ABSORPTIVE POWER OF DIFFERENT CROPS 291 



may readily be conceived to start an active transfer 

 of mineral substances into the plant. 



Plants grown in solutions of nutritive salts have 

 few or no root-hairs, but absorb through the epidermal 

 tissue of the roots. If the plant depended upon the pre- 

 pared solution in the soil-water, a similar structure would 

 doubtless suffice. The special modification by which 

 the root-hairs come in intimate contact with the soil- 

 particle, and almost surrounds it, indicates a direct 

 relation between the soil-particles and the plant, and not 

 merely between the soil solution and the plant. 



New root-hairs are constantly being formed, and the 

 old ones become inactive and disappear. The contact 

 of a root-hair with a soil-particle is not long-continued. 

 Whether the period of contact is determined by the 

 ability of the root to absorb nutriment from the particle 

 is not known. Certain it is that only a small portion of 

 the particle is removed. It may be true that only the 

 immediate surface which had been previously acted 

 upon by the disintegrating agents of the soil, and thus 

 rendered more easily soluble, is affected by the absor- 

 bent action of the root-hairs. 



149. Absorptive power of different crops. As has 

 already been pointed out (page 281), crops of different 

 kinds vary greatly in their ability to draw nourishment 

 from the soil. The difference between the nitrogen, 

 phosphorus and potassium taken up by a corn crop 

 of average size and a wheat crop of average size is very 

 striking. Corn has the longer growing period, but as be- 

 tween oats and wheat, where the growing period is nearly 

 identical, a similar relation exists. 



