2 THE AMERICAN BOTANIST 



supposed fern fossils have proven to be the leaves and stems 

 <>i" primitive gymnosperms. The apparent decadence of the 

 pteridophytes is thus not so great as was once commonly be- 

 lieved. The tree ferns probably never comprised a dominant 

 element of the Carboniferous forests. 



The arborescent ferns are today widely distributed 

 throughout the humid tropics. They give a distinctive appear- 

 ance to many of the equatorial rain-f( >rests. In favored regit ins, 

 like Hawaii, they form pure stands of considerable magnitude. 

 Australia, New Zealand, Malaya, Ceylon, Africa, Central and 

 South America, all possess fern forests. The important 

 genera are: Cyathca, Alsophila, Hemitelia, Dicksmiia, Thyr- 

 sopteris, Cibotium, and Bahiiitium. 



Although associated in the popular mind with a hot-house 

 atmosphere and a gorgeous tropic background, in reality the 

 tree ferns manifest a considerable range of thermal adaptation. 

 They literally extend from the steaming jungle, "where the 

 blazoned, bird-winged butter-flies flap through," to the icy 

 rim of the Antarctic. A number of the Tasmanian and New 

 Zealand ferns are annually subjected to very low temperatures. 

 It would appear that the critical factor governing the geo- 

 graphic distribution of the tree ferns is not primarily a high 

 temperature, but high atmospheric humidity fairly continuous 

 throughout the year. The perennial crown of gigantic fronds 

 is vitally dependent upon a moist atmosphere. It cannot long- 

 withstand drought, nor thrive in a xerophytic habitat. 



Small clumps and groves of tree ferns are abundant on 

 the mountain slopes of all the larger islands of the Hawaiian 

 archipelago, but the finest stands are limited to the island of 

 I lawaii. This i^ the largest and youngest island of the group, 

 in fact its area (four thousand square miles) is greater than 

 the combined area of the others. Hawaii's large size, its great 

 volcanic domes rising to nearly fourteen thousand feet, its 



