Botany for Gardeners, Fourth Edition

What Is Botany?

Of all the natural sciences, botany is dedicated to the study of plants, which presupposes that we know what a plant is. To a gardener, that may seem like a simple question to answer. With only a few exceptions, you would say, plants have leaves, stems, roots, flowers, and fruit or cones in which seeds are produced. But “plants” also includes ferns and mosses, both of which reproduce by dustlike spores formed in neither fruit nor cones; and as we think further, seaweeds and pond scums—both classed as algae—are also plants, even though they are vastly different from the specimens we have in our gardens.

Mushrooms and molds—types of fungi—are plantlike, but sufficiently different from the types mentioned above that they are no longer included in the plant kingdom. But there are some actual plants that lack roots, or stems, or even leaves, which seem to pose a problem if we try to write an all-inclusive definition of “plant.” So botanists are willing to accept the idea that plants are a large, very diverse group of living organisms, different from animals, which share at least a few common characteristics. Those characteristics will be made clear in the following pages.

Because plants evolved first among Earth’s living things, in one form or another they can exist without the presence of animals, but animals could never exist without plants. Plants purify the air by exchanging the oxygen we breathe with carbon dioxide, which is poisonous in too high a concentration. And plants convert the energy of sunlight into foods that sustain themselves and all animals that eat them. After humans evolved, we made use of plants in other ways, beyond just as food sources. We extract fibers from them to make cloth; drugs to cure our maladies; wood to construct houses, furniture, and ships; and plant materials are used to make paper on which we have recorded our history.

As the science of botany developed, our knowledge about plants expanded into understanding their biochemistry, anatomy, physiology, intricate reproductive methods, and systems of inheritance—all areas about which most gardeners know little. But working with plants inevitably brings curiosity about basic things like, what takes place inside a seed after we have set it in the ground? Just exactly how does water move from soil to treetops? What makes a plant become bushy with repeated pruning? What controls seasonal flowering patterns? How do plants grow, and why is light necessary to make growth happen? These and other questions will be addressed before you reach the end of this book.

The Language of Botany

As a science, botany has its language of technical terms that are used to describe intricate structures and unique behavioral patterns that plants display—a language that all too often intimidates the layperson. You may be surprised that some botanical terms are also words that gardeners use. But for want of suitable nontechnical equivalents, other words cannot be avoided when writing a botany book. Each technical word, whether common or obscure, is explained in the text and glossary and, to help you better understand their meaning, occasional reference is made to the Greek and Latin roots from which these words have been derived. In addition, numerous illustrations will add clarity to the botanical vocabulary and the sequence of ideas discussed in the course of this book.

Some of the photographic subjects are not the customary things that gardeners look for in plants, but they are plants or parts of them seen in close-up, sometimes through a microscope. A majority of the plant specimens that have been photographed were selected from those available in my own and neighbors’ gardens, local parks, and botanical gardens in southern California. But the broad principles of botany each photograph exemplifies are equally applicable to plants in almost any part of the world.

Botanical Disciplines

The science of botany is divided into various disciplines, each having its specialists, subject limitations, and technical vocabulary. Among them, cytology (Greek: kytos, “container”) is the detailed study of cells. Study of the form and structure of plants is the work of morphologists (Greek: morphe, “form”). By virtue of their practical relationships with plants, gardeners are more familiar with morphology than with cytology. Plant anatomy deals with their intimate cellular structure as best seen with microscopes. Taxonomy deals with the scientific classification of plants into large and progressively smaller subgroups. Genetics is concerned about inheritance of characteristics from generation to generation. And physiology reveals some of the mysteries of how plants function. Botanists can spend their lives studying any one of these fields and the amount of information on each is beyond comprehension. For our purposes, we shall tread lightly into each area.

The Plant Kingdom

There are close to 400,000 recognizably different kinds of plants, or species, in the world today. One-third of all plants do not have the familiar roots, stems, and leaves. About 150,000 plant species never produce flowers, and almost that same number do not grow from seeds, but from tiny spores. The vast majority of plants manufacture their own food supplies by a process called photosynthesis. Most plants spend a lifetime anchored in one place, yet a few simple, one-celled plantlike organisms are capable of swimming to different locations in the waters they inhabit.

The range of plant types can be looked at on a scale, extending from simple forms seen among many primitive algae that live in aquatic habitats, to complex structures of the most advanced groups. Between the two, there are types of plants that illustrate how intermediate forms appeared during the course of evolution. Mosses and liverworts belong to a group called the bryophytes. Mosses frequently grow in cool, shady, damp places in a garden; liverworts are less well known. Although bryophytes are small plants, their bodies are structurally more complex than algae, and are well adapted to living on land. Somewhat more advanced than bryophytes are the club mosses and horsetails, both of which may be grown by gardeners with eclectic tastes. In these, we see even more advanced body parts, including elementary stems and roots.

