92 BOTANY, 



Nitrogen gatherers. 



163. How clovers, peas, and other legumes gather ni- 

 trogen. It has long been known that clover plants, peas, 



beans, and many other leguminous plants 

 are often able to thrive in soil where the 

 cereals do but poorly. Soil poor in nitro- 

 genous plant food becomes richer in this 

 substance where clovers, peas, etc., are 

 grown, and they are often planted for the 

 purpose of enriching the soil. Leguminous 

 plants, especially in poor soil, are almost 

 certain to have enlargements, in the form 

 of nodules, or " root tubercles." A root 

 of the common vetch with some of these 

 root tubercles is shown in fig. 6q. 



Fig. 69. 



Root of the common vetch, 163a. A fungal or bacterial organism 



showing root tubercles. ,11 T r 



in these root tubercles. If we cut one 



of these root tubercles open, and mount a small portion of the 

 interior in water for examination with the microscope, we shall 

 find small rod-shaped bodies, some of which resemble bacteria, 

 while others are more or less forked into forms like the letter 

 Y, as shown in fig. 70. These bodies are rich in nitrogenous 

 substances, or proteids. They are portions of a minute organ- 

 ism, of a fungous or bacterial nature, which attacks the roots 

 of leguminous plants and causes these nodular outgrowths. 

 The organism (Phytomyxa leguminosarum) exists in the soil 

 and is widely distributed where legumes grow. 



164. 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 



