Sources of Nitrogen in Plants. 343 
in question ; moreover, the grasses cultivated under the 
same conditions showed that the soil &c. could not have 
yielded the nitrogen. 
Thus we must look to the atmosphere. Now the only 
conceivable sources of nitrogen yielded by the atmosphere 
are (1) the free nitrogen, (2) nitric acid, (3) salts of ammonia 
(carbonate and nitrate). Hence we must either assume that 
the Leguminosae have an extraordinary capacity for col- 
lecting and absorbing the nitrogen compounds from the atmo- 
sphere, or we must admit that the Leguminosae are in some 
way able to make use of the free nitrogen of the atmosphere. 
Enormous difficulties stand in the way of direct proof. 
First, the author asks us to consider the following further 
observations. When peas are cultivated in a sand devoid 
of nitrogen as above, two remarkably sharp periods of de- 
velopment are to be noticed. 
Up to the period when the reserve-materials are exhausted, 
the seedlings grow normally, luxuriantly, and with normal 
colour. But directly the reserves are exhausted, a some- 
what sudden change occurs — growth stops, the leaves turn 
pale, and the plant evidently begins to starve. 
Sooner or later, however, the pale or yellow leaves again 
turn green, and a second period of growth begins , and the 
plants go on growing normally to the end. 
The sharply marked starvation is not reconcilable with the 
view that the peas take their nitrogen directly from the above 
compounds in the air. When the reserves begin to be ex- 
hausted the plants have each about six leaflets ; how are we 
to explain that these six leaflets suddenly and so completely 
fail, and that just at this particular period the plant becomes un- 
able to use the nitrogen supplied ? and further, to explain why 
and how, after a pause, the plant begins to acquire nitrogen ? 
We are then asked to note the following observations. 
When the above cultivation experiment is repeated on a 
large scale, it is noticed that the development of the indi- 
vidual plants — all under the same conditions — is very un- 
equal. Some, usually few, grow very vigorously as said ; 
