September 30, 1904.] 



SCIENCE. 



431 



During the twenty-five years from 1866 

 to 1891 observations and experiments by 

 several investigators upon peculiar swell- 

 ings on the roots, especially of leguminous 

 or pod-bearing plants, resulted in showing 

 that these are inhabited by bacteria, and 

 that the green plants having them do in 

 some way gain their needed nitrogen from 

 the air when it is not available from the 

 soil. This was a great discovery. It had 

 long been known that these plants rendered 

 soil more fertile for other crops afterward 

 grown on the same land, and rotations had 

 long been practised with clover as one of 

 the crops for its fertilizing effects. How 

 the clover acted as a fertilizer was not 

 known iintil about fifteen years ago. Since 

 that time our knowledge iipon the subject 

 has very considerably increased and every 

 year now something more is learned. 



The facts seem to be in general some- 

 what as follows : legTiminous plants un- 

 aided have no more ability to help them- 

 selves to the nitrogen of the air under any 

 circumstances than have other green plants. 

 Certain bacteria existing in the soil pene- 

 trate the soft tissues of young roots of 

 legumes and multiply within these living 

 tissues so as to form a little mass of gela- 

 tinous substance. Responding to the irri- 

 tation produced, the plant builds a nodular 

 structure about the bacterial invaders, not 

 unlike the formation of a gall consequent 

 upon the sting of an insect. Numerous 

 nodules may be formed on the roots of one 

 plant and they have characteristics pe- 

 culiar to the species of plant on which they 

 occur. Some are characteristically much 

 larger than others, varying from the size of 

 the smallest seeds to more than that of 

 garden peas, or they grow in different 

 shapes, or, have different forms of group- 

 ing, etc. In some way not well under- 

 stood these plants with nodules or tubercles 

 are capable of getting sufficient nitrogen 



from the air abundantly to serve their pur- 

 poses when the soil has no supplies, and 

 when other plants must die from the want 

 of it. Either the bacteria are direct agents 

 in the process or they aid the plant itself 

 to do what without such aid it can not do. 



The number of bacteria in one of these 

 root tubercles is very large, altogether 

 countless, yet the total number thus dwell- 

 ing within the roots is insignificant when 

 compared with the number in the soil out- 

 side. Biit these in the root are specifically 

 dift'erent from those, and seem to have a 

 decidedly different office to fulfil. As was 

 said above, they are not common soil bac- 

 teria. They may, and sometimes do, live 

 even some years in the soil outside of living 

 roots, but they never become numerous 

 among the other kinds and are often en- 

 tirely wanting, however rich the soil may 

 be otherwise. 



There is a disputed question whether 

 these root-dwelling bacteria all belong to 

 one species with some temporary variation 

 into races each adapted to a special kind 

 of leguminous plant, or whether there are 

 numerous species, almost as many as there 

 are genera of legumes, upon which they are 

 found. This may be thought to be an un- 

 profitable inquiry, but it may mean more 

 than we see at first glance. There is no 

 longer any question but that there is some 

 kind of difference among them. Those 

 that form tubercles on the roots of clover 

 fail entirely to develop on garden peas and 

 on soy beans, and those that accompany 

 soy beans appear to have no effect upon 

 alfalfa. Some of them grow differently in 

 artificial cultures and appear to have, 

 under the microscope, various different 

 characteristics. In certain instances it has 

 been shown, however, that kinds usually 

 associated with certain legumes can become 

 adapted to others which they did not previ- 

 ously accompany, and this has been held 



