ALTERNATION AND ZONATIOX 303 



produced the primary features of the vegetation of North America. 

 Tropical forests occur where heat and water are excessive, deserts 

 where either is unusually deficient, deciduous and coniferous 

 forests where the water content is relatively high, and prairie 

 and plains where it is relatively low. 



This primary arrangement is modified by the disturbing effect 

 of three continental mountain systems. The Appalachian system 

 is not sufficiently high to produce a pronounced effect upon humidity 

 and rainfall. In consequence forests extend far beyond it into 

 the interior before gi^ing way to prairies and plains. On the 

 other hand, the influence of the Rocky ^fountains and the Sierra 

 Nevada is marked. The latter rise to a great height relatively 

 near the coast, and condense upon their western slopes nearly 

 all of the moisture brought from the Pacific. The Rocky Moun- 

 tains have a similar effect upon the drier ^dnds from the east, 

 and the two systems in consequence enclose a parched desert. 

 All three systems carry the formations of the north far beyond 

 their normal southern limit, owing to the low temperatures that 

 prevail in high altitudes. The alpine grasslands of the Sierra 

 Nevada and the Rocky ^Mountains are a southward extension of 

 arctic grasslands, and the belts of coniferous forests along the 

 slopes of the three systems are similar extensions of northern 

 forests. 



Experiment 72. Zonation of pond and meadow formations. Select for 

 study a pond of some extent and depth. Note the various zones of 

 vegetation and list the species of each. ]Make a map showing the general 

 outline of the pond, and indicate the limits of the various zones of pond 

 and meadow. 



