347 
Sources of Nitrogen in Plants. 
On the other hand, the yield of the plants in four vessels 
supplied with the £ bacteria ’ (and these were not the best) was 
in every case more than 1 5 gr. dry substance : — 
Hellriegel then points out that the uniformly and decided 
large yield in the latter cases must have been due to the 
co-operation of the micro-organisms, for since in each case 
the 25 cm. of fluid added contained less than 1 mg. of nitrogen, 
we cannot suppose it due to that. 
Two cultivations were made in soil without nitrogen, to 
which nutritive materials and 25 c.cm. of the above soil- 
washings were added, and then the whole sterilised by 
heating, then the seeds sown, and then covered with a layer 
of sterilised wadding. All went well until the development 
of the sixth leaflet, and the setting in of the starvation-phase. 
The plants never recovered, however; all died. ‘No trace 
of tubercles was to be found on their roots.’ 
Passing over other experiments, which lead to the same 
general result, I may sum up Professor Hellriegel’s results 
in his own words : — 
‘ The Papilionaceae, in contrast to the Gramineae, are not 
dependent on the soil for their nitrogenous nutrition ; the 
sources of nitrogen afforded by the atmosphere have for these 
plants the highest importance, and are alone sufficient to 
bring them to normal and even luxuriant development.’ 
‘ It is seen that not one of these observations supports 
the idea that the sources of nutriment of plants are to be 
sought in the small quantities of combined nitrogen which 
are found in the atmosphere, and thus probably the only 
remaining assumption is that the Papilionaceae have the 
power of making use of the free nitrogen of the air.’ 
‘To the nutrition of the Papilionaceae, and especially to 
B b 
