AIR NECESSARY FOR SEEDS AND PLANTS 13 



the seeds in the other tumbler will require a longer time to 

 germinate, and, if the toinpornture of the room in which the 

 two glasses are kept is low, tlie seeds will rot. The tumbler 

 which contains an excessive amount of moisture prevents 

 the access of air that is necessary to the germination of the 

 seed, while the one which is kept only moderately moist 

 allows a sufficient amount of air to come in contact with 

 the seeds to insure germination. 



Discussion : For the best growth of crops the space not 

 occupied by soil particles should be divided equally between 

 air and water. If this space becomes entirely filled with 

 water, crops will not thrive, since their roots will not be 

 able to get the air necessary for plant growth. Some plants, 

 such as the cypress and the water lily, have special structures 

 which enable them to obtain air from the water while their 

 roots are entirely submerged, but our common field plants 

 do not have this ability. 



Of the various means employed to improve land that is 

 naturally unfit for cultivation, there is none which exceeds 

 in importance the operation of tile drainage. A large part 

 of the United States is well drained naturally because of 

 the open structure of the subsoil, which permits the surplus 

 water to pass away rapidly into the underground channels 

 and from thence into the small streams and rivers. In 

 many cases, however, the subsoil is too tight and compact 

 to allow free percolation of the water and artificial drain- 

 age is needed. 



