ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 371 



slope to its dry and sunny south exposure; or we can see 

 it by walking from the north side of our house around to 



FIG. 216. Engelmann's spruce from an exposed mountain side 

 (altitude 10,800 ft.). Photo by D. M. Andrews. 



the south side. Food and water are the primary requis- 

 ites of plants ; but green plant food is nearly everywhere 

 present the carbon in the air, and the other food materi- 

 als in the soil ; but water is not so uniformly distributed 

 over the surface of the earth. So it has come about that 



