Botany for Gardeners (Science for Gardeners)

by Brian Capon

Woody perennials generally bearing large numbers of branches are classified as either trees or shrubs. What is the difference? By definition, trees have one or a small number of main trunks to support their leaf crowns; shrubs are smaller plants with many woody stems branching close to the ground.

Axillary buds, the stem’s potential branches.

The outer bark, called cork, continually flakes or peels off most woody plants but is replaced from within.

Scattered bumps that may look like scale insects on the smooth, young bark of a winter twig are actually breathing pores, called lenticels, through which gases, including oxygen, pass to and from the living cells of the inner bark.

The winter twig’s growing tip is encased by overlapping bud scales to form a tight apical (or terminal) bud. The apical bud begins to form in autumn, at the time of leaf fall, as part of the tree’s preparations for entry into its annual period of dormancy.

A. Lenticels, breathing pores in the young bark of a woody stem. B. Leaf scars remain on a stem after the leaves have fallen. The circles within each scar are the severed ends of food- and water-conducting bundles. Scattered lenticels dot the stem between leaf scars. C. A dormant bud is encased in glossy bud scales for the winter’s duration. D. The new growth of spring pushes aside the bud scales.

Rings of terminal bud scale scars indicate where the stem suspended growth for a winter season.

Leaves are elegantly crafted to harvest light, the energy source for food manufacture in photosynthesis (see chapter 8). This is typically fulfilled by the expansion of sheetlike blades, which must be thin and translucent to allow light to penetrate to their innermost cells. They must also be held in outstretched positions without the assistance of wood in their construction, because wood is opaque and heavy. The blade is frequently attached to the stem by a leaf stalk, or petiole, one of the advantages of which is to rotate the leaf blade to track the sun’s changing position over the course of the day.

The descriptive term, petiolate leaf, meaning one with a petiole, is contrasted with a sessile leaf in which the blade is directly attached to the stem (Latin: petiolus, “stalk”; sessil, “sitting on”).

Leaf parts: (A) simple, petiolate leaf; (B) sessile leaf; (C) compound leaf

Leaf blades develop as single units in simple leaves or are divided into smaller units, or leaflets, in compound leaves.

The leaflets of a pinnately compound leaf (from Latin for “featherlike”) are arranged along a central axis; those of a palmately compound leaf arise from one point at the tip of the petiole, like fingers of an outstretched hand.

Similar descriptions are given to vein patterns within leaf blades: pinnate venation and palmate venation, in addition to a parallel arrangement that is most common in the leaves of monocots (such as grasses, palms, and irises).

Margin patterns: (A) entire, (B) sinuate, (C) crenate, (D) serrate, (E) dentate, (F) lobed, (G) double serrate. Leaf shapes: (H) linear, (I) oblong, (J) ovate, (K) hastate, (L) sagittate, (M) deltoid, (N) spatulate, (O) peltate.

Everyone who has grown a lawn knows how the grass keeps growing after it has been mowed. Few gardeners notice that the pointed tip of each blade, once lost, is not regenerated. Instead, the blades continue growth from near their bases, from what are known as intercalary meristems, areas of cell division inserted between the blade and the stem. The evolution of intercalary meristems enabled various types of grasses to survive in prairie habitats in association with herds of grazing animals, such as deer, antelope, bison and, later, domesticated cows. As long as the animal’s teeth simply snip the tops off the grass blades, as a lawn mower does, the leaves continue to grow indefinitely.

Out of the vast array of plants that populate our planet, only about 2000 species are estimated to have been used by humans as food. Of these, forty species are listed among today’s major food sources; and of these, a mere fifteen species are considered the plants upon which the human race completely depends.

Distinctive cell types are not randomly arranged in leaves, roots, or stems but in groups called tissues.

For example, transportation of large volumes of water through a plant is performed by groups of specialized cells, organized into a xylem tissue (from the Greek for “wood”; pronounced “zí-lem”). Water conduction occurs only in an upward direction. Another tissue, the phloem (Greek: phloe, “tree bark”; pronounced “fló-em”) conducts food molecules in opposite directions between leaves and roots.

Proteins, in turn, are even larger molecules made from hundreds of atoms of carbon, hydrogen, oxygen, nitrogen, and sulfur.

The next most complex level of organization occurs in living organisms in which countless numbers and types of molecules congregate to form the visible structures of cells, including the organelles described in chapter 1. Cells, in turn, are united into tissues; tissues make up the next larger structures of organs (roots, stems, leaves, flowers),

From air, water, and the dust of the earth, atoms unite, albeit temporarily, into living, functioning plants and animals. And when, inevitably, the spark of life is lost, it is back to those primal forms that the elements return.

The soft, flexible tissues of an herbaceous stem are products of primary growth processes and are organized into six clearly defined areas.

The outer boundary of the stem is a single layer of cells, called the epidermis (Greek: epi, “upon”; derma, “skin”). The cuticle is a layer of waxy cutin superimposed on and impregnating the outer walls of epidermal cells. It reduces evaporative water loss and protects the stem against invasion by molds. Stems may remain smooth, or glaucous (Greek: glauco, “bluish gray,” from the presence of wax), or become hairy, or pubescent (Latin for “downy”), when numerous epidermal hairs grow from their surfaces.

