Stomata and Stomatal Clusters in Begonia: Ecological Response in Two Mexican Species

Abstract

Many species in the genus Begonia have stomata that occur in clusters; clusters are an unusual character and have an extremely limited distribution among the higher plants. Six populations of Begonia heracleifolia Cham. et Schlecht. and nine populations of B. nelumbiifolia Cham. et Schlecht. were sampled from tropical forests in Mexico to determine the ecological response of the stomata and clusters of these species. Several characters, including mean cluster size, number of stomata/duster, length, and number of stomata/mm2, were analyzed to determine variation within and between populations; the latter three characters are responsive to elevation in B. nelumbiifolia, but only mean cluster size and number of stomata/ mm2 are responsive to elevation in B. heracleifolia. Ecological evidence from this study supports the hypothesis that clusters in Begonia assist in water conservation. A distributional analysis of sympatric populations of these species at a waterfall site in Chiapas indicates that the relative density of B. nelumbiifolia is greater near the pool formed by the falls, and that the relative density of B. heracleifolia is greater away from the pool. B. nelumbiifolia has a lower mean cluster size and a lower range of cluster sizes than B. heracleifolia. The hypothesis is further supported by differences in the range of cluster sizes between populations occupying different habitats and substrate types: populations growing on rocks near waterfalls have larger clusters than populations collected from soil substrates. THIS INVESTIGATION CONSIDERS THE ECOLOGICAL RESPONSE OF STOMATA in Begonia, a pantropical genus of the Begoniaceae induding approximately 1200 species (Barkley & Golding 1974). A cluster is a group of two or more guard cells belonging to one stomatal chamber, where the individual stomata within the duster group are separated from each other by subsidiary cells (Fellerer 1892). Epidermal cells are only observed between clusters, never within the duster group. Within the genus Begonia, some species are observed to have dusters, with much variation in the number of stomata per duster, but other species are observed to have stomata occurring only singly. Most vascular plants have stomata occurring singly; the occurrence of clusters is very limited within the higher plants. Some other taxa known to have clusters besides Begonia include Pagamea (Rubiaceae), and Saxifraga (Saxifragaceae) (Fellerer 1892), Gesneria, Heppiella, Halphophyllum, and Rytidophyllum (Gesneriaceae) (Metcalfe & Chalk 1950, Wiehler 1970, Skog 1976) and some species in the Bromeliaceae (T. Givnish, pers. comm.). Possibly the first report on clusters in Begonia was by Strasburger (1866-1867). The first to suggest that dusters function to reduce transpiration was Fellerer (1892), reporting that the distribution of dusters over the lower leaf surface provides larger spaces for water vapor to be trapped, though he provided no anatomical or physiological support for this suggestion. Evidence indicating a positive correlation between the number of epidermal cell layers and the number of stomata/duster has been presented by Boghdan and Barkley (1972), though no function has been advanced. A list of 122 species of Begonia and their corresponding cluster sizes has been reported by Neubauer (1967) and Neubauer and Bissler (1971). Of additional importance, Neubauer (1967) describes the presence of water-storing tissue in Begonia. Clearly important to the evolution of clusters in Begonia is the functional significance of their size, density, and the characteristics of their component stomata. In this paper, intraand interpopulation variation of these traits are described for two Mexican species, B. heracleifolia and B. nelumbiifolia, which occupy varying habitats and elevations throughout the tropical vegetation zones of Mexico. This paper provides ecological evidence supporting Fellerer's (1892) suggestion that dusters are a means of reducing transpiration.