234 



botanical gazette. [ September, 



The puzzling and much-debated question regarding the nitrogen as- 

 similation of plants has been receiving much attention during the last few 

 years. In all the discussion the behavior of the Leguminosse cuts an im- 

 portant figure. Recall the references in this journal recently to papers 

 on the root-tubercles of these plants. The publication in November, 

 1888, of a paper by Hellriegel and Willfarth seems to have given fresh 

 impetus to the study and discussion of this interesting problem. The 

 conclusions reached by these experimenters (continuing HellriegeFs ear- 

 lier researches) were essentially these: 



The Leguminosse differ markedly from other plants (particularly 

 grasses with which they were compared) in their N assimilation, 

 since the latter depend entirely on the N in the soil for their 

 supply, while the former have an additional source in the free N 

 of the air. This they are enabled to utilize, not by any power of their 

 own, but through the low organisms (as to whose nature nothing is de- 

 cided) which enter into symbiotic relations with the roots, and thus give 

 rise to the tubercles. These tubercles are absent from the roots of 

 Leguminosae grown in sterilized soil, and are formed when to such soil 

 a small quantity of an infusion of unsterilized soil is added. Plants from 

 which such tubercles are absent can not assimilate free nitrogen. 



These conclusions as to the great difference between the N 

 assimilation of Leguminosse and other plants are strongly controverted 

 by Frank 3 , who points out that Joulie, in 1885, determined the fixation 

 of free N by ray grass and buckwheat, while he himself has demon- 

 strated the same for some of the simple algae. He adds details of recent 

 experiments with oats showing a like N accumulation. In the ex- 

 periment with oats the -N in the soil was at the beginning 0.118 per 

 cent. After raising nineteen strong plants, bearing 530 ripe seeds, an- 

 alysis showed 0.131 per cent, in soil. This increase was probably due to 

 refuse from the plants, such as the root-hairs and finer rootlets. The N 

 in the seed sown was 0.0142 gm.; in the crop, 0.487 gm. In a vessel con- 

 taining the same quantity and quality of soil treated in the same way, 

 but having no plants in it, the percentage of N at the conclusion of the 

 experiment was 0.110. Hellriegers negative results Frank ascribes to 

 the lack of vigor in his culture plants, and asserts that plants will not 

 assimilate N from either soil or air unless they are strong and healthy. 

 He also claims at length the absence of exact proof that the root-tubercles 

 of Leguminosae are organs of assimilation. Hellriegel's case on this 

 point rests on the observation that among peas grown on a soil free from 

 N some starved, while others subsequently recovered and grew well. In 

 subsequent experiments this recovery was hastened by watering the 

 plants with a minute quantity of an infusion of field soil. From this 

 Hellriegel infers that the peas were unable to assimilate the free N until 

 the tubercles were formed by the infection of the roots from the infusion, 

 after which the plants grew well. Frank, however, interprets the experi- 

 ment to mean that the peas were unable to assimilate free N because they 

 could not, under the conditions, attain sufficient vigor. He further ar- 

 gues a priori against the probability of Hellriegel's theory, and shows the 

 dissimilarity between the root tubercles and mycorhiza, with which they 

 have been compared. 



*Ueberden gegenwartigen Stand unserer Kenntnisse der Assimilation elementaren 

 Stickstoffs durch die Pflanze.— Ber. d. D. bot. Gesells., vii. 234. 



