THE ROOT 



105 



plant, and we may therefore dwell on it at some length. 

 As has already been said, saltpetre manures, while 

 they exercise such an essential influence upon cereals, 

 sometimes remain entirely without effect upon legu- 

 minous plants. One such experiment ^ is represented in 

 fig. 29 (lower half) . Two pots of oats (marked by the 

 letters KP.) received all 

 the necessary mineral man- 

 ures with the exception of 

 saltpetre ; two other pots 

 (marked by the letters 

 KPS.) received the same 

 mineral manures with the 

 addition of saltpetre : the 

 result speaks for itself. A 

 similar experiment was 

 made with peas (at the 

 top of the figure), and the 

 result was negative ; the 

 presence of saltpetre was 

 not manifested in any way 

 whatever. This means that 

 peas can procure nitrogen 

 for themselves, even if it is 

 not present in the soil. 



Evidently beans can ob- 

 tain nitrogen from the air ; 

 but under what circum- 

 stances ? In raising this 

 question investigators recalled 

 small swellings were noticed 

 on the roots of leguminous plants. These nodules 

 (fig. 30) appear in consequence of the roots becom- 

 ing infected by certain bacteria, apparently widely 

 distributed in the soil. This can be proved very simply. 

 A leguminous plant is grown in an aqueous solution in 



^ One of Professor P. Wagner's remarkable experiments. 



Fig. 30. 



the fact that certain 

 even by the ancients 



