Leaf dimorphism of Microgramma squamulosa (Polypodiaceae): a qualitative and quantitative analysis focusing on adaptations to epiphytism

The epiphytic fern Microgramma squamulosa occurs in the Neotropics and shows dimorphic sterile and fertile leaves. The present study aimed to describe and compare qualitatively and quantitatively macroscopic and microscopic structural characteristics of the dimorphic leaves of M. squamulosa, to point more precisely those characteristics which may contribute to epiphytic adaptations. In June 2009, six isolated host trees covered by M. squamulosa were selected close to the edge of a semi-deciduous seasonal forest fragment in the municipality of Novo Hamburgo, State of Rio Grande do Sul, Brazil. Macroscopic and microscopic analyzes were performed from 192 samples for each leaf type, and permanent and semi-permanent slides were prepared. Sections were observed under light microscopy using image capture software to produce illustrations and scales, as well as to perform quantitative analyses. Fertile and sterile leaves had no qualitative structural differences, being hypostomatous and presenting uniseriate epidermis, homogeneous chlorenchyma, amphicribal vascular bundle, and hypodermis. The presence of hypodermal tissue and the occurrence of stomata at the abaxial face are typical characteristics of xeromorphic leaves. Sterile leaves showed significantly larger areas (14.80cm2), higher sclerophylly index (0.13g/cm2) and higher stomatal density (27.75stomata/mm2) than fertile leaves. The higher sclerophylly index and the higher stomatal density observed in sterile leaves are features that make these leaves more xeromorphic, enhancing their efficiency to deal with limited water availability in the epiphytic environment, compared to fertile leaves. El helecho epífito Microgramma squamulosa se encuentra en el Neotrópico y tiene hojas estériles y fértiles dimorfas. El objetivo de este estudio fue describir y comparar cuantitativa y cualitativamente la organización estructural de las hojas de la M. squamulosa, investigando las características morfológicas y anatómicas, y se alando los factores que contribuyen a la adaptación al ambiente epífito. Los análisis macroscópicos y microscópicos se realizaron a partir de 192 muestras de hojas de cada tipo. Láminas permanentes y semi-permanentes fueron preparadas y las secciones fueron observadas en el microscopio de luz. Hojas fértiles y estériles no mostraron diferencias estructurales cualitativas, son hipostomáticas, presentan epidermis uniseriada, haz vascular anficrival e hipodermis. La presencia de hipodermis y aparatos estomáticos en la nsuperficie abaxial de la epidermis son típicas de hojas xeromórficas. Hojas est