PLANT FOOD 29 



must dissolve in water before the plant can absorb them. 

 It is not necessary, however, that they be very soluble. A 

 weak solution is even better for a plant than a strong one. 

 In the case of iron, for example, enough of the hydrated 

 ferric oxide dissolves in the soil moisture to nourish plants, 

 although hydrated ferric oxide is usually regarded as a 

 comparatively insoluble compound. 



16. How Absorbed. — As stated above, part of the plant 

 food is absorbed through the leaves from the air and part 

 through the roots from the soil. Considering first the ab- 

 sorption from the soil, it is to be noted that not all of the 

 roots of plants spreading through the soil will absorb water 

 and plant food. Only the very fine root hairs, located near 

 the growing tips and extending but a short distance back of 

 them, act as absorbers of plant food matter. The remainder 

 of the roots is covered with a hard, corkv laver which is 

 impervious to water. Since the growing tip keeps pushing 

 forward, the feeding ground is constantly changing. 



The root hairs are long, slender, single cells. The walls 

 are very thin, composed largely of cellulose, and are easily 

 pervious to liquids. Lining the inside of these cells is a 

 layer of protoplasm which serves as a regulator for the 

 entrance and exit of water and soluble material. Under 

 normal conditions of growth, water passes into the root 

 hairs together with the dissolved plant food. These materials 

 pass from cell to cell until they reach the long conducting 

 tubes on the interior of the root and stem called trachea 3 . 

 Here the stream of liquid is forced up to the leaves and other 

 parts of the plant. The passage of any particular plant food 

 from cell to cell is always from the region of greater con- 

 centration of that particular food to the region of less con- 

 centration. Water passes into the root hairs and from cell 

 to cell regardless of the concentration of any one plant food, 

 but from the region of less total concentration to the region 

 of greater total concentration. 



17. Selective Action by the Roots. -The protoplasmic 

 lining of the root hairs, as well as of other cells, exhibits 

 a certain amount of selective action, permitting the entrance 

 of some substances and keeping out others. To a limited 



