494 PEACTICAL BOTANY 



Corn is one of the most important of these crops, and the 

 supremacy of the corn belt in the United States is perhaps 

 as much due to the amount and intensity of its sunshine 

 during the summer months as to its admirable soil.^ 



In planting many kinds of crops (corn among others), atten- 

 tion should be given to allowing light to enter freely between 

 the plants ; otherwise the quantity and quality of the product 

 from the field will suffer. Fruit trees should be pruned so 

 that the lower limbs will not "be prevented from developing 

 by the shade, and should be planted far enough apart to pre- 

 vent injury to the entire tree from the same cause. The in- 

 tensity of sunlight in an orchard region has much to do with 

 the amount of pruning necessary to make apple trees bear 

 well-developed and ripened fruit. In the northeastern states 

 the tops of the trees should be thinned out to not more than 

 half the thickness allowable in the Middle West. The high 

 color of apples from Colorado and other regions of intense 

 sunlight is due to this abundant light supply, which brings 

 about the coloration even in trees with dense tops. 



450. Plant distribution.^ The subject of the distribution of 

 plants on the earth's surface was at first discussed almost 

 wholly as a group of geographical facts. While travelers for 

 thousands of years have known something of the diversity of 

 the vegetation of the earth, it is only recently that careful 

 attention has been given to the factors which determine the 

 kinds of plant inhabitants of any given region. 



The vegetation of any portion of the earth is usually con- 

 sidered either from the floristic or the ecological point of view. 

 In the former case the student takes into account mainly the 

 kinds of plants — that is, the species, genera, families, and 



1 See Warren, Elements of Agriculture, chap. vii. The Macmillan Com- 

 pany, New York. 



2 Within the limits of a chapter like the present one this subject can only 

 be touched upon. Much information can be found in the larger physical 

 geographies, in Schimper's Plant Geography on a Physiological Basis (see 

 p. 477), and in the popular writings of naturalists like Humboldt, Darwin, 

 J. D. Hooker, Wallace, Belt, Bates, Hudson, and others. 



