THE SOURCES OF PLANT-FOOD. 



43 



plants growing in it were laid bare. Roots were ex- 

 posed in this way in a field of rye, also in one of 

 beans, and in a bed of garden-peas, which were 

 found to present the appearance of a mat of white 

 fibres to a depth of four feet from the surface of the 

 ground. He found the roots of winter wheat as deep 

 as seven feet in a light sub-soil forty-seven days 

 after sowing. Another German, who has studied 

 the subject, calculated the total combined length 

 of the roots of a vigorous barley plant in a rich 

 garden soil to be one hundred and twenty-eight 

 feet, and in a coarse-grained, compact soil eighty 

 feet. 



92. The absorbing surface of roots is greatly in- 

 creased by very small root-hairs which are lost as 

 the roojis become old. It was once supposed that 

 the ends, or tTps of roots consist of delicate tissue, 

 or organs called " spongioles," through which alone 



rption of food takes place, but such is not the 

 se ; no organ or structure of the kind exists. 



93. Agricultural writers distinguish three kind of 

 roots, viz., soil roots, water roots, and air roots. 

 Nearly al^P^ our useful plants have soil roots which 

 perish if kept for a length of time in air or water, 

 while stfSfS"* plants, as rice for instance, have roots 

 which gan grow either in soil or water. The com- 

 mon mulberry and China tree will extend a portion 

 of their roots down to the bottom of a well thirty or 

 more feet in depth, and mat over the bottom com- 

 pletely, while the cornstalk will put out air roots at 



