NUTRITION: NITROGEN GATHERERS. 



319 



some of which resemble bacteria, while others are more or less forked into 

 forms like the letter Y, as shown in fig. 421. These boilies are rich in 

 nitrogenous substances, or proteids. They are portions of a minute organism, 

 of a fungus or bacterial nature, which attacks the roots of leguminous plants 



Fig. 421. Fig. 422. 



Root-tubercle organism from vetch, old con- Root-tubercle organism from Medicago 

 dition. denticulata. 



and causes these nodular outgrowths. The organism (Phytomyxa legumi- 

 nosarum) exists in the soil and is widely distributed where legumes grow. 



601. How the organism gets into the roots of the legumes. — This ininute 

 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 ver}- 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." 



603. 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 ma)- be. This is why 

 such plants can thrive in soil with a poor nitrogen Cfjntent. 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 the)' are oval, 

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

 other genera. 



