THE ROOT— TL'XCTIOX AND STRUCTURE 



39 



of fertilizers. Certain substances, as common salt, will kill 

 the roots. 



Roots absorb Substances only in Solution. — Substances 

 cannot be taken in solid particles. These materials are 

 in solution in the soil water, and the roots themselves 

 also have the power to dissolve the soil materials to some 

 extent by means of substances that 

 they excrete. The materials that ^xZ 

 come into the plant through the 

 roots are water and mostly the min- 

 eral substances, as compounds of po- 

 tassium, iron, phosphorus, calcium, 

 magnesium, sulfur, and chlorine. 

 These mineral substances compose 

 the ash when the plant is burned. 

 The carbon is derived from the air 

 through the green parts. Oxygen 

 is derived from the air and the soil 

 water. 



Nitrogen enters through the Roots. 

 — All plants must have nitrogen ; 



yet, although about four fifths of Fig. 41.— Nodules on Roots 



.,..., , of Red Clover. 



the air is nitrogen, plants are not 



able, so far as we know, to take it in through their leaves. 

 It enters through the roots in combination with other ele- 

 ments, chiefly in the form of nitrates (certain combinations 

 with oxygen and a mineral base). The great family of 

 leguminous plants, however (as peas, beans, cowpea, 

 clover, alfalfa, vetch), use the nitrogen contained in the air 

 in the soil. They are able to utilize it through the agency 

 of nodules on their roots (Figs. 41, 42). These nodules 

 contain bacteria, which appropriate the free or uncom- 

 bined nitrogen and pass it on to the plant. The nitrogen 



