Chapter 10
Plant Biology 101
In This Chapter
• Identifying plant parts, inside and out
• Photosynthesis explained
• The drinking habits of plants
• The sex life of plants
• Annuals versus perennials
Before you can begin sowing seeds and tending plants, you probably should know a little about how plants grow. If you remember everything you learned in freshman biology, you can probably skip this chapter. If not, read on.
And be prepared, this stuff can get pretty technical.
Plant Parts
To understand how plants grow and reproduce, you need to be familiar with their various parts. It’s actually pretty easy to understand, but to make it even easier, we look at this in terms of vegetables.
A Little About Leaves
Leaves are lateral outgrowths of a plant’s stem. Most leaves are green (at least at some stage of their lives). Edible leaves include lettuce, cabbage, kale, mustard greens, bok choy, spinach, and parsley, among many others. The role of the leaf is to soak up the sun’s energy.
Most leaves are green because they contain chlorophyll, the plant’s green pigment. (Chloro means “green” in Greek.) The chlorophyll traps the energy from the sun. This is part of a process called photosynthesis, which we look at a little later in this chapter.
Garden Guru Says
Bulbs like onions and garlic are a collection of modified leaves that live underground. If you’ve ever peeled an onion, you can see how it’s made of tightly wrapped layers. These modified leaves don’t have the same role as green leaves that are exposed to sunlight. Instead, they store energy.
The Story of Stems
The stem is (usually) a slender growth of plant material that supports or connects one plant part to another. Stems function like arteries and veins, distributing water and nutrients throughout the plant. For example, a stem connects leaves to each other and supports a flower or fruit. Some of the stems we eat include celery, asparagus, and rhubarb.
The story of stems can get a little more complicated. Stems can also grow underground as stolons. The runners of strawberries are a good example of stolons. Some other underground stems are called tubers. Potatoes, yams, jicama, and Jerusalem artichokes fall into this category.
Get Back to Your Roots
The root is the part of the plant that usually lives below the soil. It serves as a sort of anchor for the plant and is the “machinery” the plant uses to absorb water and nutrients.
Roots also can enlarge and become a kind of storage tank of nutrients the plant will call on later in its life cycle. Usually, people eat these little warehouses before the plants get to use them. Among the edible roots we enjoy are carrots, radishes, beets, turnips, and parsnips.
Most plants have a primary root that goes more or less straight down into the soil with smaller lateral roots that reach out from the main root. At the ends of lateral roots, tiny root hairs are responsible for absorbing nutrients and water from the soil. Finally, at the very end of the root is a root cap, a little protective covering.
A plant’s root system helps it stay stable in the soil and also soak up water and nutrients.
Some plants develop one very large main root with some lateral roots. This main root is called a tap root. Carrots, parsnips, and radishes are good examples. Other plants have fibrous roots, which are what you see with tomatoes, peppers, beans, and most other vegetable plants. The fibrous roots have a much smaller main root and lots of lateral roots.
Beautiful Buds
The bud is either a stem or a flower in the very earliest stages of its development. Some buds are vegetative and can become new stems and leaves. Others develop into flowers. Some flowers eventually become fruit.
Food for Thought
Vegetables like peppers, tomatoes, eggplant, squash, and cobs of corn are, botanically speaking, fruits, which, as you now know, are fully developed buds. Some other vegetables are actually flowers. Artichokes, for example, are flower buds. We eat the unopened petals one by one. The parts of broccoli and cauliflower that we eat are clusters of flower buds. Fruits including berries and watermelons are, well, fruits.
Inside Plants
The really interesting parts of plants are buried—but not hidden—inside the cells that make up plant organs. Most of these parts are very much like those in human and other animal cells, such as the nucleus, which contains a cell’s DNA. But some of the tiny structures inside plants are very different from anything found in animals.
Chloroplasts
The one big exception where plants differ from animals is that plants have chloroplasts. These are little green engines that carry out photosynthesis (be patient, we’ll get to it). Chloroplasts are green because they contain chlorophyll. (Remember, chloro means “green” in Greek.) Chloroplasts typically appear in leaves, which is why leaves are green, but they can also be found in other green organs like buds (broccoli), stems (celery), and fruits (bell peppers).