For our purposes, we shall largely be concerned with the two most highly evolved groups of plants, and those we, as gardeners, most often work with. One is the gymnosperms, plants that produce seeds in the open spaces of cones—between the flaplike parts that make up a pine cone, for example. The Greek words gymnos, “naked,” and sperma, “seed,” describe this form of development. Gymnosperms are the more primitive of the two major plant groups, but are of considerable economic importance, as well as of interest to landscapers for their compact forms and richly colored, needle-shaped or scalelike leaves. Gymnosperms have woody stems, and are classified as softwoods. Pine and fir are used to make paper, lumber, and plywood, and are the source of pitch, turpentine, and rosin. The gymnosperms include all the conifers: cedar, redwood, juniper, cypress, fir, pine, and the largest living things on earth, the giant sequoias (Sequoiadendron giganteum). Members of this group include many ornamental shrubs, such as varieties of Chamaecyparis (false cypress) and Thuja occidentalis (American arborvitae); the beautiful maidenhair tree, Ginkgo biloba, a broad-leaved species; and the least typical of gymnosperms, the cycads.

Most importantly, we shall be looking in detail at the second and most advanced group, the flowering plants, or angiosperms. It is the largest group in the plant kingdom, and consists of about 250,000 species. The name angiosperm refers to the fact that seeds from these plants are formed inside containers that we call fruits (Greek: angeion, “vessel”; sperma, “seed”). The flowering plants most often decorate our homes and landscapes, supply almost all of the vegetable matter in our diets, and are the source of the world’s hardwoods. They are the most sophisticated of plant forms, and are best adapted to survive in a wide range of climates, from the tropics to the Arctic.

Characteristics of Living Things

All living things share certain characteristics that make them distinctly different from nonliving objects such as rocks. Consider these features of plants. A living plant has the ability to make seeds or spores from which other plants of the same species can be grown. In other words, a living plant can reproduce. A dead one has lost that capacity.

If you have the opportunity to look at any part of a plant through a microscope, it becomes obvious that plants are composed of countless numbers of small units called cells, which are invisible to the naked eye. This is another characteristic of living things: they are composed of one or more cells. A bacterium is a single cell, whereas a tree is composed of countless numbers of cells.

It may be argued that cells still exist within a leaf when it is dead and dried. But when the leaf was a part of a living plant, its cells were actively engaged in a complicated chain of chemical reactions, grouped together under the term metabolism. We can be quite sure that, as long as a cell or a whole creature is alive, it is going to display some sort of metabolic activity. When their chemistry irreversibly stops, cells die.

Perhaps the most obvious difference between a rock and a rose is that the rock doesn’t grow. In fact, it progressively becomes smaller as its surface erodes. Plants and animals, on the other hand, begin life as single, fertilized eggs, and become larger as they mature. And as a plant or animal grows, it must respond to changes and challenges in its environment if it is going to survive. The ability to reproduce, being constructed from one or more cells, displaying cellular metabolism, growth during at least a part of a lifetime, and being responsive to the environment: these are the fundamental qualities of living things.

Growth Patterns

In the case of animals, including ourselves, a determinate growth pattern dictates a prefixed, maximum size that the body may reach. This pattern is implicit in and established by genes, cellular instructions inherited from parents, and is more or less related to the number of cells that the body is programmed to produce. Strenuous exercise may enlarge cells, but relatively few new cells are added. Full growth potential is realized if an animal receives adequate nutrition and its muscles are exercised, especially during the formative years.

For the most part, plants have no definite size toward which they grow. That is, they display indeterminate growth, or at least their stems and roots do. When left untouched and growing in an unrestricted volume of soil, a plant’s roots will never reach an established size, nor do its branches in the freedom of an open-air space. Any restrictions on plant growth are related to reduced availability of light, water, minerals, or oxygen. Life span is genetically determined—one year for annuals, two for biennials, and indefinitely for perennials.

Compare the indeterminate growth pattern of roots and stems with the pattern found in leaves, flowers, fruits, and seeds. The latter are characteristically ephemeral and determinate in growth. Their maximum possible sizes are rarely displayed in nature, but can be realized under a skillful gardener’s control of the plant’s environment. With plenty of fertilizer, careful watering schedules, optimum illumination, and thinning—removal of parts that may compete for available nutrients—a plant can be pushed to the limits of leaf, flower, and fruit size.

