>18 Agricultural Joutmal of Victoria. 



opinion being that members of this family, owing to the unnsual 

 length and strength of their root system, were able to draw upon a 

 store of food that was not available to wheat and corn and other crops 

 not belonging to the pod-bearing group. It is only within a com- 

 paratively recent time that the real cause of the beneficial effect of 

 these legumes has been fully understood, and it seems that here again 

 the bacteria are responsible for the nitrogen-gathering power ; for it 

 is because these plants are able to fix and use the free nitrogen of 

 the air that they are of such benefit in rotation and in reviving 

 poor and exhausted land. The immense yields of wheat following 

 alfalfa or clover are easily understood when it is realized that there 

 has actually been added to the soil a certain definite amount of 

 nitrogen in such form that the wheat can be benefited by it. Such 

 etticient users of the atmospheric nitrogen are clover and peas and 

 similar crops that they can actually live and thrive in a soil that has 

 not the first trace of combined nitrogen within it. If quartz sand be 

 ignited to red heat, thus burning out all the nitrates, and then be 

 ])lanted with peas or beans, it is possible to bring these plants to full 

 maturity without in any way allowing a particle of fixed nitrogen to 

 find its way into the soil. On the other hand, wheat or ])otatoes, 

 or crops not legumes, will die as soon as the small amount of nitrogen 

 available from the seed is exhausted. What is the reason for this? 

 It can not be merely a difference in the length or extent of the root 

 system, because plants flourish where it is certain there are no avail- 

 able nitrates whatever. For a long time the pi-esence of certain 

 peculiar nodules or tubercles upon the legumes has been noted and 

 speculated upon. These formations are always present upon the 

 roots of leguminous plants grown under proper conditions, and may 

 vary in size from that of the smallest pin head, in some clovers, to a 

 cluster as large as a potato. They have been thought to be due to 

 the bites of worms or insects, or to be caused by conditions of the 

 soil and various abnormal climatic effects, and only within very recent 

 years has it been learned that these formations ai*e due to the pre- 

 sence of innumerable bacteina, and that unless these tubercle-pro- 

 ducing bacteria exist the plant is no more able to use the nitrogen 

 from tlie air than wheat or any of the other crops which do not have 

 such nodules on their roots. 



Microscopic Appearance of Tubercles. 



Just where the nitrogen is fixed and how it is used by the plant 

 have been debated questions. Some have supposed that the presence 

 of the bacteria in the roots simply acted as a stimulus, and that the 

 leaves of the plant were thus able to take in nitrogen as a gas and to 

 elaborate nitrates from it in some such way as carbon is formed from 

 carbon dioxid. It seems much more probable, however, that the 

 bacteria themselves fix the nitrogen in the roots of the plant and that 

 it is then used as nitrates would be used from the soil. It is certain 

 that these tubercle organisms can fix free nitrogen in cultures, and 

 there is no reason to suppose that this power is lost when within the 



