Scientific Agriculture. 



326 



[October, 1909. 



added, in addition, just at the stage 

 when the seedlings were dying of 

 nitrogen hunger, small quantities of a 

 watery extract of ordinary fertile soil. 

 As a result, the young plants so treated 

 recovered and grew to maturity, their 

 roots in every case developing nodules 

 similar to those already referred to. 



Hellriegel and Wilfarth were, there- 

 fore, bound to conclude that the power 

 possessed by peas and other leguminous 

 plants of appropriating free nitrogen 

 from the air, and utilizing it in building 

 up their own tissues, was dependent 

 upon the formation of nodules upon the 

 roots of these plants. The experiments 

 in which the addition of watery extracts 

 of fertile soil resulted in the revival and 

 renewed growth of the seedlings pre- 

 viously dying from nitrogen starvation, 

 led further to the belief that the 

 nodules themselves were due to inocu- 

 lation of the soil— and hence of the roots 

 of the plants — with some species of 

 micro-organism. Other investigators, 

 notably Lawes and Gilbert, confirmed 

 the work of Hellriegel and Wilfarth, 

 the results of which were published in 

 1886. A further step in advance was 

 made by Beyerinck, who demonstrated 

 that the root nodules on leguminous 

 plants were full of bacteria, which could 

 be cultivated also on various artificial 

 media. Beyerinck gave the name 

 Pseudomonas radicicola to the bacteria 

 in question. 



The Pseudomonas bacteria appear to 

 be widely distributed in most fertile 

 soils, although they naturally exist 

 in much greater quantity in laud on 

 which legume crops are frequently 

 grown. The organisms, as grown in 

 artificial non-nitrogenous culture media, 

 are observed to be exceedingly minute 

 in size, rod-shaped, and generally in 

 rapid motion. They exist free in the 

 soil in this form, and enter the host plant 

 through the root hairs, forming tuber- 

 cles or nodules on the younger rootlets 

 only. Within the nodules, however, 

 the bacteria frequently undergo consi- 

 derable modification, and in different 

 plants they assume rather large rod- or 

 Y-shaped forms. The irregular forms 

 are especially numerous in the older 

 tubercles. At the same time they in- 

 crease enormously in number. 



The investigations carried out, as des 

 cribed above, enabled a conclusion to be 

 finally reached which afforded a satis- 

 factory explanation of the observed 

 facts, and of the contrary results which 

 had at different times been obtained in 

 experiments on the matter. It was 

 evident that the bacteria in the root 

 nodules were the agency by which legu- 

 minous plants were enabled to assimilate 



nitrogen from the air, and that, in the 

 absence of these organisms, peas, clover, 

 etc., were as dependent upon the supply 

 of combined nitrogen in the soil as plants 

 of other botanic families, e.g., sugar- 

 cane, cotton, etc, Since the relation- 

 ship of the nitrogen-assimilating bac- 

 teria to the legume cannot be regarded 

 as in any way parasitic, but is rather a 

 physiological partnership of mutual ad- 

 vantage, the term symbiosis (literally 

 ' living together ') has been applied to 

 describe it. The plant supplies the 

 necessary carbohydrate material which 

 the bacteria require for their life pur- 

 poses, and the micro-organism in turn 

 gathers from the air nitrogen which is 

 ultimately utilized by the legume plant. 

 It may here be mentioned that the 

 mechanism of the actual process by 

 which the plant avails itself of the nitro- 

 gen compounds elaborated by the micro- 

 organisms is still incompletly under- 

 stood. It is possible that the host plant 

 may attack and absorb the highly 

 nitrogenous bacteria, or, on the other 

 hand, it may avail itself of the soluble 

 and diffusible substances formed within 

 the bacterial cell. The latter appears 

 the more probable explanation, since 

 before the plant could utilize the nitro- 

 genous compounds forming the sub- 

 stance of the bacterial organisms, it 

 would be necessary that the latter be 

 first broken down and dissolved. They 

 would, in tact, have to be brought into 

 a condition in which the plant could 

 absorb them as food, by a process ana- 

 logous to that in which the insoluble 

 food supply in seeds is dissolved for the 

 benefit of the developing embryo on the 

 germination of the seed. This process 

 of solution is effected by means of 

 various ferments or enzymes, but, so far, 

 no proteolytic ferment (i.e., one capable 

 of dissolving proteid or nitrogen com- 

 pounds) has been discovered in legu- 

 minous plants, and therefore there is 

 little basis for assuming that these plants 

 secure the nitrogen collected by the 

 bacteria in their root nodules by dissolv- 

 ing and absorbing the latter. 



The figures which have been placed on 

 record showing the actual amounts of 

 uitrogen added to the soil, as the result 

 of growing various leguminous crops, 

 are useful as giving a clear idea of the 

 value of including such crops in farm 

 and estate rotations. Even in the 

 course of a single season these quantities 

 of nitrogen may be quite large. Experi- 

 ments with velvet beans in Alabama 

 showed a gain of nitrogen amounting to 

 213 lb, per acre, while with the same 

 crop, gains of 172 lb. and 141 lb. per acre 

 were obtained in Louisiana and Florida 

 respectively, Trials with cowpeas have 



