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 



■^',S-^< 



■:i*f .«f1 





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

 (altitude lo.Soo 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 



