NO. 2 gentry: land plants 77 



paid to the life histories and the ecology of the plants that live upon 

 them. We are still ignorant of the community make-ups and the inter- 

 relationships of the biota. Certainly the terrain is sufficiently diversified 

 to support various associations, although the elevation is not spectacular 

 and the present island area is not an ancient one. The area of the island, 

 its tropical nature, the observations of visitors, and the lack of summer 

 and winter collecting all indicate that the known flora of about 300 

 plants is probably little over half of what actually exists. Many of the 

 plants listed, due to inadequate material, have been determined to genus 

 only, others are tentative. The Euphorbiaceous genus, Calaenodendron, 

 appears to be endemic. Should more be found, we would have to revise 

 our opinion regarding the age of the island, which seems to have been 

 little more than a large rock until some time in the Pliocene. Until the 

 amount and nature of endemism and the identity of most of the species 

 are known, all inferences regarding the development and relations of 

 the flora must be very tentative. There is no group of islands in the 

 Eastern Pacific more worthy of thorough investigation. 



Maria Magdalena Island lies about 11 kilometers across chan- 

 nel from Maria Madre. It is approximately 150 square kilometers 

 in area, 16 kilometers long, 10 kilometers wide, and with a central 

 peak of about 450 meters elevation. Physiographic habitats include 

 beaches, sea cliffs, canyons, rocky slopes and ridges. Hanna (1926:73) 

 mentions various canyons and a water hole in the next canyon west 

 of the one in which they were camped, near the center of the north 

 side of the island. They were there in May, when the dry season is 

 well advanced, so that the water must be nearly or actually perma- 

 nent. Concerning the geology he wrote (1926:72) "Maria Magdalena 

 has had an entirely different history from Maria Madre. Basement 

 rocks are volcanic and are overlain by a great series of cherts, sand- 

 stones, and mud shales. These we took to be Cretaceous in age but 

 definite paleontologic proof was not found. Miocene appeared to be 

 absent and Pliocene was not positively identified. Pleistocene, however, 

 is exposed near the sea and on the beach at the creek mouth and the flat 

 eastern end of the island is probably an elevated terrace of this age. The 

 dangerous reefs projecting from the north side of the island are com- 

 posed of resistant layers of the supposed Cretaceous rocks, the softer 

 shale layers having been eroded away. Many of these resistant layers 

 weather out as huge flagstones. The high western end of the island, the 

 Pacific side, with its enormous sea cliffs, is composed of highly altered 



