Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Barbara S. Carlsward


The subtribe Oncidiinae is a diverse group of mostly epiphytic orchids within the tribe Maxillarieae, consisting of 55 genera and 1700 species. In this group, there have been many studies examining morphological and anatomical variation as well as metabolic pathways of carbon fixation, but most have not integrated morphological and anatomical variation with a physiological aspect of inquiry. The objective of my research was to establish a suite of anatomical characteristics that can be used to distinguish between C3 and CAM species. Secondarily, I hoped to use that suite of characteristics in determining whether intermediate CAM species (those that have the ability to switch between C3 and CAM) are more similar anatomically to established C3 plants or CAM plants. In my study, 19 species of Oncidiinae were selected for anatomical description and comparison in relation to their photosynthetic pathway. Of these species, seven were identified in previous studies as having a C3 photosynthetic pathway, six were identified has having an intermediate photosynthetic pathway, and six were identified as having a CAM photosynthetic pathway. Cross sections, longitudinal sections, and paradermal sections were prepared for observation of overall leaf thickness, mesophyll thickness, mesophyll cell size, stomatal complex length, stomatal complex width, and stomatal density. A principle components analysis of the structural variables was performed using a correlation matrix to address all traits simultaneously. The analysis showed that stomatal density and mesophyll cell size were the key anatomical identifiers in distinguishing between photosynthetic pathways. None of the leaf traits I examined differed enough between C3 species and intermediate CAM species to anatomically distinguish between the two photosynthetic pathways. Phylogenetically, the occurence of CAM seems to have evolved independently several different times within the Oncidiinae, probably via intermediate pathways.