Several structures are closely related to chloroplasts. These other structures are not green and they don’t carry out photosynthesis, but they have distinct functions critical to the life of plants.
Amyloplasts
One relative of chloroplasts is the amyloplast, which contains starch. (Amylo is Greek for “starch.”) Amyloplasts are found in very large quantities in seeds and tubers. The starch they hold is where much of the energy we get from food originates.
The amyloplasts fuel the plant’s reproduction. Developing seedlings harvest energy from the starch stored in amyloplasts as they grow until they are mature enough to soak up sunlight.
Chromoplasts
Chromoplasts are another relative of chloroplasts. (Chromo means “color” in Greek.) They accumulate large amounts of yellow, orange, and red pigments (called carotenoids), which are found in foods that are high in antioxidants like carrots and tomatoes, in nasturtium flowers, and in squash blossoms.
How Seeds Grow
Seeds, which plants make so they can reproduce themselves, are basically tiny undeveloped new plants with some extra things added on. Each seed contains an embryo (the undeveloped new plant) and some tissue that’s packed with food that will feed the developing seedling until it’s able to survive on its own.
The seed is enclosed in a tough jacket that protects it from the elements until the conditions for germination are just right. Usually this is when there’s enough moisture and warmth in the seed’s environment. When the seed senses that it’s warm enough and that there’s enough moisture, it breaks out of dormancy and begins to grow.
Prof. Price’s Pointers
Dormancy is a rest period in a plant’s life cycle. Germination is when a seed’s dormancy ends and the seed begins to develop as a new plant. Plants also have an internal clock that’s set by the length of the night. So when nights become shorter in the spring, the seeds of some plants, such as lettuce, say “Wake up! It’s time to germinate.” Other plants use this clock to decide when it’s time to flower.
As the seed grows into a seedling, it uses its little storehouse of food. The stem of the seedling pushes up through the soil toward sunlight. When it finally “sees” the sun, it puts its energy into expanding its tiny leaves. At this point, the colorless leaves begin to turn green.
The Mystery of Photosynthesis
One of the great triumphs of plant science in the mid-twentieth century was unraveling the mystery of photosynthesis. Photosynthesis takes place inside the plant’s chloroplasts in two separate processes called “the dark reaction” and “the light reaction.”
Prof. Price’s Pointers
Photosynthesis is the conversion of carbon dioxide and water into glucose and other sugars and starch using the energy of sunlight. Oxygen is produced as a waste product. Photosynthesis is how plants manufacture their own food.
The dark reaction converts carbon dioxide to sugar. This process can happen in the dark, but only for a few seconds. To keep on going, it needs a huge input of light energy. That’s when the light reaction kicks in. This process starts when the chlorophyll in leaves soaks up the sun’s energy, which is then converted through chemical processes to electrical energy. For all you chemistry buffs, this is when an electron is separated from a hydrogen atom, a lot like when you charge a battery.
The electrical charge is then converted to a complex chain of reactions that eventually fuels the dark reaction, which makes the sugar. Finally, the waste product of this process—oxygen—is released. Simple, right?
In a nutshell, photosynthesis …
• Produces food for plants.
• Occurs in cells that contain chloroplasts.
• Uses water and carbon dioxide.
• Requires the energy of the sun.
How Plants Drink
Do you remember the science “experiment” you did in third grade when you put a stalk of celery in a glass of water mixed with red food coloring? After a few hours, the celery had magically turned red!
Even if you didn’t realize it at the time, that was a lesson on how plants drink. You might even remember that the process was called osmosis. That elementary school lesson gave you part of the story about how plants drink, but it’s a little more complicated than that.
Why Plants Wilt or Don’t Wilt
Plant cells are filled with solutes—these are salts, sugars, organic acids, and so on. The solutes create what’s called an osmotic potential, which decreases the energy of the water being absorbed. As a consequence, plants absorb water until the pressure inside the plant cells matches the osmotic pressure.
The balance of osmotic pressure and internal water pressure is what makes plant stems get stiff when they have enough water and makes them wilt when there’s not enough. The firmness of the plant’s tissues is called turgor.
More Drinking Stories
Osmotic potential pulls water into a plant only so far, so plants have another trick: evaporation. The leaves of plants have thousands of tiny openings called stomata that allow water vapor to diffuse out. (They also permit carbon dioxide, needed for photosynthesis, to diffuse into the leaves.)
