Herbs Up Close

If you were suddenly transformed into a tiny bug with big eyes crawling across the surface of a leaf, you might be astonished at the complexity of the landscape that unfolds. What humans see as a smooth, flat leaf leaps into another dimension when seen from a bug’s-eye view: leaf veins look like giant storm drains, pores become craters, and barely visible hairs turn into towering structures. At The Herb Companion, we found a way to accomplish this surreal shift of perception–with the technology of scanning electron microscopy.

We recently took an array of herbs to Colorado State University’s department of an­atomy and zo­ol­ogy, where one of its electron microscopes is housed. A researcher clipped leaf pieces from the plants, treated them with a fixative, and dehydrated them in alcohol. He then dried the specimens in a high-pressure chamber, dusted them lightly with powdered gold, and fed them to the microscope. This is not a conventional microscope–it looks more like a large computer station, with the operator twiddling knobs to change the scenery on what looks like a television screen.

Sitting at the microscope, zooming in and out of different magnifications, roaming at random over each specimen, we discovered how easy it is to get utterly lost in the stark and beautiful lunarlike vistas. The peaks, ripples, spikes, crevasses, and other mystifying features of the terrain billow and jut and undulate across the screen. Familiar rosemary becomes an alien being when seen crisply in focus 800 times as large as life. The delicate detail of a lavender bud suggests underwater creatures or a fairy-tale primeval ­forest.

When we found a view we liked, the operator captured it with the microscope’s camera. Back in the office, we had fun playing “Name That Herb” with the photographs. The unexpected visual effects dazzled us, but we turned to botanist Art Tucker of Delaware State University to help understand what we were seeing.

We were most interested in the ­essential oils, the source of the plants’ flavor and fragrance. Tucker directed our attention to a photo of the underside of a peppermint leaf and told us that 80 percent of the herb’s oil glands are located on the lower leaf surface, with another 19 percent on the upper surface (shown on page 55) and 1 percent on the stems. Two types of oil glands can be seen: glandular hairs, made up of three cells, and the conspicuously larger, ten-celled glandular trichomes. The glands each consist of a foot, a stalk, and a head, although some are sunken, with only the head visible. The heads of the glands sometimes look like globs of oil plopped on the rough-looking surface of epidermal cells.

A film of wax, or cuticle, forms a cap over the oil in the gland. The oil is released when the wax layer is broken by rubbing, for example, or when it is pelted by rain or overhead irrigation. On a warm day at high noon, an herb garden is fragrant with volatile oils that have diffused through the porous wax layer.

Most of the herbs we examined–including catnip, rosemary, lavender, sage, thyme, basil, and lemon balm–belong to the mint family, Lamiaceae (Labiatae). Their glands are similar in structure, as are those of lemon verbena, which belongs to the Verbenaceae, but the number of cells in each kind of gland varies from one genus to the next. Basil, for example, has trichomes with four-celled heads, compared to the eight-celled heads of peppermint glands. Some herbs, such as rue, have no visible oil glands on the surface because they ­secrete their essential oils internally into spaces between the cells.

Some of the photographs also show nonglandular hairs, which may outnumber and mask the glandular ones. According to Tucker, these hairs are the plant’s way of coping with the stress of low moisture. The hairs create a shell of humidity around the stomata, the tiny pores through which the plant takes in carbon dioxide and gives off oxygen during photosynthesis and through which water vapor passes in and out. In general, the grayer (hairier) the plant, the more it can withstand drought but the less it can tolerate overwatering and high humidity.

Wander with us through the herb photographs on the following pages and ponder the possibilities. Imagine that you’re a bug. . . .

Our thanks to John Chandler, of Colorado State University, who operated the microscope.