92 GENEKAL CONSIDERATIONS 



extreme root tip it is covered with what looks like a 

 delicate down or a weft of mold. On magnification 

 this down is seen to consist of immense numbers of 

 short delicate root hairs. These are the organs for 

 absorbing water, together with the small amounts of 

 mineral salts that the soil water always holds in 

 solution. They are pressed close to the soil parti- 

 cles and so are in immediate contact with the films 

 of water that, as we have previously seen, cling to 

 each of these particles. After a few days or weeks 

 the oldest of these root hairs die, but others are con- 

 stantly forming near the younger root tips during 

 all the period when the plant is in active growth. 

 Although each single root hair is so small as to be 

 invisible to the naked eye, their aggregate number 

 is so great that their combined surface usually con- 

 siderably exceeds the entire leaf surface of the plant. 

 The growing of root hairs for absorbing the films of 

 moisture that cling to the soil particles is thus seen 

 to be the chief function of the roots of agricultural 

 plants. Incidentally they also serve as holdfasts 

 for anchoring the plant firmly in the soil and pre- 

 venting its being blown away by the wind. In some 

 special cases, as with the turnip and sweet potato, 

 roots also serve as storage organs for holding re- 

 serve food supplies. This is always the case with 

 these crops in which the root is the part having com- 

 mercial value. All that has been written, therefore, 

 about the soil and methods for its preparation for crops 

 and its improvement is simply to insure that it be in 

 proper condition for the roots to ramify freely and de- 

 velop root hairs abundantly ; and that the films of soil 



