238 EVOLUTION OF PLANTS 



as the Gulf of Mexico is approached, is unfavorable for 

 the growth of the strictly desert plants. With the 

 increase in the rainfall, the desert gives place to a prairie 

 formation, with coarse grasses and many showy flowers, 

 phloxes, evening primroses, and gay Compositse. Still 

 further east the amount of moisture is sufficient for the 

 growth of a few low-spreading trees like the " mesquit " 

 of the Texan plains, and in eastern Texas open forests 

 of pines appear, which presently give way to the dense 

 forests and swamps of the Gulf region of Louisiana. 

 Here the conditions are almost tropical. An extremely 

 heavy rainfall and high temperature combine to produce 

 a rank vegetation, forming deep swamps and jungle-like 

 forests. Very few of the plants occurring in these wet 

 forests and swamps are at all related to the desert plants 

 of the same latitude, but have their nearest allies among 

 the plants of the Atlantic side of the continent. The 

 eastern forests, unlike those of the Pacific slope, contain 

 few Conifers, but there is a remarkable variety of 

 angiospermous trees, most of which are deciduous. 

 Hickories, gums, magnolias, tulip-trees, elms, beeches, 

 and many other trees, quite absent from the Pacific 

 coast, are important constituents of this magnificent 

 forest flora, while the herbaceous plants associated with 

 them are quite as diverse in character. Among the 

 plants of the Gulf region are a number of genuine tropi- 

 cal types, like the " Spanish moss," the most northerly 

 representative of the pineapple family, and the palmet- 

 toes, the most northerly palms of the United States. 



California, owing to its peculiar topography, has the 

 most varied flora of any region within the United States, 

 and is extremely interesting in respect to the origin of 



