Botany for Gardeners (Science for Gardeners)

by Brian Capon

As we delve into the science of botany, we shall largely be concerned with the two groups of plants with which we, as gardeners, most often work. One, known as the flowering plants, or angiosperms, 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 and places.

Second are 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. On the evolutionary scale, gymnosperms are more primitive than angiosperms but are of considerable economic importance as well as interest to landscapers for their compact forms and richly colored, needle-shaped, or scalelike leaves. Softwoods such as pine and fir are not only used to make paper, lumber, and plywood, but are the source of utilitarian products such as pitch, turpentine...

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.

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 will its branches in the freedom of an open-air space. Limits of plant growth are proportional to the availability of light, water, minerals, and oxygen. Life span is genetically determined—one year for annuals, two for biennials, and indefinitely for perennials.

tempestuous

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.

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.

Other organelles include mitochondria that extract energy from foods by the process of cellular respiration and those that specialize in protein production, the ribosomes.

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 by the exchange of foods and other materials by way of interconnecting stands of cytoplasm, called plasmodesmata (Greek: desmos, “chain”).

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.

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.

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.

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.

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 (meaning literally “colored body,” from the fact that they readily stain with artificial dyes).

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.

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.

Bean seeds have thin coats, easily peeled off after being soaked for a couple of hours. The bulk of these seeds is occupied by two, kidney-shaped, food-storage structures called cotyledons, or seed leaves (Greek: kotyledon, “cup-like hollow” or “concave,” as some cotyledons are). Only when these are carefully pried apart do we find the reason for the seed’s being: an embryo, a miniature plant waiting for the moment of its germination.

apical meristems

Cotyledons are attached to and are a part of the embryo, but what happens to them is entirely different. Rather than growing, they progressively shrink as stored foods are transferred to the seedling.

The seeds of flowering plants contain either one or two cotyledons. Botanists use this characteristic to subdivide the angiosperms into two major groups, the dicots (di, “two”) and the monocots (mono, “one”).

Compared with dicots, monocots are believed to be the more recent products of plant evolution and include grasses, cereal grains (wheat, oats, barley, rice, rye), sugar cane, bamboo, palms, lilies, irises, and orchids. Dicots, the larger group, encompass everything from roses and rhododendrons to ash trees and asters. In addition to cotyledon numbers, other features, to be described later, characterize these two groups of angiosperms.

A corn grain is actually a seed surrounded by a thin fruit wall to which the seed coat is tightly bonded. The seed contains an embryo, one cotyledon (corn is a monocot), and a second food-storage structure called the endosperm (endo, “within”; sperma, “seed”) that also nourishes the seedling during germination. The soft, white pulp in each grain of fresh corn-on-the-cob is endosperm.

The size of the food-storage structures in a seed determine the maximum depth to which it can be planted and successfully germinate. If, for example, a small seed is set too deeply, the seedling will use the reserve foods before it reaches the soil surface. Seed packages give specific instructions on depth of planting, but a useful rule of thumb is to bury a seed no deeper than its length. Too shallow is generally better than too deep.

The white, edible meat of a coconut is seed material, and the rich-tasting juice in a fresh coconut is endosperm that has turned to liquid during seed maturation.

Both plants and animals use exactly the same process (cellular respiration) to extract energy from foods by breaking them down in the presence of oxygen. The gas simply diffuses into plants from their surroundings, including from pores in loose-textured soils.

Throughout its early stages of growth, the seedling is completely dependent upon food supplies from the storage structures of the seed, the cotyledons and endosperm.

Germination officially ends when the shoot emerges from the soil. Subsequent seedling development includes stem growth, complete expansion of the first leaves—the minute pair, first seen inside the bean seed—and, underground, proliferation of the root system by repeated branching.

Such seed germination before dispersal is called vivipary.

It would be wasteful for seedlings to start growth late in the year because none could survive winter. To avoid such an outcome, the seeds must be stratified before they can germinate; that is, they must be moistened and given an extended period of low temperatures. In nature, this happens in the course of the normal seasonal cycle. Seeds are produced in late summer, moistened by autumn rains, chilled throughout the winter, and are ready to germinate in the mild, sunny days of spring. Seeds possessing this requirement can be artificially stratified by placing them between layers of moist paper in a refrigerator for a month or two.

In a forest of deciduous trees (ones that lose their leaves in winter), light-sensitive seeds remain dormant until early spring when the leaf canopy has not yet regrown but temperatures and soil water conditions are favorable for seedling growth. In evergreen tropical rain forests, germination may be delayed for years until the collapse of a large, old tree creates an opening where full sunlight can reach the ground and stimulate the waiting seeds.