Inside the epidermis, several layers of cells constitute a cortex (Latin for “shell”). The green color of herbaceous stems results from chloroplasts located in cortex cells.

At the stem’s center, a large pith seems to be connected to the cortex due to the similarity of their constituent cells.

The most obvious features of a young stem are bundles of vascular tissues, held in place by the surrounding pith and cortex. The word vascular is derived from the Latin vasculum, “little vessel,” and refers to the fact that water, minerals, and food molecules are transported through ductlike cells in plants. The inner half of each vascular bundle consists of large, water-conducting cells of xylem tissue; toward the outside are food-conducting phloe...

Plant stems that are herbaceous throughout their lives either lack a vascular cambium or have a lateral meristem that remains inactive.

All tissues in an herbaceous stem are established by the apical meristem during primary growth. Thus, they are called primary tissues. The food-and water-conducting tissues in the vascular bundles are designated primary phloem and primary xylem to distinguish them from tissues formed later by the vascular cambium, namely the secondary phloem and secondary

In the three-dimensional structure of a stem, the epidermis, cortex, and vascular cambium form concentric cylinders around a central core of pith.

When cells of vascular cambium divide, they do so in three directions, resulting in different fates for their products. New cells laid down on the inner side of the cambium layer develop thick, lignified walls and their protoplasm dies. They are thereby destined to become water-conducting cells of the secondary xylem, better known as wood cells.

When the vascular cambium divides in an outward direction, secondary phloem is formed. Many secondary phloem cells have thin walls, remain alive, and conduct food; others develop thick walls and give physical support to the flimsier food-conducting tissue.

The third direction in which cambium cells divide is sideways, to add more cells to the meristem as it increases its circumfere...

The vascular cambium forms the dividing line between wood and bark. The inner portion of bark is secondary ...

Vascular cambium cells (shaded) divide in three directions. The long rows of vascular ray cells also originate in the vascular cambium.

In addition, cork cells are naturally water-proofed with a substance called suberin (Latin: suber, “cork”) that prevents evaporative losses from the bottles.

Grafting involves the permanent union of a branch (called a scion, from an early English word meaning “offshoot”) taken from one plant, with another plant (a stock, Old English for “stump”) that bears roots.

Alignment of the vascular tissues of stock and scion permits free exchange of nutrients, food, and water during the period when the tissues of the two parts fuse together. Grafts can only be made between closely related species of plants, with incompatible organs being rejected.

The outcome of secondary growth is dramatically revealed in the stump of a felled tree. The obvious line separating bark and wood is the location of the vascular cambium. The thick, woody core is secondary xylem. The most recently formed wood, closest to the cambium, conducts water up the tree trunk and is called sapwood.

Frequently, sapwood is a lighter color than the inner area, the heartwood, the cells of which are plugged with chemical substances and cellular debris. One occasionally sees a large tree still growing after its heartwood has been burned out by a forest fire. This is possible because only sapwood is needed to sustain the tree’s life.

Along with materials essential to the plant’s well-being, metabolic activities in living cells unavoidably create some waste products. When deposited in the heartwood, such substances discolor the tissue. The rich color of naturally stained heartwood from some tree species makes it particularly desirable for the manufacture of fine furniture. The same waste substanc...

Waste substances are passed into the heartwood from the inner bark by way of narrow rows of cells called vascular rays, which are easily recognized in the accompanying photographs. When the vascular cambium adds cells to the secondary vascular tissues, it also adds cells to the progressively lengthening rays.

The world’s hardest and heaviest woods are from lignum vitae (Guaiacum officinale), a native of the Caribbean region, and South African black ironwood (Olea laurifolia), a member of the olive family.

Each annual ring consists of several layers of large xylem cells, referred to as springwood (earlywood), followed by progressively smaller cells, the summerwood (latewood).

A variety of studies indicate that many jungle specimens, all angiosperms, are hundreds of years old but none rival the astonishing longevity of the great gymnosperms.

As the fastest growing plants, bamboos have been measured increasing in height 47.6 inches (121 cm) in 24 hours. Many bamboos undergo mass flowering after 30 or more years, with all the plants of a species flowering simultaneously regardless of geographic location or climate, and then the plants die.

Primary tissues of a root

In phloem tissue, long, narrow food-conducting cells are arranged end to end in ranks. Their end walls (called sieve plates) are pierced with holes, a characteristic giving columns of these cells the name sieve tubes. Threads of living cytoplasm pass from cell to cell through the sieve plates. An interesting feature of sieve tube members is that they lack nuclei, leaving their cytoplasm free to transport food. Sustaining the living cytoplasm of these cells is the work of nuclei located in adjacent companion cells.

Many arctic and alpine perennials are evergreen, although minimal photosynthesis occurs during winter. Because their growing season is short, the plants can ill afford the time and expenditure of food reserves needed to make a completely new set of leaves each spring.

Correctly, a rose stem’s protective structures are not thorns but prickles, arranged in irregular patterns within internodes. Prickles are short, woody outgrowths arising from the epidermal tissue of stems, leaves, and some species of fruit.

Litmus, a dye that indicates pH (acidity or alkalinity), is extracted from a lichen.