RAISED STOMATAL CLUSTERS ON COLEUS (LAMIACEAE) STEMS

Stomata on stem internodes of Coleus blumei occur only in clusters on hollow epidermal bulges. We illustrate them here with light and scanning electron microsocpy. Some stomatal clusters are sites for lenticel formation on older internodes. Three Plectranthus species were also examined; two had discrete stomatal clusters while the third showed a more scattered arrangement of stomata, indicating that the stomatal pattern on stems may have some potential as a taxonomic character in Lamiaceae. STOMATA are usually evenly distributed in the epidermis of leaves; however, they have been reported as clustered in discrete groups in leaves of representatives of 17 dicot families (Metcalfe and Chalk, 1979). Whatever their distribution pattern, foliar stomata are most often flush with the surrounding epidermis. Stomata sunken in depressions or crypts are relatively common, particularly in xerophytic plants. This position reduces water loss because the chamber of relatively humid air above the stomata reduces the diffusion gradient of water vapor. Stomata raised above the surface are less common, but they have been reported for some taxa from moist environments (Metcalfe and Chalk, 1979; Solereder, 1908). Trecul (1871) surveyed the arrangement of stomata on young stems of about 50 species of woody dicots. In most of these he found small, widely separated green or white spots, each with a single raised stoma. The light color of the spot is caused by a substomatal chamber and surrounding parenchyma. In six of the species, Trecul found clusters of stomata over each spot. Weber (1949) examined the stems of seven herbaceous species and noted clustered stomata on three species. These are the only two investigations of cauline stomata that we are aware of. We have found that the internodes of Coleus blumei Benth. bear stomata in discrete clusters that are widely separated and conspicuously elevated above the stem surface. Weber's description of clustered stomata on Coleus scutellarioides differs somewhat from what we found, and since cauline stomatal arrangements appear to have been generally neglected, ' Received for publication 20 December 1982; revision accepted 24 March 1983. Microscopy and photographic work was performed in the Bessey Microscopy Facility, Department of Botany, Iowa State University. a brief description of this curious stomatal arrangement seems in order. MATERIALS AND METHODS -Stems, petioles, and leaves from living plants were examined at 70X under a dissecting microscope, and freehand sections of these plant parts were examined with a compound microscope. Portions of young internodes were fixed in FAA, dehydrated in an ethanol series, and embedded in Spurr's resin (Spurr, 1969). One-,um-thick sections were made with a glass knife on the LKB ultramicrotome and stained with toluidine blue. Other samples were later fixed in a 3% formaldehyde (from paraformaldehyde) and 1% glutaraldehyde mixture in phosphate buffer at pH 7.2 overnight at 4 C. These were then post-fixed in buffered 1% osmium tetroxide, dehydrated in an ethanol series, critical point-dried, sputter-coated with gold-palladium, and examined at 15 kv with a JEOL JSM35 scanning electron microscope (SEM). OBSERVATIONS-At low magnifications, fusiform epidermal ridges are evident on all sides of the stem (Fig. 1). The SEM revealed that stomata are restricted to these ridges, and that each ridge contains 3-15 stomata depending on its size (Fig. 2, 3). These figures also show that stomata are absent from the surrounding epidermis. Figure 4 shows two stomata in detail. The stem is square in transectional view. Vascular bundles are positioned uniformly 78 cell layers beneath the epidermis. Immediately external to them are about five layers of chlorenchyma. About two cell layers of collenchyma extend from the chlorenchyma out to the epidermis. Where stomatal clusters occur, collenchyma is replaced by an extension of the chlorenchyma outward to about the level of the surrounding epidermis. The raised epi-