THE FOREST ORGANISM. 



213 



anywhere in the north woods. In general, it is true that mixed for- 

 ests, Fig. 57, i. e., those having a variety of species, grow on good 

 loamy soil. The great central, deciduous Atlantic Forest grew on such 

 soil until it was removed to make room for farms. On the other hand, 

 pure stands i. e., forests made up of single varieties of pine occupy 

 poor sandy soil. Within a distance of a few yards in the midst of a 

 pure stand of pine in 

 the south, a change in 

 the soil will produce a 

 dense mixed growth of 

 broad-leaves and coni- 

 fers. 



The soil in the for- 

 est is largely deter- 

 mined by the forest it- 

 self. In addition to 

 the earth, it is com- 

 posed of the fallen and 

 decayed leaves and 

 twigs and tree trunks, 

 altogether called the 

 forest floor. It is 

 spongy and hence has 

 the ability to retain 

 moisture, a fact of 

 great importance to 



tllO forest. Fi ff- 56 - Virgin Stand of Red Spruce. White 



Mountains, New Hampshire. U. S, Forest 

 Service. 



MOISTURE. 



Some trees, as black ash and cypress, Fig. 58, and cotton gum, 

 Fig. 59, grow naturally only in moist places; some, as the pinon and 

 mesquite, a kind of locust, grow only in dry places; while others, as 

 the jumper and Douglas fir, adapt themselves to either. Both excess- 

 ively wet and dry soils tend to diminish the number of kinds of trees. 

 In many instances the demand for water controls the distribution alto- 

 gether. In the Puget Sound region, where there is a heavy rain-fall, 

 the densest forests in the world are found, whereas on the eastern 

 slopes of the same mountains, altho the soil is not essentially different, 

 there are very few trees, because of the constant drouth. 



