io6 



BOTANY OF THE LIVING PLANT 



necessary supplies be present, the growth of the plant shows. that its 

 requirements are fully met. But if no combined nitrogen be given, 

 the plant will show signs of starvation, and remain dwarfed. The 

 conclusion is that though bathed by atmospheric air, of which the 

 chief constituent is nitrogen, the plant cannot lay hold of the nitrogen 



in the free state, but must be supplied with 

 it in the combined form. This is so for all 

 ordinary green plants. 



But the case is different with some green 

 plants, and especially with the Pea Family 

 (Leguminosae). It has long been known 

 to farmers that it is an advantage to have 

 a crop of peas, beans, clover, vetches, or 

 some such Leguminous plants in regular 

 rotation with other crops in successive 

 years. The roots of these plants are 

 ploughed in after the surface crop has been 

 taken, with the result that the soil appears 

 to be enriched. Culture experiments on 

 such plants have shown that they are able 

 to obtain their nitrogen -supply from the 

 atmosphere, and that this capacity is re- 

 lated to certain swellings (tubercles) present 

 upon their roots. (Fig. 77.) These consist 

 of tissues connected with the cortex of the 

 root, and traversed by vascular strands. 

 Their cells are densely protoplasmic, and 

 are crowded with foreign organisms of 

 b acterial nature (Baiillus radicicola] . These 

 gain access to the tissues of the young root 

 through the root-hairs, and multiply rapidly 

 in the tissue outside the stele. The invaded 

 cells grow, and divide till a large tubercle is formed. When the roots 

 bear these tubercles, it is found by culture experiments that the plant 

 as a whole can increase its supply of combined nitrogen, and the only 

 source of its supply -must be the free nitrogen of the air. The way in 

 which this is effected is by a mutualism between the Bacillus and the 

 cells of the root. The latter supply the carbohydrate which the 

 Bacillus requires. This like a few other bacteria, can use atmospheric 

 nitrogen in the formation of proteids, and flourishes greatly. But 

 finally a large proportion of the hypertrophied bacterial cells are 



FIG. 77- 



Root of Vicia Faba, with nu- 

 merous root-tubercles. Reduced. 

 (After Strasburger.) 



