6o 



Science of Plant Life 



(i) The plants that natu- 

 rally Uve where the evapora- 

 tive power of the air is intense 

 and the available water is 

 limited are called xerophytes 

 (Greek : xeros, dry, and phy- 

 ton, plant). These are the 

 plants that are adjusted to a 

 nearly continuous dearth of 

 water; the cacti, agaves, 

 yuccas, and sagebrush of our 

 Western plains and deserts 

 are striking representatives. 

 In the eastern United States 

 there are less marked ex- 

 arnples of xerophytes in the 

 plants that live on dry cKffs 

 and sand beaches, and in the 

 mosses and lichens that grow 

 on trees and rocks. 



(2) The plants that live 

 partly or wholly submerged in the water are known as 

 hydrophytes (Greek : hudor, water, and phyton, plant) . These 

 plants have an excessive water supply, and transpiration is 

 reduced or entirely wanting. In this class are included the 

 water liHes, pondweeds, cat-tails, bulrushes, and many sedges. 

 They are the common plants of fresh-water ponds, swamps, 

 and marshes throughout the world. 



(3) Between these extremes are the mesophytes (Greek: 

 meso, middle, and phyton, plant), by far the largest class of 

 plants. They have a medium rate of transpiration and grow 



Fig. 40. Evergreen trees growing under 

 extreme xerophytic conditions in Oregon. 

 These "Pillars of Hercules" are by the 

 side of the Lincoln Highway. 



