488 BIOLOGY FOR BEGINNERS 



falfa, and lentils, have the power of converting the free nitrogen 

 of the air into nitrogen compounds, available for plant use, so 

 clover crops actually benefit the land so far as nitrogen is con- 

 cerned. 



Other bacteria help in decay of organic matter and return it 

 to the soil in useful forms; all dead tissue and natural manures 

 are acted upon in this way. 



3. Rotation of crops merely applies what has just been said. 

 The farmer cannot use the same field for the same crop, year after 

 year, without removing the special soil compounds which that 

 crop requires and thus diminishing his return. He therefore 

 varies his crop so that clover or peas shall have a chance to replace 

 nitrogen compounds which wheat or corn may have removed. 

 He also alternates between crops that require hoeing and those 

 that do not, so that the soil may benefit by the different methods 

 of cultivation. Often the clover crop is plowed under so that the 

 organic matter as well as the nitrogen is returned to the soil. 



Plant Breeding. Not only does biology bear upon soil condi- 

 tions .but also upon all that relates to seed planting, germination, 

 and growth. Especially is this true in the matter of testing and 

 selection of seed and in crossing and breeding of new varieties. 

 A glance at any seed catalog will show the great advances that 

 are being made by applying biologic methods to bettering the 

 varieties of plants. 



In this same connection, all other methods of plant propaga 1 

 tion are concerned. Cuttings and grafts, pollenation, trans- 

 planting, and pruning all involve the use of biological information. 



In 1900 the British Millers Association decided that the wheat 

 that was then raised in England was so unsatisfactory that they 

 engaged Prof. R. H. Biffen of Cambridge University to try to 

 improve the quality. 



Professor Biffen obtained seed of all the different wheats, which 

 had any one desirable characteristic, such as stout straw, full 

 heads, immunity to rust, or resistance to cold weather. These 

 he raised separately, and cross-pollenated by hand, combining 

 their desirable features, till after years of effort, selection, crossing, 



