Skip to content
Quick Guides

Plant Biology - Growth of Plants

Preview 7 of 22 pages

See the first 7 pages exactly as they appear in the document.

  1. Page 1
    Read page 1 as text

    Study Guide Plant Biology – Growth of Plants 1. Responsive Growth Movements: Tropisms Plants may not move from place to place like animals, but they do respond to their environment by growing in specific directions. These directional growth responses are called tropisms . • When a plant grows toward a stimulus, it shows a positive tropism . • When it grows away from a stimulus, it shows a negative tropism . Different types of stimuli — such as light, gravity, or touch — cause different kinds of tropic movements. 1.1 Phototropism (Response to Light) Phototropism is the growth response of plants to light coming from one direction . • Shoots usually grow toward light, so they are positively phototropic . • Roots grow away from light, making them negatively phototropic . How does this happen? Experiments with oat seedlings have helped scientists understand this process. A plant hormone called auxin (IAA) plays a key role. • Auxin moves to the shaded side of the shoot. • Cells on the shaded side elongate more than those on the lighted side. • This uneven growth causes the shoot to bend toward the light . In green stems , the process is slightly different. Here, an IAA inhibitor prevents cells on the lighted side from elongating, while cells on the shaded side keep growing. This difference again causes the stem to bend toward the light. Role of light color Not all light affects growth equally. Blue light , especially wavelengths below 500 nm , is the most effective in triggering phototropic responses.

  2. Page 2
    Read page 2 as text

    Study Guide 1.2 Gravitropism (Response to Gravity) Gravitropism is a plant’s growth response to gravity . • Shoots grow upward, away from gravity, so they are negatively gravitropic . • Roots grow downward, toward gravity, making them positively gravitropic . How do plants sense gravity? The exact mechanism is still not fully understood . Scientists have proposed many ideas, but none explains everything. Over millions of years, plants likely developed multiple ways to respond to gravity. Certain substances are known to be involved in this response: • Auxin (IAA) • Calcium ions (Ca² ⁺ ) • Possibly abscisic acid (ABA) A long - standing theory suggests that starch grains inside the root cap move downward when a root is placed horizontally. This movement may help the plant sense gravity and trigger growth changes, but this idea has not yet been conclusively proven . 1.3 Thigmotropism (Response to Touch) Thigmotropism is the growth response of a plant or plant part to touch or contact with a solid object . A common example is seen in climbing plants : • Their tendrils wrap around supports like sticks or wires. • Cells on the side touching the support shorten , while cells on the opposite side elongate . • This uneven growth causes the tendril to curl around the object. Plant hormones such as auxin (IAA) and ethylene are believed to play an important role in this response.

  3. Page 3
    Read page 3 as text

    Study Guide Summary Tropisms help plants adapt and survive by growing in directions that improve their access to light, water, support, and stability. Even though plants appear still, their growth responses show how actively they interact with their environment. 2. Other Plant Movements Even though plants are fixed in one place by their roots, they are far from motionless. Many plant parts — such as leaves, flowers, and traps — can move when they receive signals from their environment. These movements help plants survive and respond to changi ng conditions. What are nastic movements? These plant movements are called nastic movements . Unlike tropic movements, they are not directed toward or away from a stimulus . Instead, the movement happens no matter where the stimulus comes from. Types of nastic movements • Touch - based movements are called thigmonastic or seismonastic movements. • A classic example is the rapid closing of traps in insect - eating plants like the Venus flytrap. • Daily “sleep” movements of leaves are called nyctinastic movements . • These occur because of regular cycles of light and darkness, such as leaves folding at night and opening during the day. How do these movements happen? Most nastic movements occur due to changes in turgor pressure . After a plant senses a stimulus, water moves in or out of special parenchyma cells , causing the cells to swell or shrink. This change in pressure leads to visible movement of plant parts. 2.1 Thigmomorphogenesis: Growth Changes Caused by Touch Plants constantly experience mechanical forces in nature. Wind, rain, animals brushing past, and even repeated shaking all affect how a plant grows. What is thigmomorphogenesis? Thigmomorphogenesis is the growth response of plants to repeated mechanical disturbances like touching or shaking.

