76 



SCIENCE-GOSSIP. 



ROOT-NODULES OF LEGUMINOUS PLANTS. 



By Rudolf Beer, F.L.S. 



TF the roots of a bean, pea, clover or almost 

 any other leguminous plant be examined, 

 a number of curious nodules or tuberous swell- 

 ings will be noticed upon them 

 (fig. i). These tuberosities have 

 an intimate connection with a 

 physiological process of great 

 importance which takes place in 

 the leguminous plant. 



All vegetation, high or low, 

 requires nitrogen as an item of its 

 food -stuff. The form in which 

 this nitrogen is available to the 

 plant, whether in the free con- 

 dition or combined with other 

 elements, differs to some extent 

 with the plant we are considering. 

 For the present all we need know 

 of this is that green plants, from 

 the Algae upwards, cannot utilise 

 free nitrogen as a food material, 

 and that it is only when this 

 element is united with oxygen in 

 a certain proportion, to form what 

 is known to the chemist as a 

 nitrate, that it is of nutritive 

 value. From this it follows that 

 the nitrogen of the air in which 

 the plant grows is, so to speak, 

 thrown away upon at least the greater part of the 

 vegetation of the earth. This is a point which has 

 been put beyond all doubt by 

 the experiments of Boussingault, 

 Lawes and Gilbert, Pugh and 

 others. 



The useful nitrates are evidently 

 obtained by the plant from the 

 soil, but since no great store of 

 these is to be found here, a 

 somewhat difficult problem was 

 presented to the physiologist. 

 The only reasonable explanation 

 seemed to be that the nitrates 

 should be re-formed as fast as 

 they were taken up by the plants. 

 But although this shuffled the 

 difficulty off the shoulders of the 

 biologist, it was only to place it 

 all the more heavily upon those 

 of the chemist. The formation 

 of nitrates, either from the 

 elements nitrogen and oxygen or 

 from oxygen and ammonia (a compound of nitrogen 

 and hydrogen), is, chemically speaking, a most 

 difficult undertaking. Much fruitless speculation 



Fig. i. — Root of red clover showing 

 root-nodules. 



Fig 2. — Transverse section from root- 

 nodule of scarlet-runner {Phaseolus 

 mnltiflorus). 



took place in explanation of this, but it was not 

 till 1877 that the observations of two chemists, 

 Schloesing and Miintz, gave quite a new aspect 

 to this tangled question. They 

 kept samples of soil under 

 observation | for many weeks ; 

 analysing each sample after the 

 experiment, and knowing its 

 constitution before, the result 

 was that they found a noticeable 

 nitrate-increase in every instance. 

 If, on the other hand, they treated 

 the soil at the commencement of 

 their experiments with an anti- 

 septic, or subjected it to great 

 heat, the quantity of nitrates in it 

 remained the same after as before 

 the experiment. From these 

 observations they inferred that 

 the power of nitrate formation 

 resided in the soil and was due 

 to living organisms in it, probably 

 bacteria. This hypothesis, as 

 unexpected as it was strange, 

 opened up an hitherto untraversed 

 path of research, which was 

 followed in the ensuing years by 

 Winogradsky and Frankland. In 

 1890, both these investigators 

 almost simultaneously, succeeded in isolating these 

 soil-bacteria which previously had only existed in 

 theory. 



A little further observation 

 showed, however, that only half 

 the problem had been solved. 

 What these isolated bacteria 

 could effect was the partial 

 oxidation of ammonia, which is 

 abundantly present in the soil, 

 to the intermediate stage of a 

 nitrite. In order to furnish the 

 nitrogen compound available to 

 the plant it was necessary still 

 further to oxidise this first-formed 

 compound, so that instead of 

 containing only two atoms of 

 oxygen of the nitrite it contained 

 the three of a nitrate. It was stated 

 just now that only half the problem 

 had been solved, but it was by 

 far the most unaccountable half 

 which was now cleared up. 

 Only the most powerful agents at the command 

 of the chemist (such as ozone) were known to 

 oxidise ammonia to a nitrite, NH 3 + 3 = HN0 2 