Another unusual requirement for germination of some seeds is the need to be scarified by fire. Obviously, such extreme measures apply only to seeds with very thick coats and are most common among species living where periodic lightning-caused fires are a part of the balance of nature. In Mediterranean-type climates, including the American Southwest, a group of plants is classified under the name chaparral. These are low-growing shrubs bearing small, leathery leaves, rich in highly flammable resins. Their leaf litter and dry branches make perfect tinder for fast-moving fires, especially on the steep slopes where chaparral normally grows. Seeds from these plants survive the fires with nothing more than a scorching, but it is sufficient to ready them for water uptake during subsequent rains.

Regrowth of chaparral shrubs takes place from underground root crowns. It is more vigorous growth than that which it replaces and more palatable to animals that fled the fires but soon return to start a new life. In the blackened, nutritious soils, seedling growth is rapid and, with the leaf canopy removed, many species of sun-loving plants, especially annuals, occupy formally unfavorable sites, at least temporarily.

They anchor the plant in the soil; absorb water and minerals; and store excess food for future needs underground, where animals are least likely to find it.

Roots anchor the plant in one of two ways or, sometimes, by a combination of the two. The first is to occupy a large volume of shallow soil around the plant’s base with a fibrous (or diffuse) root system, one consisting of many thin, profusely branched roots. Because these grow relatively close to the soil surface, they effectively control soil erosion; grasses are especially well suited for such a purpose. Fibrous roots capture water as it begins to percolate into the ground, drawing their mineral supplies from the surface soil before the nutrients are leached to lower levels.

A tap root system sends one or two rapidly growing, sparsely branched roots straight down into the soil to draw from deep water tables and mineral supplies. Tap roots are especially good anchors in shifting soils or windy locations. A few species simultaneously grow both root systems, and others adopt one form or the other, depending on soil and water conditions: fibrous roots when the surface soil is moist, tap roots when it becomes dry.

Unexpectedly, some large trees have only shallow roots, but because they are spreading and matted, the roots form a broad base for support of the trunks. This form of root system is common among trees in tropical rain forests, where even the forest giants, as much as 180 feet (60 m) tall, have roots penetrating little more than 3 feet (1 m) into the soil.

In temperate zones, conifers are generally anchored by deep tap roots that develop large, horizontal branches. Although the roots of most trees grow to moderate lengths, they rarely exceed the height of their uppermost stems.

Forest trees, ornamentals, and fruit trees frequently distribute their roots in a wide circle, where water-absorbing root tips occupy a drip zone, an area beyond the leaf canopy to which rain is channeled from the foliage above. This pattern of root growth should be recalled when irrigating and fertilizing garden trees.

But among wild plant species, tap roots with lengths of 30–45 feet (10–15 m) are not uncommon. Those of some desert shrubs grow to a vertical depth of more than 90 feet (30 m). Cacti, on the other hand, have shallow, spreading fibrous roots to intercept the small amounts of rain penetrating hard, baked desert soil surfaces. ROOT

Because a damaged meristem cannot be regenerated, for protection the meristem also produces cells ahead of itself forming a root cap. Root cap cells are readily rubbed off but are quickly replaced from within, much like our skin when it dries and peels from the surface. When root cap cells are ruptured by sharp soil particles, their protoplasm forms a slimy coat lubricating the root tip as it works its way through the soil and around large objects.

Each branch is an exact copy of the root that produced it, with an apical meristem, the same methods of growth, a set of root hairs, and the capacity to form branches of its own.

A shoot system consists of the plant’s principal aerial stems, their branches, and attached leaves. All have their origins in the stem’s apical meristem. A stem’s growing tip, its apical bud, is much more complex than that of a root, both in structure and activity.

To further reduce crowding of leaves on a stem and their competition for light, the meristem places successive leaf primordia in different directions from those it previously made. Three basic leaf arrangements exist: alternate, opposite, and whorled (arranged in a ring around the stem).

The principal function of stems is to support leaves in such a way that maximum amounts of light can be captured for photosynthesis.

Shoot systems are generally capable of producing unlimited numbers of branches. Only a small percentage of their axillary buds grow at any one time; the remainder lie dormant, perhaps for years, to act as points of reserve growth in case apical buds are destroyed by disease, frost, wind, or animals. They are also there, ready to grow, after gardeners prune their plants. New growth may appear even from tree stumps, arising from buds long hidden inside the bark.

Leaf arrangements: (A) alternate, (B) opposite, (C) whorled.

Some plant groups do not form axillary buds, relying instead on one apical bud. It may seem a chancy strategy, because damage to the single stem tip eliminates all possibility for future growth. Most palms follow this growth pattern, although a few species may sprout new shoots from their bases, but not higher up on established trunks or upright stems.