Botany for Gardeners (Science for Gardeners)

by Brian Capon

But if left to overwinter, the plants make chemical preparations for the burst of stem growth, called bolting, leading to flower development by late spring.

Photoperiodism, the response of plants to changing lengths of day and night, was discovered about 1920 by W. W. Garner and H. A. Allard.

Unfortunately, later studies indicated that plants measure night lengths, rather than the daylight hours; but, for convenience, the “photo-” and “day” were left unchanged in the scientific vocabulary.

Angiosperms are roughly divided into three categories, based on their photoperiod requirements: short-day plants, long-day plants, and day-neutral plants. Day-neutral species simply flower after some specific period of vegetative growth and are unaffected by day length;

Short-day plants flower when day lengths are less than the critical photoperiod. Long-day plants flower when day lengths exceed the critical photoperiod.

Most species require several successive days of photoinduction in order to change the activity of apical meristems from leaf to flower production—an astonishing switch, the mechanism of which continues to keep botanists guessing. In most species, once floral initiation has begun, the meristem never returns to making leaves.

Discovering the chemical nature of the flowering hormone (florigen, as it has been named) continues to be one of the most sought-after goals of plant scientists. The elusive florigen moves across grafts between photoinduced and uninduced plants, causing both to flower. It also moves between certain grafted long-day and short-day plants; when either is photoinduced, the other also flowers. But, as with all plant hormones, florigen is effective in extremely low concentrations and is therefore difficult to detect. A prevailing theory among plant physiologists is that florigen is actually a special combination of some of the known hormones, such as gibberellin, auxin, and cytokinin, which makes its isolation even more difficult.

Fashioned from earth, air, and water, their life-sustaining processes kindled by the sun, plants build intricate bodies and manufacture foods to supply their every need. To such a self-sufficient lifestyle, the name autotrophic (“self nourishment”) has been given. Heterotrophic (“different nourishment”) animals, fungi, and microorganisms depend on products made by photosynthesizing plants or resort to eating other heterotrophs to obtain such nutriments in second-hand form.

During osmosis, water molecules attempt to equalize their concentration on both sides of cell membranes when they move into or out of living protoplasm.

In most soils, small quantities of salts are dissolved in large volumes of water. Conversely, the protoplasm of epidermal cells contains lesser amounts of water in which salts, sugars, and other substances are concentrated. When water moves (diffuses) from the soil, where it is most abundant, it seeks to dilute the cells’ solutions.

The system of equalization should also apply to salts and other substances that try to diffuse from the root’s cells to the soil. However, cell membranes are selective in their permeability, permitting free inward movement of water but denying passage outward to most dissolved substances. It is such ...

turgor

Water entering a cell is stored in the large, central vacuole (chapter 1), which expands and presses the cytoplasm against the rigid cell wall. When a cell becomes turgid (fully inflated), the rate of water uptake is slowed but does not come to a complete stop. Water continues to diffuse into the cell and simply displaces a comparable volume, while the cell wall, counteracting internal turgor pressures, squeezes water out. Turgid cells are thus equipped with a safety valve that keeps them from inflating to the bursting point.

In cells excreting large amounts of water, the vacuoles shrink and the cytoplasm is pulled from the cell walls, a condition called plasmolysis (Greek: lysis, “loosening,” of the cytoplasm). Prolonged plasmolysis results in cell death. Yet, the cells of seaweeds and angiosperms adapted to coastal and desert salt flats are able to thrive in saline conditions without suffering plasmolysis. Such an ability is attributable to these species’ capacities to store salts at even higher concentrations than the external medium, thereby sustaining osmotic water uptake.

When the epidermal cells are turgid, they discharge water into spaces between cortex cells, which is the line of least resistance for the escaping liquid. After the water works its way across the cortex, a second osmotic pump, the endodermis, directs it into the hollow, tubular cells of the xylem at the root’s center.

Together, the epidermal and endodermal pumps push water across the root and up the xylem with a slight pressure, called root pressure, the effect of which is seen when liquid oozes from the cut stump of an herbaceous stem. Root pressure is also responsible for the droplets of water appearing early in the morning at leaf tips or on leaf margins. Such exudations are called water of guttation (Latin: gutta, “drop”) and emerge from special pores (hydathodes) evolved by certain species to rid themselves of excess dissolved salts.

Although root pressure may push water to the leaves of low-growing plants, it is insufficient to elevate water to hundreds of feet above the soil as in some trees. To accomplish such an engineering feat, a pulling forc...

