BIOLOGY OF THE ROOT-NODULES 95 



into the inner tissues of the root (Fig. 21, a & b\ And now the plant begins 

 visibly to react to the stimulus. Many of the cells of the root become 

 larger, and carbohydrates and asparagin are brought down from the upper 

 parts of the plant to be given unstintingly to the micro-organisms, whose 

 accelerated growth and multiplication soon become manifest in the fast 

 developing nodules. The root-nodules are nothing else than pure cultures 

 of bacteria maintained by the plant for its own ends. The microbes, which 

 grow at first at the expense of the tissues they live in, become before long 

 more independent and draw their nitrogen from the atmosphere. Carbon 

 they most likely obtain the whole summer through from the plant, which 

 probably also secretes the ferment by which the starch is changed into 

 assimilable sugar. 



The root-nodules are now in full activity, the bacteria in the cells 

 absorbing nitrogen from the air in the intercellular spaces (63), and storing it 

 up in their own bodies. But this state of exuberant vitality does not last. 

 From overcrowding and other causes the multiplication of the bacteria 

 becomes less rapid and bacteroids (involution forms) appear, a sign that the 

 conditions of life are becoming less favourable. The plant asserts itself, and 

 begins to absorb the substance of the bacterial cells, conveying the nitrogen 

 from the root-nodules into the growing seeds. In the lupine at the time of 

 flowering the nodules contain 5-2 per cent. N, which then becomes gradually 

 less until, by the time the pods are ripe, only 1-7 per cent, is present. It is the 

 nodules alone which yield up their nitrogen in this way, the amount contained 

 in the other parts of the root remaining from first to last unchanged, namely 

 about 1-6 per cent. The precise way in which the leguminous plant absorbs 

 the bacterial cells is not clearly understood, but in all probability it dissolves 

 them by means of a peptonizing enzyme. Only a remnant of the bacteria 

 return to the soil when the root decays, the greater number are literally 

 eaten up by the plant. To call this symbiosis is certainly a misapplication 

 of the term *. The carbohydrates and asparagin that the plant offers the 

 bacteria in the beginning are nothing but a usurer's loan, for in the shape of 

 valuable nitrogen they are demanded back again later on with heavy 

 interest. Looked at from this standpoint, the view that the leguminous plant 

 is parasitic on the bacteria no longer appears absurd. It is obliged to ' put 

 itself outside ' its hosts, just as the fungus is obliged to enclose the algae in 

 the lichen thallus. But, whilst the lichen fungus is completely parasitic, the 

 leguminous plant is only partly so. It only derives its nitrogen from its host, 

 supplying its other needs (carbohydrates, mineral salts, &c.) in the same 

 way as all ordinary plants, from which it differs indeed only in being unable 



* [It should be pointed out that the author's views of the relations between the organisms here 

 and in the Lichens are not those accepted by botanists generally. See, for example, Pfeffer, 

 Physiology of Plants, Vol. I, Engl. ed., pp. 364 and 371.] 



