Botany for Gardeners (Science for Gardeners)

by Brian Capon

Suberin, a substance in the walls of cork cells, inhibits water loss from woody stems, whereas tannin, another chemical present in cork, acts as a natural fungicide and insecticide.

The name rubber dates from 1770, when a piece of the material was found good for rubbing out pencil marks on paper.

Even before an infected leaf separates from its stem, a tannin-containing cork layer forms across the soon-to-be-exposed leaf scar to secure it against the spread of microorganisms.

Gums are used as sizing agents, food thickeners, and stabilizers in emulsions, such as chocolate milk, in which the gum holds chocolate particles in suspension.

Chewing gum is manufactured from another type of latex, called chicle, tapped from the bark of the chicle tree (Manilkara zapota), a native of Central America.

Tannins are produced by many species of higher plants—especially angiosperms, gymnosperms, and ferns—and are generally absent from lower forms.

For example, heartwood’s greater resistance to invasive insects and microorganisms, compared with sapwood, is directly attributable to a five- to ten-fold difference in tannin content between the two tissues.

During fruit ripening, tannin molecules disintegrate and are replaced by increasing amounts of sugar.

The dryness is due to salivary proteins being bound together by the tannins and, thereby, reducing their lubricating action. Interestingly, it is that same astringency that lends appeal to such beverages as tea, wine, and cocoa.

When animal skins are soaked in a concentrated tannin solution, the chemical permeates and attaches itself to the protein fibers and gives the leather the same degree of resistance to microorganisms as that of cork.

From the first use of plants in folk medicine to the development of our present pharmaceutical industry, alkaloid-containing plant species have played a prominent role in human medicine. Extracts of alkaloids are employed as pain relievers, cardiac and respiratory stimulants, muscle relaxants, blood vessel constrictors, cures for malaria, and pupil dilators used during eye examinations. Some alkaloids have mild or strongly addictive side effects: caffeine from coffee and tea, nicotine from tobacco, cocaine from the leaves of the tropical coca plant, and morphine from the opium poppy. Heroin, a synthetic derivative of morphine, is an even more powerful narcotic than the parent chemical. Many of the so-called psychedelic drugs are alkaloids, including mescaline from the peyote cactus and psilocybin from a species of mushroom.

The hallucinogen LSD is a chemically modified form of natural lysergic acid, an alkaloid from ergots, which are abnormal fungal growths on grasses.

It is not explainable why rhubarb petioles may be safely eaten, when their attached leaf blades contain sufficient oxalic acid to cause muscle and kidney damage, coma, even death. The roots and shoots of a tomato plant, but not its fruit and seeds, contain the violently toxic alkaloid solanine, possibly to fend off herbivores but not harm animals that help disperse the seeds.

All parts of poison hemlock (Conium maculatum) are charged with the alkaloid coniine. The most famous victim of this poison was the Greek philosopher Socrates who, having offended the Athenian government and according to the custom of the times, was forced to drink a hemlock brew.

Ricin, one of nature’s most lethal substances, is present throughout castor bean (Ricinus communis), especially in its attractive seeds. Only one to three seeds, if eaten, can be fatal to a child; two to eight can kill an ad...

And it has been suggested that the purple-black color of some toxic fruits, such as those of nightshade (Solanum spp.), give a clear “do not eat” signal to birds and other vertebrates.

Horizontal stems growing aboveground are called runners, or stolons (Latin for “shoot”); those growing underground are rhizomes (Greek: rhizoma, “root”).

Stolons generally emerge from near the crown of a plant, bend under their own weight, touch the soil, and develop plantlets at their tips. These, in turn, send out more runners in a stepwise fashion to claim an ever-widening circle of ground. Thus, a strawberry patch may grow from a single, spreading plant by this natural method of vegetative propagation. It is this same stoloniferous habit that makes some ornamental species well suited for use as ground covers.

(A) A stolon, or runner; (B) a rhizome (an underground stem); and (C) a sucker (an upright shoot, arising from a horizontal root).

Other species, growing in wet forest habitats from the tropics to temperate zones, are epiphytes (Greek: epi, “upon”; phyton, “plant”), plants that spend their lives clinging to tree branches. In such elevated locations, their leaves receive optimum illumination. Epiphytes’ roots are used more as grasping rather than absorbing organs; few ever reach the

B. All but the green tip of an orchid’s aerial root is covered with a spongy, white velamen through which water vapor is absorbed from the atmosphere.

The succulent leaves and stems of such genera as Mesembryanthemum, Sedum, Crassula, and Echeveria contain enlarged water-storage cells capable of supplying the plants’ basic needs for many months.

An onion is a typical bulb.

Axillary buds, developing between the bulb’s scale leaves, enlarge to become new bulbs. Garlic cloves are formed in such a manner.

Adventitious

In some species, when root tips become firmly anchored to the soil, the upper root region contracts by the shortening and thickening of cortex cells. Such contractile roots serve to pull the bulb to an appropriate depth in the soil for protection.

