IN TROD UC TION. 289 



ing water at some seasons and for losing water at 

 others. The plants are all perennial. The peren- 

 nial parts are xerophytic, while the annual parts 

 are hygrophytic. Examples: trees and shrubs 

 which possess foliage leaves in summer and in the 

 winter the shoots are devoid of leaves. The plants 

 are thus enabled to turn from one condition to 

 another. (The first part of the word tropophyte 

 means to turn, while the latter part means plant.) 

 Compare such plants astrillium, jack-in-the-pulpit, 

 etc., with underground perennial shoots, and aerial 

 annual shoots. 



The pines, spruces, etc., are protected from rapid 

 transpiration during the winter by having 

 narrow and thick leaves, and also by some in- 

 ternal changes in the leaf as winter comes on. 

 This division of plant forms into classes as xerophytes, hygro- 

 phytes, and tropophytes is often very marked in w r ide regions. 

 The coastal plains and the mountain regions of the tropics are 

 characterized by hygrophytes; the steppes, deserts, polar 

 regions, and alpine regions of the temperate zones by xero- 

 phytes; while the greater part of the North Temperate zone is 

 characterized by tropophytes. 



Between these classes there are intermediate forms which 

 break down any attempt to draw a hard and fast line between 

 them; yet such a classification, even if it is arbitrary, is con- 

 venient. Also the plants of one class may occur in regions 

 where another class is dominant. For example, the touch-me- 

 not (impatiens) is a hygrophyte, and it occurs in the region 

 dominated by the tropophytes. The parsley (portulaca), the 

 mullein (verbascum) are xerophytes, and they also occur in the 

 same region; while the heaths, the labrador tea, etc., which 

 occur in sphagnum moors are also xerophytes, and yet occur 

 in the region dominated by the tropophytes. (See Chapter 

 LII.) 



