| Adiantum vivesii Proctor is a rare fern endemic to the northwestern part of Puerto Rico, known only from its type locality on limestone substrate on privately owned land in the Municipality of Quebradillas. The distribution, abundance, morphology, and cytology of the species were studied to clarify its status as a distinct species or of hybrid or polyploid origen. The population of A. vivesii occupies a total area of 21 m x 10 m; eight sympatric species of Adiantum are found in the immediate area. Following several deep excavations it was concluded that the entire population of A. vivesii is one individual with proliferation by rhizomes. Morphological measurements and light and scanning microscopy were used to describe some parts of A. vivesii and A. tetraphyllum. Morphometric studies were conducted for 22 characters of the species and the possible parent, Adiantum tetraphyllum. Greater variability was obtained within the A. tetraphyllum population, and statistically significant differences between the two species were observed for 17 characteristics. Meiotic studies were inconclusive, but no evident difference in chromosome number was observed between the two species. The spores of A. tetraphyllum showed normal appearance and were rarely abortive, while those of A. vivesii showed greater variability in size and were almost always abortive, suggesting that A. vivesii is a sterile hybrid. |