Coordination of Plant Growth

Animals grow and spend their lives in a variety of places. Mobility enables them to choose habitats that are most favorable for existence under changing conditions at different times of the year. A plant is anchored in one place throughout its life. Half of its body, its root system, is buried in the soil. Despite being surrounded by a legion of potentially destructive grubs and soil microorganisms, such as fungi and bacteria from which the roots can’t escape, and their passage through the soil manipulated by encounters with immovable rocks, roots are wonderfully adapted to this strange, hidden environment.

In contrast, shoot systems, consisting of stems and leaves, occupy a sunlit, airy but frequently tempestuous world. Growth impediments are different from those below ground, and may range from insects and larger animals with voracious appetites out to survive at the plant’s expense, to the drying effect of wind and sun, or even damage from fire.

Roots and shoots are frequently thought of as different entities growing in opposite directions. To a plant, they are parts of the whole body that must be as well coordinated as growth of torso and legs in the human form. Root growth and shoot growth are harmonized events, one complementing the other, with energy reserves and raw materials for body building equally allocated to the two halves. And when daily or seasonal environmental changes affect one part, the other must respond in sympathy.

What Is a Cell?

Robert Hooke, an English physicist, was understandably excited when in 1665, he wrote about having used a crude microscope to look at a slice of cork. He probably thought he’d simply confirm the prevailing idea that plants are composed of some sort of amorphous material, like clay shaped by the Creator’s hands. But contrary to such expectations, Hooke was the first person to find that plants are actually constructed of tiny units, which he named cells. His choice of word more likely reflected his acquaintance with Latin (cella, “a small room”) than with the interior of a jailhouse.

What subsequently became known as the cell theory—that all living things are composed of one or more cells—was as revolutionary to scientific thought as was, in our own time, the discovery of DNA (deoxyribonucleic acid), the chemical substance controlling biological inheritance.

To get an idea of what a typical plant cell is like and what it can do, think of a large factory, capable of manufacturing thousands of different and elaborate products from simple raw materials—water, air, and soil. The factory uses sunlight, rather than electricity or oil, as an energy source. It is designed to exert considerable autonomous control over what goes on within its boundaries and, whenever increased productivity is called for, it simply builds an exact copy of its entire physical structure—within a day or two. Now, mentally squeeze the factory into a box, each side approximately 1/2000 of an inch (0.05 mm). That is a cell.

The living part of a cell, the protoplasm, consists of two parts: a nucleus, which is the center of inheritance and cellular control, positioned in the cytoplasm, a soft, jellylike material (a colloid) in which most of the cell’s metabolism takes place. The cytoplasm is enclosed within a sac called the cytoplasmic membrane. This, like other membranes in a cell, is composed of protein and fatty substances, and has the ability to control the passage of water, foods, and selected minerals across the boundary that it defines.

Suspended in the semi-liquid cytoplasm are numerous small bodies, or organelles, which specialize in the cell’s separate functions. Some organelles are the same in both plant and animal cells, hinting at ancient ancestral ties. Chloroplasts are organelles unique to plants, and they are the place where photosynthesis takes place, where light energy is used to manufacture foods. The green pigment chlorophyll, essential for the process, is located within the chloroplasts, as its name indicates (Greek: chloros, “green”; plastos, “body”; phyll, “leaf”). Obviously, chloroplasts are not found in most roots or other parts of a plant that are not green. The color of a leaf is actually the combined appearance of millions of chloroplasts discernible only with the aid of a microscope.

Other organelles include mitochondria that extract energy from foods by the process of cellular respiration and those that specialize in protein production, the ribosomes. The functions of some organelles, visible only with powerful electron microscopes, are still not fully understood.

The nucleus of a cell is its control center from which instructions for the cell’s operation, maintenance, and reproduction emanate. It is comparable to the main office in the imaginary industrial plant. Inherited chromosomes, bearing genes that are composed of the DNA mentioned earlier, are located in the nucleus. These are the blueprints for making more cells.

A vacuole occupies a large part of the volume of most plant cells. Although the word vacuole means “empty space,” it is a membrane-bound inner sac containing much of a cell’s stored water, and serves as a repository for excess mineral nutrients as well as toxic waste products from the cell’s metabolism.

Each cell is designed to function most of the time as an independent unit. Yet their metabolism and other activities are enhanced when groups of cells act in concert via the exchange of foods and other materials by way of interconnecting strands of cytoplasm, called plasmodesmata (Greek: desmos, “chain”).

