought; keeps seeds dormant |
Tropisms: growing toward or away from a stimulus
A tropism is a growth response toward (positive) or away from (negative) a stimulus. In phototropism, light on one side of a shoot causes auxin to move to the shaded side. Cells on the shaded side elongate more than cells on the lit side, so the shoot curves toward the light. Charles Darwin and his son Francis showed that the tip is the sensor: grass seedlings with an opaque cap over the tip did not bend toward light, while seedlings with the region below the tip covered still did, so a signal must travel down from the tip. In gravitropism, shoots grow up (negative) and roots grow down (positive). Auxin collects on the lower side of both; in the shoot it speeds elongation there so the shoot curves up, but root cells are far more sensitive, and the same auxin level slows their elongation, so the root curves down. In thigmotropism, a response to touch, the tendrils of plants such as peas and cucumbers coil around whatever they contact.
Sexual reproduction in flowering plants
- Sepals protect the flower bud; petals often attract animal pollinators with colour and scent.
- The stamen is the male part: an anther, which makes pollen, held up on a filament.
- The pistil is the female part: a sticky stigma that catches pollen, a stalk called the style, and an ovary containing one or more ovules, each holding an egg.
- Pollination is the transfer of pollen from an anther to a stigma, by wind (grasses and corn have small, dull flowers and make huge amounts of light pollen; the silks on an ear of corn are its long styles and stigmas) or by animals such as bees.
- Fertilization comes after pollination. A pollen tube grows down the style into an ovule and delivers two sperm. One fuses with the egg to form the diploid zygote, which becomes the embryo. The other fuses with two polar nuclei to form the triploid endosperm, a food store. This is called double fertilization.
- After fertilization, each ovule becomes a seed and the ovary becomes the fruit, which protects the seeds and helps disperse them.
Seeds and germination
A seed is a dormant embryo packed with food and wrapped in a protective seed coat. The embryo has a radicle (embryonic root), a shoot tip, and one or two cotyledons. In beans, the two cotyledons store most of the food; in corn, a monocot, food is stored mainly in the endosperm. Germination needs water, oxygen and a suitable temperature; some seeds also need light or a period of cold first, which stops them sprouting at the wrong time of year. Germination begins with imbibition: the dry seed soaks up water, swells and splits its coat. Water activates enzymes, and in grains such as barley the embryo releases gibberellins that signal the aleurone layer (the outer cells of the endosperm) to make amylase, an enzyme that breaks stored starch into sugars. Cellular respiration of those sugars (which is why oxygen is needed) supplies energy for growth. The radicle emerges first, anchoring the seedling and absorbing water, and then the shoot pushes up toward the light.
Asexual reproduction and propagation
| Method | How it works | Example |
|---|---|---|
| Runners (stolons) | Horizontal stems along the ground grow roots and new plants at nodes | Strawberry |
| Tubers | Swollen underground stems whose buds (eyes) sprout | Potato |
| Bulbs | Short underground stems wrapped in fleshy leaf bases | Onion, tulip |
| Stem cuttings | A piece of stem is planted; rooting powder containing an auxin helps roots form | Geranium, coleus |
| Leaf cuttings | A leaf, or part of one, grows roots and new shoots | African violet, snake plant |
| Root cuttings | A section of root sprouts new shoots | Horseradish |
| Grafting | A shoot (scion) from the desired plant is joined to the rooted stem (rootstock) of another so their vascular tissues grow together | Apple and other fruit tree varieties |
| Tissue culture | Small pieces of tissue grow into many plantlets on a sterile nutrient gel containing hormones | Orchids, many nursery plants |