OUR PRESENT KNOWLEDGE OF THE FOREST FORMA- 

 TIONS OF THE ISTHMUS OF PANAMA^ 



By Dr. H. Pittier 



This is a brief account of some results of the study of the flora of 

 Panama made in connection with the general biological survey organ- 

 ized by the Smithsonian Institution. 



When, in 1910, I started the systematic botanical exploration of the 

 country, about 1,115 species had already been catalogued, due mainly 

 to the efforts of Seemann, Hayes, Fendler, and a few others. Of these 

 plants, nearly four-fifths had been collected along the transisthmian 

 railway, at the mouth of the Chagres River, around the city of Panama, 

 and in the savanna and park-like formations along the Pacific coast. 

 From 1910 to 1912 and in 1914 the survey was extended over the 

 whole Isthmus, excepting the stretch of the coast between Colon and 

 the Almirante Bay. A little over half of the collections made have 

 been worked up and about 3,000 species have been listed. It is expected 

 that after all the materials brought together have been examined the 

 total number of known species will reach far above 3,500. This is 

 about 2,500 less than the number of species recorded for the neighbor- 

 ing Costa Rica, but this large difference is easily explained by the fact 

 that while the territory of the latter country is distributed in fair pro- 

 portions between the sealevel and an altitude of nearly 3,900 meters, 

 nearly nine-tenths of the Isthmus is included in an altitudinal belt of 

 less than 1,000 meters. 



Six-tenths, at least, of the territory of Panama is covered with 

 forests, while the rest is given to savannas, park-like formations, and 

 the very small openings resulting from human activity. Leaving the 

 latter aside, the respective distribution of the natural formations, 

 forests, savannas, and park-like landscapes is the result mainly of the 

 regime of the rainfall, dependent itself on the dominating winds. 



As is well known, the Isthmus of Panama stretches in a west-easterly 

 direction between the Caribbean Sea and the Pacific Ocean, and is thus 

 fully exposed on one side to the northeast trade-wind, on the other 

 to the southwest monsoon. Both these atmospherical currents reach 

 the respective coast saturated with humidity, which is dropped as they 



* Read to the Biological Society of Washington, November 18, 1916. 

 76 