Rhizomes and stem and root tubers are used extensively by horticulturists as propagative organs because they can be cut into pieces and planted. However, the gardener must be sure that at least one axillary bud (or adventitious bud in the case of root tubers) is present on each piece. As with all methods of vegetative propagation, the offspring are clones of the parent plant and, therefore, possess identical characteristics. SAPROPHYTES AND

Many parasites penetrate the host plant’s tissues with a special structure called a haustorium. In fungal parasites, the haustorium is an extension of the mycelium.

Because viruses are not composed of cells, they are not considered living organisms in the same way as bacteria.

The beautiful, bright red snowplant (Sarcodes sanguinea), a springtime inhabitant of pine forests in western North America, is an angiosperm lacking the ability to photosynthesize. It survives through the intercession of a mycorrhizal fungus that transfers food from the roots of nearby trees.

From an ecological point of view, nitrogen-fixation is a crucial link in the nitrogen cycle, a worldwide process in which the nutrient is cycled and recycled between the atmosphere, oceans, soil, and living organisms. As long as the cycle continues, nitrogen will always be available for plants and animals. The same does not hold true for the world’s supplies of noncyclable nutrients, such as phosphorus, potassium, magnesium, and iron. As these elements slowly wash out of the land and into the oceans, they become unrecoverable by natural means and, therefore, unavailable to terrestrial plants.

In an age when people have walked on the moon and lives are saved with transplanted organs, it is humbling to admit that routine functions in seemingly simple plants still baffle us. For example, there is no completely satisfactory explanation for how food molecules move through phloem cells;

Regardless of the outcome, the gathered information is of use to other scientists only if it is reported factually and without bias on the part of the investigator. Nowhere, in all human knowledge, must truth be accounted for more rigorously than in the world of science.

The first plant hormone to be discovered was the substance causing stems to grow toward light—the physiological process of phototropism.

In stems illuminated from above, cells undergo equal rates of elongation, resulting in vertical growth. But when lit from one side, stems change direction because cells on the shaded side grow faster than those toward the light.

Phototropism is a common response in sun-...

Most shade-loving species display little or no phototropic responses, an important factor in their selection as houseplants.

Growth of stem internodes is promoted by another hormone, named gibberellin after the fungus Gibberella in which it was first discovered. The action of gibberellin on internode cells is also related to light intensity. In full sun the hormone’s effect on growth is somewhat restrained. Thus, while gibberellin promotes sufficient internode elongation to space the leaves, the structural stability of a squat growth form is maintained. In low light intensities, however, gibberellin becomes more active, causing internodes to stretch. By so doing the upper leaves are elevated to a position where they are better able to locate light, especially in competitive situations with surrounding plants.

When roots and shoots grow in opposite directions, they reflect contrary responses to Earth’s gravitational field. Geotropism (Greek: ge, “earth”), or gravitropism, are the names given to this physiological process. Most roots are positively geotropic; that is, they grow in the direction of gravity. Stems, for the most part, are negatively geotropic, growing opposite to the gravitational force. When rhizomes, stolons, and some roots grow horizontally, they display diageotropism (Greek: dia, “across”). And branches from roots and stems, developing at an angle from the vertical plant axis, are plagiotropic (Greek: plagios, “oblique”).

Under the influence of gravity, auxin collects in the lower side of the stem, where it stimulates the cells to grow more rapidly than those across the top. The mechanism of hormone migration to the underside of the stem is a mystery, however, because the weight of auxin molecules is less than the smallest objects that can usually move under the influence of gravity.

Geotropic (gravitropic) responses in a plant axis and its branches

The compound leaves of sensitive plant (Mimosa pudica) respond rapidly to vibrations or touch by folding their small leaflets together and collapsing their leaf stalks (petioles) against the stem. When shaken by a wind, folded leaves have reduced evaporative water loss. This phenomenon, unusual in the plant kingdom, is known as seismonastic motion.

Studies with the insectivorous Venus’ flytrap have shown that a similar system controls the movements of its leaf traps, the closure being an exceptionally rapid growth response. Each time a trap closes and opens the leaf increases in size because changes in cell dimensions are not reversible.

Six green persimmon fruits were picked the same day. At the end of one week, the three that were placed in a bag with apple peelings—a source of ethylene—had ripened; the others, stored on an open shelf, had only begun to change color.

And, primarily as a result of ethylene’s influence, drastic changes occur in cell structure, including the breakdown of membranes and the softening of cell walls. The outcome of such processes is seen in the rapid deterioration of a fruit as it becomes overripe, ready to release its seeds.

Ethylene is used commercially to ripen fruits, such as bananas, that are picked green for shipment. They are treated with the gas prior to being sold in retail stores.

As long as apical buds are present they suppress the growth of axillary buds, especially toward the top of the stems.

This physiological process is called apical dominance.

An intact oleander stem (left) shows the normal pattern of apical dominance and suppressed axillary bud growth. With the stem tip removed, axillary buds freely develop into branches.

Gibberellin controls seed germination, and ethylene promotes stem thickening, especially in seedlings. In some species at least, abscisic acid brings on dormancy, whereas gibberellin revives them from winter sleep. In some species, artificially applied gibberellin promotes flower formation and increases fruit size. Ethylene initiates flowers in pineapple and its relatives (the bromeliads), which, according to some reports, need only be enclosed in a bag with apple peelings for several days to produce flowers. Unfortunately, neither ethylene nor gibberellin is universally effective as a flowering hormone.