And because water molecules move in unbroken chains through the connected xylem of roots, stems, and leaves, the drawing force of transpiration, called transpirational pull, is felt throughout the length of the plant.

To appreciate the combined effects of root pressure and transpirational pull, imagine a vertical tube in which water is both pumped under pressure at its base and pulled by suction from above. The water would move at a considerable speed; but for the flow to continue, water must also be removed from the top of the system. In most plant species, about 98 percent of the water entering the roots is lost in the form of transpired water vapor from the leaves. One can readily perceive the outpouring of transpiration when, on a hot, dry day, the air beneath a shady tree feels cooler and is more comfortable because of its higher moisture content.

The magnitude of transpiration is impressive. A 48-foot (16-m) silver maple tree is estimated to transpire as much as 58 gallons (220 L) per hour. A forest of temperate-zone, broad-leaved trees transpires about 8000 gallons (30,000 L) of water per acre per day. An average-size tomato plant transpires about 30 gallons (115 L) during its growing season; a corn plant, 55 gallons (210 L). ...

The suction of transpirational pull places water under tension in the xylem of an actively transpiring plant. Thus, when stems are cut, air is drawn into the exposed vessels and blocks the flow of water. In the garden, flower stems should always be cut longer than desired and recut to the correct length while holding the stems under water. If the stems are then quickly transferred to a vase, the transpiration stream continues uninterrupted, without the blossoms wilting. Scissor-type pruning shears or a sharp knife should be used to make clean cuts so the ends of the xylem vessels remain open.

Peat is decaying plant material in marshy areas that is dug from the ground and dried for consumption.

In winter, when the leaves of deciduous species have fallen, water movement comes to a standstill. If the remaining water freezes in the cells, its expansion ruptures the delicate cell membranes—a condition from which there is no recovery. Plants prepare for winter with a process called cold hardening, part of which involves the accumulation in the protoplasm of sugars that function as antifreeze.

Plant physiologists divide the required mineral elements into two groups: Macronutrients are those used in greatest quantities by plants; micronutrients are used in lesser amounts and, in some cases, are simply introduced as impurities in fertilizer mixes or are dissolved in tap water.

In addition to those listed in the table, some species require traces of chlorine (Cl), aluminum (Al), sodium (Na), silicon (Si), or cobalt (Co).

MINERAL NUTRIENTS NEEDED BY PLANTS The elements used in greatest quantities by growing plants—carbon, hydrogen, and oxygen—are derived primarily from air and water. In nature, all other nutrients have their origins in Earth’s rock materials. Erosion slowly releases t...

Clues to mineral nutrient functions are obtained, in part, from observable and predictable symptoms resulting from deficiencies of the individual elements. For example, the characteristic deficiency symptom of magnesium and iron is chlorosis (yellowing of the leaves) due to curtailment of chlorophyll synthesis.

Nitrogen is incorporated into the structure of chlorophyll, as well as amino acids, the small molecular units from which large protein molecules are made.

Mineral nutrients return to the soil when leaves and branches periodically fall to the ground and decay, thereby completing one of nature’s most important cycles. Prior to leaf abscission, some of the nutrient elements (including nitrogen, potassium, and magnesium) are released from their bound form in protein, chlorophyll, and other molecules and transferred from leaves to the plant’s growing tips for reuse. Because mineral relocation is in progress when older leaves turn yellow, gardeners can help their plants conserve nutrients by not removing discolored leaves for several days, until the minerals have been transferred.

Such relationships between nitrogen (N), phosphorus (P), and potassium (K) and the development of specific organs underlie the reason for printing three numbers, the N–P–K ratio, on packages of fertilizer. For lawn grasses and most houseplants, recommended fertilizers have a proportionately high nitrogen content to promote leaf growth (20–5–5, being one example: 20 parts nitrogen to 5 each of phosphorus and potassium). Whereas a product having a 0–10–10 ratio is a typical formulation designed for flower and fruit set. A fertilizer for root crops may have an N–P–K ratio of 2–12–10, and an all-purpose mix, 5–10–5.

Soil is a complex mixture of inorganic materials, derived from the erosion of rock, and organic matter or humus, the decomposed remains of plants and animals.

The inorganic fraction is divided into three classes, defined by particle size (1 mm = 0.04 inch): Sand particles have a diameter between 0.02 and 2 mm, silt ranges from 0.002 to 0.02 mm, and clay particles are smaller than 0.002 mm. Mixtures of sand, silt, and clay are called loams; a sandy-loam, for example, contains proportionately more sand. In humus-loams various proportions of organic matter are mixed with the other components.

