282 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



a relatively short passage over the land. With this high humidity it 

 might seem paradoxical that the sea winds could be the master factor in 

 the evaporation rate, yet such is the case. This is partly due to the 

 high average velocity of the winds and partly to the adabiatic warming 

 of the air on the descent of the lee side of the island. This may explain 

 why the evaporation rates on the lee side of the island are about equal 

 to those on the windward side. Repeated direct observation has shown 

 that the velocity is much greater on the sea bluffs, headlands, and the 

 ridges than on the terraces or in the canyons. The average evaporation 

 rates obtained from the former locations were markedly higher than for 

 any station located elsewhere. The profiles of the island, figures 7 and 

 8, show the relationships of the evaporation rates at different locations, 

 particularly the greater rates of the sea bluffs as compared with those 

 of the terraces. 



The sea bluffs present an almost unlimited variety of exposure to 

 wind and sun, and of slope angles. There is also considerable divergence 

 in the depth and character of their soils, and great variety in the floristic 

 composition of their plant communities. Yet they have many character- 

 istics in common. They possess uniformly coarse soils, low extremes of 

 temperature, and high evaporation rates. There is a general uniformity 

 in respect to the life-forms of the plants and in their structural adap- 

 tation to the environment. In every particular the sea bluffs form a 

 distinct contrast to the interior of the island. The plants of the sea 

 bluffs are mostly low perennials, usually very compact in habit. A great 

 number of the plants are succulent or have developed a heavy pubesence. 

 This is due partly to the need for the conservation of moisture in their 

 arid environment, and partly to the presence of sea salts in the soil. 

 (Braun-Blanquet, 1932, p. 193). 



Many of the differences in the floristic composition of the sea bluff 

 communities' in closely adjacent areas are the result of chance inequalities. 

 The vicissitudes of erosion with frequent slides, due mainly to under- 

 cutting by wave action, bring about different conditions and different 

 plant life. This combined with the steepness of the slopes makes lateral 

 migration difficult. Unless windborne, or carried by animals, seeds 

 cannot travel laterally across a steep slope. The vertical range of edaphic 

 factors is very narrow in so far as the ecesis of plants from zones in an 

 intensity or degree favorable for the ecesis of plants above or below 

 these slopes may often be very limited. The extent of the sea bluffs and 

 the large variety of plant communities, with the general similarity of 

 edaphic and climatic factors and of the life-forms of the plants, may 



