HOW PLANTS OBTAIN FOOD. 93 



197. How the organism gets into the roots of the legumes. This minute 

 organism in the soil makes its way through the wall of a root hair near the 

 end. It then grows down the interior of the root hair in the form of a 

 thread. When it reaches the cell walls it makes a minute perforation, 

 through which it grows to enter the adjacent cell, when it enlarges again. 

 In this way it passes from the root hair to the cells of the root and down to 

 near the center of the root. As soon as it begins to enter the cells of the 

 root it stimulates the cells of that portion to greater activity. So the root 

 here develops a large lateral nodule, or "root tubercle." As this "root 

 tubercle" increases in size, the fungus threads branch in all directions, 

 entering many cells. The threads are very irregular in form, and from cer- 

 tain enlargements it appears that the rod-like bodies are formed, or the 

 thread later breaks into myriads of these small " bacteroids. " 



198. The root organism assimilates free nitrogen for its host. This 

 organism assimilates the free nitrogen from the air in the soil, to make the 

 proteid substance which is found stored in the bacteroids in large quantities. 

 Some of the bacteroids, rich in proteids, are dissolved, and the proteid sub- 

 stance is made use of by the clover or pea, as the case may be. This is why 

 such plants can thrive in soil with a poor nitrogen content. Later in the 

 season some of the root tubercles die and decay. In this way some of the 

 proteid substance is set free in the soil. The soil thus becomes richer in 

 nitrogenous plant food. 



The forms of the bacteroids vary. In some of the clovers 'they are oval, 

 in vetch they are rod-like or forked, and other forms occur in some of the 

 other genera. 



199. NOTE. So far as we know the legume tubercle organism does not 

 assimilate free nitrogen of the air unless it is within the root of the legume. 

 But there are microorganisms in the soil which are capable of assimilating 

 free nitrogen independently. Example, a bacterium, Clo.rtridium pasteur- 

 ianum. Certain bacteria and algae live in contact symbiosis in the soil, the 

 bacteria fixing free nitrogen, while in return for the combined nitrogen, the 

 algas furnish the bacteria with carbohydrates. It seems that these bac- 

 teria cannot fix the free nitrogen of the air unless they are supplied with 

 carbohydrates, and it is known that Clostridium pasteurianum cannot assim- 

 ilate free nitrogen unless sugar is present. 



6. Lichens. 



200. Nutrition of lichens. Lichens are very curious plants which grow 

 on rocks, on the trunks and branches of trees, and on the soil. They form 

 leaf-like expansions more or less green in color, or brownish, or gray, or they 

 occur in the form of threads, or small tree-like formations. Sometimes the 