The loss of water by evaporation through the stomata creates a negative pressure that literally sucks water up through the stems. This process is called transpiration.
Food for Thought
Because leaves don’t photosynthesize at night, the stomata in the leaves close down and save water. When the sun comes out, they open again for business. If the water pressure in the plant drops too low, the stomata close again to save water. So if you forget to water the tomatoes, the plants themselves will bail you out. But don’t do it again; there’s only so much a plant can do on its own.
How Plants Reproduce
When it comes to reproduction, plants are a bit more flexible than animals. Plants can reproduce using sexual reproduction or asexual, or vegetative, reproduction.
It’s All About Sex
Sexual reproduction in plants happens when the ovules (egg) and pollen (sperm) from flowers are combined and form an embryo. Most plants produce “perfect” flowers that contain both pollen and ovules, but in some plants, the sexes are separated. In members of the squash family, for example, the first flowers to appear on the stem are strictly male, producing only pollen. Later flowers are strictly female, with the baby fruit looking like a slight swelling behind the petals.
Birds, bees, other insects, the wind, and sometimes even humans all help plants with sexual reproduction. This occurs when pollen, which is usually carried on the stamen (the little yellow bundle inside the flower), is mixed with the ovule, which is usually at the center of the bloom inside the ovary. Bees and other insects and some birds visit flowers to drink the nectar in the petals. As they pass by the stamen, a little of the pollen rubs off on their wings or feet and then lands at the stigma, where it can enter the ovule.
When bees are scarce, pollination might not happen as often as it should, and plants produce less fruit. Sometimes gardeners can help with the process by hand-pollinating their plants.
A Vegetative State
Some plants are sterile (they cannot reproduce on their own) so the only way to make new plants is by vegetative reproduction. This is usually done with stem or root cuttings.
Most vegetative reproduction is done with woody plants or houseplants and rarely with vegetables. Fruit shrubs and grape vines can be reproduced with root or stem cuttings, as can some herbs. Vegetative reproduction is considered asexual reproduction.
Growing Seasons
So far in this chapter we’ve looked at the parts of plants and what makes them tick. Now it’s time to talk about how they actually grow. Instead of thinking of growing seasons in terms of simply summer, spring, fall, and winter, gardeners look at seasons of development, growth, fruit production, and death or dormancy.
Plants have a growth cycle that basically goes like this:
1. The seed germinates.
2. It becomes a seedling.
3. It becomes a full-grown plant.
4. The plant grows.
5. It produces food (photosynthesis).
6. It reproduces itself by growing a fruit that produces a seed.
7. The plant dies.
8. The seed grows into a new plant.
Some plants, notably woody plants like trees and shrubs, go as far as producing the seed and then go into a dormancy period that is usually related to temperature and daylight. At the end of the dormant stage, they wake up and start over again. And of course, trees such as maples or redwoods will go on for years, even centuries, with this cycle.
But herbaceous plants—that is, plants with a soft, nonwoody stem, like tomatoes or oregano—have a different life cycle. They are usually classified as either annual or perennial plants.
Annuals
Just about all the common garden fruits and vegetables we grow from seeds or seedlings (tomatoes, peppers, squash, watermelons, and so on) are annual plants. This means their entire life cycle is spent in one growing season. They sprout, grow into a plant, produce fruit, make seeds, and then die.
Perennials
Perennial herbaceous plants, on the other hand, have a longer life cycle. They sprout, grow into a plant, produce fruit, make seeds, and then die back to the ground. Their roots or stems, especially underground stems, remain in a dormant state. At the start of the next growing season, new growth forms and they start over again.
Perennial edible plants include asparagus, artichokes, rhubarb, and many herbs.
The Least You Need to Know
• Most plants have the same basic parts: leaves, stems, roots, buds, and flowers.
• Plants use photosynthesis to harness sunlight in the process of manufacturing food.
• Plants pull water into themselves using osmosis.
• Bees, other insects, birds, the wind, and humans help plants reproduce by distributing pollen to the stigma where it might eventually reach the ovule of a plant.
• Most vegetable plants are either perennial and live through several life cycles or annual and have one life cycle.