The proportions of each material determine the water-holding capacity of a particular soil. Water-holding capacity (or field capacity) is defined as the water content of a thoroughly wetted ...

Sandy soils retain little water, whereas the addition of humus increases the water-holding capacity, the moisture being held in tiny spaces (capillary spaces) within and between the organic particles. Capill...

A special problem is posed by soils containing large proportions of clay, the particles of which bear electrical charges that attract water molecules. The bond between water and clay, comparable to the attraction between the opposite poles of two magnets, is a difficult union for roots to break. Consequently, much of the water in a clay soil is unavailable to plants.

Another problem with clay soils is their lack of porosity. Dense packing of the fine particles leaves little space through which gases can be exchanged between the soil and the atmosphere; yet it is essential that carbon dioxide and other gases escape from belowground and that oxygen penetrates to a plant’s roots.

Acidity decreases from pH 1 to 7; alkalinity increases from pH 7 to 14.

Most horticultural species grow favorably in soils at or close to neutrality. However, ferns, azalea, rhododendron, and camellia, for example, require an acidic pH between 4.5 and 5.5, whereas asparagus, spinach, and cacti and other succulent species prefer mildly alkaline soils, to about 7.5 on the pH scale. Hydrangea tolerates a wide pH range, but the flower colors indicate the soil pH—the flowers become blue in acidic soils, pink in alkaline.

Chelates (soluble organic compounds to which iron is bound) make the element available to plants without toxic effects. The chelate is eventually broken down by microorganisms. Two commonly used chelating agents are known by the acronyms EDTA and EDDHA.

The leaves of higher plants contain various types of photosynthetic pigments. Chlorophylls exist in two forms—chlorophyll a and b—both of which are green. Carotene is an orange-yellow pigment that is also abundant in carrot roots, and several xanthophylls range from shades of yellow to almost colorless, depending on their molecular structure. A-B. Chlorophyll molecules break down in autumn, unmasking the yellow carotene and xanthophylls in leaves. Some, such as those of liquidambar (left), turn red when anthocyanin pigments add the final touch to the tree’s colorful spectacle.

A rainbow’s spectacle reveals that sunlight is composed of several colors. Of these, red and blue are captured by chlorophyll, whereas carotene and xanthophylls intercept only the blue-green part of the visible spectrum. At wavelengths represented by these colors, the energy of light is transferred, via the pigments, into the synthesis of foods.

Artificial illumination is only effective if it provides the blue and red wavelengths absorbed by chloroplast pigments.

During photosynthesis, plants absorb light and channel its energy into the formation of chemical bonds uniting atoms into molecular structures, as described in a previous section. The large-scale storage of abundant sunlight in the form of compact, energy-rich food molecules is the unique function of Earth’s flora. Although scientists are able to transform light into electricity, attempts to mimic photosynthesis have been in vain.

Food molecules—carbohydrates (sugars and starch), fats, and proteins—contain many chemical bonds, each representing a small package of stored energy. When foods are used in the biochemistry of cells, the bonds are broken and energy is released—energy to construct other molecules, such as cellulose and lignin needed for growth, to make chromosomes dance through cycle after cycle of mitosis, to transport food in the phloem, to regulate membrane permeability, and to power countless other functions.

The process shared by all living things, of extracting energy from foods, is called cellular respiration and occurs...

In higher animals, after food materials are digested, the energy-rich molecules are carried in the bloodstream to the body’s cells. Among many physiological functions, the energy derived from foods makes muscle...

During photosynthesis, carbon dioxide and water molecules enter a chloroplast. Light splits water into its component hydrogen and oxygen atoms. The oxygen atoms are combined to form oxygen gas (O2) that escapes into the atmosphere. The hydrogen and carbon dioxide are incorporated into molecules of sugar (glucose is shown here).

Photosynthesis takes place in two stages. In the light reaction, chlorophyll b, carotene, and xanthophylls absorb and channel light energy to chlorophyll a, whose electrons (negatively charged, subatomic particles) are boosted to a high energy potential. In such an excited (energized) state, chlorophyll’s electrons are diverted into a system that extracts and stores their energy for later use in the synthesis of substances such as sugars. The pigment’s lost electrons are quickly replenished with a fresh supply obtained from the breakdown of water, making the chlorophyll a ready to be excited again and repeat the process.

Splitting water molecules (H2O) to donate electrons breaks the molecules into their component hydrogen (H) and oxygen (O) atoms. The oxygen, in gaseous form (O2), escapes into the atmosphere through open stomata as another important end product of the reaction. Astonishingly, the sequence of events described above is completed in a fraction of a second.

An outline of photo...