ASSIMILATION OF FREE NITROGEN 89 



detailed investigations into the nutrition of the Leguminosae. That the Legu- 

 minosae could grow in soil poor in nitrogen and thrive thereon, even without 

 nitrogenous manuring, had long been known, and it has now been demon- 

 strated that they take up nitrogen from the air (59) and convey it to the soil. 

 This enrichment is particularly evident when the plants are ploughed in. , 

 All other plants, all our cereals and food crops, are, as regards the soil, 

 merely consumers of nitrogen, since they are unable to take it up in any 

 form but that of the nitrates of the soil. 



The difference is seen even in the amount of nitrogen present in the 

 tissues of the two groups of plants. Lupine seeds contain 5-7 per cent. N, 

 wheat grains only a-i per cent., lupine haulms 0-94 per cent., wheat straw only 

 0-5 per cent. An experiment with peas showed that a quantity containing 

 16 mgr. of nitrogen gave rise to a crop containing 499 mgr., whilst the 

 quantity of nitrogen in the 4 kilograms of mould they grew in had increased 

 from 22 to 57 milligrams ; a total gain of 518 mgr. of nitrogen. Calculated 

 for larger quantities of plants it will be at once seen that these figures 

 represent an enormous profit. One hectare (=2-4 acres) of lupines is cal- 

 culated to enrich the earth by 227 kilograms of nitrogen annually. The 

 amount gained by the meteoric fixation of nitrogen (production of HN0 2 

 and HNOg during thunderstorms) would for the same area be only 0-09 to 

 i-8 kilogram per annum, so that there is no doubt that it is the atmosphere, 

 and the atmosphere alone, from which these plants extract the gas. 



Seeing that no other cultivated plants* (not even Brassica alba, the 

 White Mustard) are able to act in this way, we apparently have in the Legu- 

 minosae a remarkable and unique group of organisms. But we shall be 

 wrong if we attribute the power of fixing free nitrogen to the tissues of the 

 plant itself. They are powerless to do this, and behave towards nitrogen in 

 no way differently from the tissues of non-leguminous plants. Not until 

 they enter into partnership with certain bacteria, the bacteria of the root- 

 nodules, do the plants act as usurers of nitrogen, gathering and storing it 

 and growing ever richer. The nodules (60) which arise upon the roots 

 of the seedling plants when they are a few weeks old are minute white or 

 pinkish excrescences that soon increase in size and sometimes give the roots 

 a distorted appearance, as though they were attacked by galls (Fig. 19, 

 a and b). At first hard and smooth, they become wrinkled as the foliage 

 of the plant grows and, by the time the pods are ripe, are shrivelled and 

 cracked. Their dried remains rot in the ground with the rest of the root. 

 The nodules are either separate excrescences on the surface of the root, 

 with whose vessels they are connected by a tiny strand of vascular fibres, or 

 the root itself swells up in places. In both cases there is always a close 

 connexion between the cells of the nodule and the conductive tissue of the 



* The nodule-bearing alder and Elaeagnus are perhaps exceptions. 