  4. Page 4
    Read page 4 as text

    Study Guide How does the plant change? When plants are regularly disturbed: • They grow shorter in height • Their stems become thicker • They develop stronger supportive tissues These changes make the plant sturdier and better able to survive harsh conditions. What happens inside the plant? Mechanical stress activates certain genes inside plant cells. One important gene produces calmodulin , a protein that binds calcium and plays a key role in regulating growth. This process is similar to other growth responses that involve calmodulin. The plant hormone ethylene , which helps regulate growth, also plays an important role in this response. 2.2 Solar Tracking (Heliotropism) Some plants can move their leaves or flowers to follow the sun across the sky during the day. This behavior helps them absorb maximum sunlight for photosynthesis. How solar tracking works A well - known example is the sunflower : • In the morning , the flowers face east • At midday , they face upward toward the sun • In the evening , they turn west toward the setting sun What is this movement called? This movement is known as heliotropism . However, it is not a true tropic movement because it does not involve growth . What causes the movement? Like many nastic movements, solar tracking happens due to changes in turgor pressure in parenchyma cells. Water movement within these cells allows the plant parts to bend and follow the sun smoothly throughout the day.

  5. Page 5
    Read page 5 as text

    Study Guide Key Takeaway Plants may seem still, but they are highly responsive organisms. Through nastic movements, growth changes, and solar tracking, plants constantly adjust to their environment — using clever internal mechanisms rather than muscles or nerves. If you’d like, I can also: • Turn this into exam - ready notes • Add diagrams with explanations • Create AP - style questions • Or simplify it even more for quick revision 3. Circadian Rhythms 3.1 What Are Circadian Rhythms? Many plants show activities that repeat on a regular 24 - hour cycle . For example, some flowers always open in the late afternoon, while others open at night or during the day. This repeating pattern of growth or activity that happens about once every 24 hours is called a circadian rhythm . Examples You Can Easily Notice Circadian rhythms control many important processes in plants, such as: • Sleep movements of leaves , where leaves fold or droop at night and reopen during the day • Opening and closing of flowers , especially night - blooming and day - blooming plants • Cell division in root tips • Production of proteins and hormones These activities happen on a daily schedule, even when conditions seem unchanged.

  6. Page 6
    Read page 6 as text

    Study Guide 3.2 Controlled from the Inside Circadian rhythms are endogenous , which means they are controlled from inside the plant . Plants have an internal timing system known as the biological clock . This clock allows plants to “know” when to carry out certain activities during the day or night. 3.3 How the Environment Helps Even though circadian rhythms are internally controlled, the environment helps keep the clock accurate . Light and darkness act like signals that adjust the plant’s internal clock so it stays in sync with the outside world. This process of resetting the biological clock is called entrainment . 3.4 Why Entrainment Matters Entrainment ensures that plant activities, such as flowering, stay aligned with: • Day and night cycles • Seasonal changes in day length Because of this, plants can respond correctly in different seasons by measuring changes in daylight . 3.5 Role of Phytochrome Plants use a special light - sensitive pigment called phytochrome to detect light changes. Phytochrome helps the biological clock track day length and plays a key role in controlling circadian rhythms, especially those related to flowering. Summary Circadian rhythms help plants stay on a daily schedule. An internal biological clock controls these rhythms, the environment keeps the clock adjusted, and phytochrome helps plants sense light and seasonal changes.

  7. Page 7
    Read page 7 as text

    Study Guide 4. Photoperiodism Photoperiodism is how plants respond to changes in day length and night length over a 24 - hour cycle. In simple terms, plants use light and darkness to tell what season it is. This helps them time important events like flowering , so they bloom at the best possible time of year. 4.1 Phytochrome: The Plant Light Sensor Plants detect light using a special pigment called phytochrome . This pigment exists in two forms: • Pr – absorbs red light • Pfr – absorbs far - red light These two forms can change into each other: • In daylight, Pr changes to Pfr , and Pfr can change back to Pr . • By the end of the day, plant cells usually reach a balance of about 60% Pfr and 40% Pr . At night, something important happens: • Pfr slowly converts back to Pr or breaks down • Pr stays stable in the dark 4.2 Why Night Length Matters Pfr is the active form of phytochrome. It acts like a switch that turns on plant responses such as flowering and seed germination . • When enough Pfr is present, certain plant responses begin. • Because Pfr breaks down at night , the length of the night (not the day) is what really controls these responses. Here’s how it works: • Short nights (long days): More Pfr remains → favors processes that need a lot of Pfr • Long nights (short days): More Pfr turns into Pr → favors responses that need only a small amount of Pfr Even though Pr is made from amino acids, it is inactive until it converts to Pfr .

15 more pages in the full document

Unlock it once and it stays in your library, ready to chat with or turn into flashcards.

Unlock the full document

More Biology study materials

See all

Study this document with CramX

Once it is in your library, every tool can work from it.