Cell Walls

The protoplasm of each plant cell is surrounded by a rigid cell wall that protects the living contents. Between adjacent cell walls the substance pectin forms a thin layer, a middle lamella (a sheet), which binds the cells together. This same substance, when commercially extracted from plants and sold in supermarkets, is used to thicken jams and fruit jellies.

Collectively, cell walls give structural support to a plant, the degree of rigidity of any part being proportional to the relative thickness of its constituent cells’ walls. The lightweight, delicate structure of a leaf, for example, indicates that it is composed of thin-walled cells, whereas in woody stems supporting heavy loads, cells with extra-thick walls are developed.

When a cell is first formed, its wall is thin and largely composed of the substance cellulose. This is called the cell’s primary wall. With increased age, the wall may thicken by addition of more cellulose and by the introduction of lignin, a hardening substance. Hardwoods like oak and ash are made up of cells with heavily lignified walls. All of these extra layers constitute the cell’s secondary wall. Cellulose is laid down in microscopic threads called microfibrils; lignin forms deposits on the cellulose surface. Each new layer of wall material, produced by the living cytoplasm, is set in place inside the previously formed layer.

Obviously, as walls thicken, the space occupied by the living contents decreases, and the ability of water and oxygen to reach the cytoplasm is diminished. It is literally an act of suicide that kills the protoplasm and ends wall thickening. Even so, the remaining hollow cell walls continue their supportive roles throughout the life of the plant. Most people are surprised to learn that, in a living tree, as much as 98 percent of its trunk and branches are composed of dead cells, including those that conduct water.

Wall Structure and Cell Growth

Most cells in a plant, especially those in roots and stems, grow in a specific direction, dictated by the way in which cellulose microfibrils are arranged in the walls. If one thinks of a cell as a slightly elongated box, placed in an upright position, the four sides are formed from microfibrils placed parallel to one another and coiled around the box. Microfibrils in the top and bottom have a very different, crisscross pattern.

When a cell enlarges, its walls temporarily soften. At the same time, cytoplasmic swelling takes place as a result of water uptake. Bonds between sidewall microfibrils are loosened, and the cellulose threads are spread apart by the internal pressure. Because the microfibrils in end walls are interwoven, similar stretching is not possible. That is why cells principally grow in length, paralleling the general direction of vertical growth of stems and roots. (Thickening of these plant parts results from a different growth process that shall be discussed later.) Once a cell reaches a predetermined maximum length, the addition of secondary wall thickening prevents further enlargements.

Growth Processes

Two processes taking place at a cellular level contribute to a plant’s growth. In the first, new cells are formed by the division of cells already in the plant body. Each time a cell divides, two complete cells are produced. Every cell in a plant, with the exception of the original fertilized egg, has had its origin in this process.

The most important part of cell division is providing each new cell with a nucleus containing a complete set of genes. This is accomplished during a process called mitosis (Greek: mitos, “thread”) in which the nuclear DNA becomes organized into sets of threadlike chromosomes (literally, the word chromosome means “colored body,” from the fact that they readily stain with artificial dyes). The chromosomes go through an elaborate sequence of movements, culminating in matched chromosome parts being segregated into the two newly developed cells.

The second growth process, in which plants’ cells undergo a limited period of elongation, was described in the previous section.

Occasionally, the well-regulated growth of plant parts is disrupted by the localized invasion of a foreign organism that causes abnormal swellings or outgrowths on stems or leaves. These are called galls, and are caused by mites, midges, thrips, and other insects, as well as nematodes, bacteria, or fungi. A common gall, the oak apple, is caused by a wasp. Galls are constructed of plant cells stimulated into rapid growth by the invading organism. They are generally not harmful to the whole plant.

Meristems

The two phases of growth—addition of new cells and cell enlargement—occur in well-defined places within a plant rather than as scattered, random events. Cells divide in areas called meristems (Greek: meristos, “divided”); close by lie regions of cell enlargement. At the tip (apex) of each stem and root, an apical meristem contributes cells to the length of these plant organs. Such increases in stem and root length, before thickening, are referred to as the plant’s primary growth process. Primary growth ensures that leaves are quickly elevated into sunlight and roots penetrate deeply into the soil. The rapid growth of a seedling, after it has emerged from the soil, is a familiar display of primary growth that continues as long as roots and stems lengthen.

When stems have gained moderate height, it is important that they begin to thicken toward their bases to give added stability and support for the leaf mass. This is called secondary growth, and results from cell divisions in meristems located inside, throughout the length of the stems. These lateral meristems also extend into the roots of larger plants. Secondary growth in a tree creates the slow but measurable thickening of its trunk and branches, as well as the upper portions of roots that may emerge above the soil surface.