One of the unusual shrubs cultivated at MEMBG is located at the corner of South Tiverton and Young Drive South, right where the vanpool vehicles collect riders at the end of each workday. The plant is Plectranthus comosus Sims, a member of the mint family (Lamiaceae) from tropical east Africa and Sri Lanka. Plectranthus, you may already know, is the genus that contains such horticultural favorites as creeping Charlie (not Pilea) and Swedish ivy, which are widely used as entryway hanging perennials, with glossy and fleshy evergreen leaves. Homeowners prize these hanging plant mostly for their foliage, although the flowers are pleasant enough. But P. comosus has truly stunning flowers.
During winter our shrub is nearly three meters tall, but after flowering ends it has been pruned back almost in half to make its size and form more manageable. And it is amazing how quickly that plant has recovered from each drastic hair cut. The site receives almost no direct sunlight, shielded as it is during most morning hours by the Botany Building and the giant floss-silk tree (Chorisia speciosa) and from midday on by other trees and the Factor Building. Not surprisingly, many species of the genus thrive in shade or dabbled sunlight.
Like all members of the mint family, this species has opposite decussate leaf arrangement, meaning that leaves grow in pairs, and each pair is positioned ninety degrees from the pair above and below it on a shoot. Hence, when you look down on a vigorous new shoot, four vertical rows of leaves can be seen. This is called four ranked. The dull green, ovate leaves of P. comosus are 15 to 20 centimeters in length, including the petiole. The leaf margin has coarse, rounded teeth, and the raised veins on the leaf underside form a textbook-perfect display of netlike venation. The leaf is somewhat like velvet to the touch, because there are short, soft hairs on the surface. When the leaf is crushed, a volatile vapor (monoterpenes) with a faint mint fragrance is released from spheroidal glandular hairs also attached to the leaf surface. The renowned aromas of mint family herbs--basal, sage, oregano, rosemary, thyme, peppermint, spearmint, hoarhound--originate from glandular leaf hairs of species, each species with different mixtures of monoterpenes.
When you take a closer look at the shoot, notice that the reddish stem is virtually square, not cylindrical. The square stem is characteristic of nearly all species of mints and is often used as one distinguishing family feature. There are a few notable exceptions within Lamiaceae, including white sage (Salvia apiana), which grows in our local mountains and has cylindrical stems. Several other plant families have species with square stems, such as vervains (Verbenaceae) and melastomes (Melastomataceae); not unexpectantly, these also have opposite decussate leaf arrangement. The stem corners are produced by the extra buildup of a special cell type, called collenchyma.
For eight months per year, beginning in October, this plectranthus produces stalks of bluish-purple flowers. Once again--stunning. Flowering stems may grow to 35 centimeters in length and bear rings of flowers every 2.5 centimeters. At one time there may be 15 to 50 open flowers per inflorescence, and at peak flowering 40 to 50 flowering stems on the plant. (Get out your calculator now!). The ring of flowers may appear to be a whorl, but it is technically referred to as a verticil, which is a branched inflorescence itself. The verticil is a derived character of the members of the mint family, another diagnostic feature. In fact, in P. comosus one can clearly see that each verticil is actually just two branched clusters of flowers, each having developed from one bud along the axis, just as a part of foliage leaves would each have an axillary bud.
Concentrate now on the lovely flower of this mint. Mints have highly modified flowers with bilateral symmetry, and the petals are fused in such a way as to produce a well-developed upper and lower lip, i.e., a bilabiate flower (L. labrum = lip). Labiatae is the older, former name of the mint family. Stamens of the mint family have been reduced to four, two being somewhat longer (didynamous), fused to the corolla. The remnant of a fifth stamen (a staminodium) may be present. The pistil of mints has an ovary with four distinct lobes, and the style, which is attached at the ovary base between the lobes, has a split stigma that resembles a minute snake's tongue.
Plectranthus comosus fits nicely into the basic mint flower model. Its flower is about three centimeters long, and the showy portion looks like a gardener's trowel. The horizontal lower lip is the prominent one, and to this are fused four stamens whose filament bend upward so that the anthers are placed about two millimeters above the lower lip. The bifid stigma, borne at the tip of an upward-curved style, projects twice as far, presumably so it comes into contact with pollen on an arriving animal pollinator. A series of stiff, colorless hairs adorns the underside of that lip. In P. comosus, the upper lip is relatively small, whitish, and bend sharply upward.
Some of you may have seen this plant before and therefore observed that on its label we were calling itColeus grandis. Shocking as it may seem, this is a legitimate name for the same plant, but we had to change the sign to use the oldest correct name, Plectranthus comosus. This aspect of plant taxonomy simply drives amateurs mad, because once you learn a plant's name, it seems silly to replace it with another. The technical reason is that most species of the genus Coleus Loudon have now been "submerged" into the genus Plectranthus L'Héritier, the older genus name. Experts were not convinced that the key feature, whether the stamens fuse or do not fuse around the style, was a reliable character for separating the two groups. When two genera are combined, taxonomists follow international rules for determining the proper genus-species combination. Seems simple, but it is not, and there is a small army of botanical lawyers trying to investigate the proper names of plants while another army of scientists is trying to determine whether species should be assigned to one genus or another. Combined, the genus Plectranthus now includes about 350 species, many of which come from southern Africa looping around to southern Asia and Australia, home of creeping Charlie, P. australis.
Don't get too comfortable yet. The variegated plant that you likely know as coleus, namely Coleus blumei or C. hybridus, does not have that name any more, nor has it been assigned to Plectranthus. To find that plant species in the most up-to-date books, look up Solenostemon scutellarioides. Now there is a tongue twister! Be that as it may, the coleus plant is one that is easily propagated by making cuttings, dipping the cut stem in rooting compound, and placing the stem end in damp soil. Adventitious roots will develop and grow from near the stem base. Similarly, its close cousins in Plectranthus, including our plant novelty here, will clone easily from shoot cuttings. We grew our plant from a cutting in the home garden of Mildred Mathias, and Mildred apparently got hers from a cutting on a plant grown by MEMBG patron Professor Harlan Lewis in Pacific Palisades, and he got his plant from a rooted cutting in Escondido, California. Keep this plant chain letter alive and ask us for a cutting next time you stop by.
ARTHUR C. GIBSON, MEMBG Director