October 1. 1900.] 



KNOWLEDGE. 



237 



Suffering of Veget-ables to die into the Ground ; And 

 so to Fatten it; As the Stubblo of Corne, Especially 

 Pease." Equally beneficial with " Pease " are other 

 crops, such as clover, belonging to the Natural order 

 Leguminosje (so-called because its members produce a 

 fruit known as the "legume" or "pod"). The roots 



A Lupine Plant, reduced from a drawing by Mi?s E. E. Praft, 

 ahowing the tubercles \t) on the roots. 



of many plants belonging to this order are swollen or 

 knotted at intervals like that of the garden Lupine 

 shown in the figure. Similar tubercles are sometimes 

 found also on the roots of the Alder and a few other 

 less familiar trees and shrubs. Needless to say, they 

 were observed long before their importance to the plant 

 was realised. On examining a section cut through one 

 of them, it is seen that the tissues of the root have been 

 stimulated to increased growth, causing the part affected 

 to swell. Such an abnormal growth of an organ is often 

 caused by the presence within it of a foreign organism. 

 And so it is in this case, for in the cells of the tubercle 

 IS a multitude of minute residential organisms to which 

 we hesitate to apply a name. In some stages of their 

 existence they resemble bacteria, by which name they 

 are frequently called. Their exact nature is, however, 



still under discussion ; it is certain that they are not 

 true bacteria, but probably the degraded descendants 

 of ancestors belonging to one of the lower groups of the 

 Fungi. The work done by these tubercle-organisms has 

 been studied only in a few members of the Leguminosa?, 

 but it is probable that wherever they are found they are 

 equally important to the plant in whose roots they live. 

 On some roots the tubercles are found in large numbers ; 

 more than 4,500 have been counted on a. single pea- 

 plant. 



In a previous articlcft it has been noticed that most 

 green plants obtain their Nitrogen in the form of 

 Nitrates (i.e., oxidised Nitrogen), and that they are 

 unable to make use of the free Nitrogen of the atmo- 

 sphere. When certain leguminous plants, such as the 

 Lupine, arc gi-own in soil which contains no Nitrates, 

 they may die from Nitrogen starvation, or, on the other 

 hand, they are more likely to grow in normal luxuriance. 

 If the plants which die under these circumstances arc 

 examined their root.s are found to be (jiiite free from 

 tubercles; while the roots of those which flourish are 

 invariably provided with tubercles, in the cells of which 

 is the usual population of organisms. If the soil is 

 strongly heated and then allowed to cool before the 

 Lupine is planted in it, and afterwards carefully pro- 

 tected so that no impurities from outside may reach it, 

 the roots grown in it do not become swollen, i.e., they 

 are not invaded by the organism which causes the 

 swellings. The organism is present in the soil, and 

 under ordinary circumstances " infects " certain roots, 

 such as those of the Lupine; but if the soil has been 

 previously heated, bacteria and other living iiihabitaiits 

 are killed, and roots grown in it afterwards do not 

 become infected. What the Lupine and similar plants 

 require in the soil is not an abundant supply of Nitrates 

 but the presence of the minute organism which infects 

 and establishes colonies in its roots. 



The facts of the last paragraph clearly indicate that 

 the work of the rootorganism consists in the production 

 of compounds of Nitrogen which can be made use of 

 by the plant which gives it lodging and partial " board." 

 As a result of much careful and reliable investigation 

 we now know that the free Nitrogen of the atmosphere|]: 

 is oxidised by the activity of these humble guests, and 

 the resulting Nitrates are passed on to the hostplant. 

 The latter not only houses its guests during their life- 

 time but apparently also consumes their remains after 

 death, thereby doubtless making use of the stores of 

 accumulated nitrogenous compounds in their bodies. 

 This is one of the most promising fields of future investi- 

 gation, and many interesting results may yet be expected 

 from it. For the present, we must realise that we know 

 very little about the details of the co-operation between 

 the flowei-ing plant and the soil-organism — a union 

 which was not su.spccted two or three decades ago. We 

 are at least certain that some leguminous plants are 

 rendered independent of supplies of Nitrates in the soil 

 by reason of the activity of minute soil-organisms which 

 reside in their roots, assimilate free atmospheric Nitro- 

 gen and pass it on to their host in an oxidised state. 

 At the end of the growing season the root of such a 

 plant contains a store of nitrogenous substances ready 

 to be absorbed by plants whoso existence depends upon 

 a supply of oxidised Nitrogen. The roots of a crop 

 of leguminous plants such as " Pease," left to decay in 



tt July, 1900. 



XX AH Boils contain air, and therefore free Nitrogen. ,See figure 

 in Kmowledqe, July, 1900, p. 160. 



