Nearly every plant you see outdoors had its beginnings as a seed. A plant begins to grow when a seed germinates, usually when water causes the embryo inside the seed to swell and break out of the tough outer seed coat.
During the earliest stages of plant growth, the first root, called the taproot, stretches downward into the soil in search of water and to establish a firm structural foundation for the plant. The root grows both by adding cells and by elongating cells that already exist. Before long, the root forms branches, which improve the plant's support system and its ability to absorb water from the soil.
Not long after the taproot has become established in the soil, the shoot, or stem, of the seedling begins to stretch upward in search of light. The time-lapse video of this reveals the seedlings' need for light: They bend first one way and then the next in what becomes a repetitive waving motion as they follow the sun's movement throughout the day.
So far, all of the energy that the plant requires for growth comes from food stored in the part of the seed called the endosperm. As the stem stretches skyward it carries with it what is left of the food stores in the endosperm. Before long, however, this food will be depleted and the plant will need to create its own food. To do this, the plant must first grow leaves.
Leaves contain a chemical called chlorophyll -- a pigment that gives green leaves their color and allows plants to collect the sun's energy and use it to convert carbon dioxide and water into sugars and other carbohydrates. This chemical conversion, called photosynthesis, is the process the plant will use to produce energy for the rest of its life.
Most plants can survive with a combination of these three parts: roots, stems, and leaves. To reproduce (produce their own seeds and, eventually, their own plant offspring), however, most plants must first produce flowers. Flowers produce the pollen and eggs that, when combined with another flower's pollen or eggs, become seeds that can germinate and grow into a new plant. Many flowers, including the sunflower seen in the video, lure insects and birds to them with rewards of pollen and nectar. In exchange, these animals unwittingly carry pollen to other flowers and fertilize the eggs they contain.
Do plants move? How do you know?
How do different kinds of movement benefit plants?
It's not surprising that early scientists hypothesized that plants ate dirt. They didn't know, as we now do, how energy-rich sunlight is. Still, it seems remarkable that plants have evolved photosynthesis--the ability to harness the sun's energy to produce their own food.
Photosynthesis is the process by which plants transform water and carbon dioxide (a gas that's plentiful in the air) into carbohydrates (sugars and starches), using the energy of sunlight. While sunlight provides the energy needed to drive this reaction, a chemical in the leaves of plants makes the reaction possible. That chemical is a green pigment called chlorophyll. Chlorophyll is found inside the photosynthetic cells of plants, attached to the membranes of small, round structures called chloroplasts. Chlorophyll absorbs light in the red and blue-violet portions of the visible spectrum, and reflects the green portion of the spectrum; this is what gives chlorophyll its characteristic green color.
As remarkable as photosynthesis is, the process is not very efficient. Studies show that prairie grasses in the western United States are some of the most efficient plants at harnessing the sun's energy, but even they capture little more than about 3 percent of the energy that reaches the prairie surface. The rest of that energy is reflected away, absorbed by humidity in the air or by the ground, or simply lost in myriad other ways before the plants can use it.
One of the most critical factors influencing the efficiency of photosynthesis is the amount (intensity and duration) of light that hits a leaf. Generally, the more light that strikes a leaf, the greater the rate of photosynthesis in that leaf. For example, a leaf that is exposed to direct sunlight will photosynthesize at the highest rate, while a leaf directly beneath it and in its shadow will photosynthesize at a much lower rate. Because of this, many plants have evolved leaf and branch structures that minimize overlap and shading, and thus maximize the plant's overall rate of photosynthesis.
Do you think that the factory is a good analogy for the process of photosynthesis in plants?
Why did von Helmont think that plants got their nourishment from soil?
Why did he eliminate soil as a source of nourishment and focus on water?
What did he measure to find out if the willow plant got its nourishment from soil?
What do you think von Helmont concluded when he measured the change in weight of the plant and the soil?
Although a growing plant needs access to the right proportions of sunlight, water, and nutrients to be healthy, plants are not simply passive recipients of nature's bounty. Despite their fixed positions in the earth, they move and grow in response to a variety of environmental cues.
Chief among any plant's requirements is light. Without this energy source, plants are unable to create the tissues that support them and the food that sustains them. Through the process of photosynthesis, plants use sunlight to convert water and carbon dioxide into carbohydrates. Mature plants can survive no more than a few days without light. Young plants that are not yet able to photosynthesize rely on energy stored in seeds to sustain them until their leaves develop.
As plants develop, they follow distinct growth and movement patterns, called tropisms. These directional movements occur in response to environmental stimuli, such as gravity, light, and touch. Because most seeds germinate underground and in darkness, gravity is one of the earliest and most important cues. As the first root emerges from the seed during germination, it grows downward in the direction of gravity's force—where there is apt to be a steady source of water and nutrients.
The plant's first shoot grows in the opposite direction, away from the force of gravity and up in the direction of sunlight. This upward growth proceeds rapidly as cells grow both in number and length. Only when a shoot encounters light does upward growth slow or deviate at all, as leaves open and photosynthesis begins. Patterns of growth that maximize the exposure to light continue for the life of most plants. Upward growth is usually sufficient to meet a plant's energy needs, but a plant may also bend, twist, or climb other plants and/or objects if a better source of light exists somewhere other than straight up.
Plants rely on specialized structures in their cells to sense environmental stimuli such as gravity and light. These structures, called amyloplasts, rest at the low end of the specialized cells in which they reside. A change in the orientation of a plant causes amyloplasts to shift, thus informing the plant's cells as to their direction in relation to gravity. Photoreceptors called phototropins enable plants to detect differences in the intensity of the light striking various cells.
A group of hormones, called auxins, enable plants to alter their direction of growth according to the stimuli they receive. These hormones promote cell division and elongation. They typically accumulate in growing plant stems on the side opposite the direction of a light source or other valuable resource. The increased growth on this side of a stem causes the stem's tip to bend and grow toward a stronger source of light or other resource.
Do plants always grow "up"? Which parts of the plant show gravitropism? How does gravitropism help a plant meet its needs?
Can plants grow in the dark? What are the differences between plants grown in darkness and those grown in light? What do you think is the reason for these differences?
How do the motions of the plants that you see in these videos benefit the plant? Give specific examples.