The Sources of the Nitrogen of onr Le<jumi7iovs Croj)s. 693 
nitrogen is fixed, to any material extent, by microbes within the 
soil, independently of leguminous growth, there is evidence that 
in soils and subsoils containing organic nitrogen, lower organisms 
may serve the higher plants by taking up or attacking and 
1 bringing into a more readily available condition combined nitrogen 
I not otherwise, or only very slowly, available for the higher plants. 
For example, it is probable that fungi generally derive nitrogen 
I from organic nitrogen ; and in the case of those of fairy rings 
there can be little doubt that they take up from the soil organic 
nitrogen which is not available to the meadow plants, and that 
j on their decay their nitrogen becomes available to the associated 
! herbage. Then in the case of the fungus mantle, observed by 
Frank on the roots of certain trees, it may be supposed that the 
: fungus takes up organic nitrogen, and so becomes the medium 
j of the supply of the soil nitrogen to the plant. More pertinent 
i still is the action of the nitrifying organisms in rendering the 
organic nitrogen of the soil and subsoil available to the higher 
plants. It may well be supposed, therefore, that there may be 
other cases in which lower organisms may serve the higher, 
bringing into a more available condition the combined nitrogen 
already existing, but in a comparatively inert state, in soils and 
subsoils. 
But to return to the question of the explanation of the un- 
doubted fixation of free nitrogen in the growth of leguminous 
crops under the influence of suitable microbe infection, and of the 
development of nodules on the roots of the plants. 
As in the exact quantitative series of experiments made at 
Rothamsted in 1888 and since, some of the results of which have 
been given, the plants were not taken up until they were nearly 
ripe, it is obvious that the roots and their nodules could not be 
examined during growth, but only at the conclusion, when it is 
to be supposed that the contents of the nodules would be to a 
great extent exhausted. Another series was therefore com- 
menced in 1890, and is still in progress, in which the same four 
annuals — peas, beans, vetches, and yellow lupins — and the same 
four plants of longer life — white clover, red clover, sainfoin, and 
lucerne — were grown in specially-made pits, so arranged that 
some of the plants of each description could be taken up, and 
their roots and nodules studied, at successive periods of growth : 
the annuals at three periods — namely, first when active vegeta- 
tion was well established, secondly when it was supposed that 
the point of maximum accumulation had been approximately 
reached, and thirdly when nearly ripe ; and the plants of longer 
life at four periods — namely, at the end of the first year, and in 
the second year when active vegetation was re-established, when 
