Temperature and Moisture Together 183 



expression or an indicator of climate. This idea has been extensively 

 developed by the Danish botanist Raunkiaer (1934), whose work 

 should be consulted for a thorough treatment of the subject. Raun- 

 kiaer stresses the significance of the adaptations of buds and shoot 

 tips for withstanding adverse temperatures and surviving drought. 

 On this basis Raunkiaer deHneates different groups of plants. Trees 

 and shrubs are placed in one group in which the surviving buds pro- 

 ject into the air. The perennial grasses are included in another group 

 in which the buds are situated on or near the soil surface. Plants 

 with bulblike buds that are protected during adverse seasons by being 

 buried in the ground are placed in a third group. 



Dry cold 



Wet cold Dry cold 



Wet cold 



Dry hot Wet hot 



Precipitation-evaporation ratio 



Dry hot Wet hot 



Precipitation-evaporation ratio 



VEGETATION SOIL 



Fig. 5.22. Schematic representation of the interaction of temperature and 

 moisture effectiveness in controHing the general distribution of vegetational Ufe 

 forms and soil groups in North America. (Modified from Thornthwaite, 1931.) 



Deciduous trees, conifers, shrubs, succulent xerophytes, mosses, 

 grasses, annual plants, and epiphytes represent contrasting life forms. 

 As we have seen the major life zones are distinguished by the life form 

 of the vegetation. Plants of distinctive life form characterize the 

 major plant communities, and these communities are the dominant 

 feature of the biotic formations, or biomes, of the world, as described 

 in Chapter 12. 



Thornthwaite ( 1931 ) has worked out indices of the effectiveness of 

 thermal and moisture elements of climate as related to the range of 

 temperature and the rate of evaporation in influencing the distribution 



