Sources of Nitrogen in Plants. 341 
blem ; and the following is a short summary of his chief 
experimental results. 
When graminaceous plants were sown and allowed to grow 
in a soil devoid of nitrogen, but to which all other necessary 
minerals were added in proper quantities, they developed 
normally until the third leaf appeared and the reserves were 
exhausted. The experiments were conducted in the open, 
care being taken that no rain fell on the plants, &c. Then 
the ‘ production ’ ceased suddenly. But the plants did not 
die — they lived as long as normal plants, only their vegetation 
was dwarfed. The stunted plant developed stunted and 
miserable organs (even barren ears), and struggled on through 
the season : the total dry weight increased very little, and 
this concerned the non-nitrogenous constituents only. 
If nitrates are added at the moment when the above arrest 
of development sets in, the grasses go on growing normally 
again, and if sufficient is added the recovery is complete ; if 
insufficient, a gradual passage to the starved condition sets in 
again. Hellriegel also finds that there is a direct proportion 
between the amount of nitrates added and the yield of grain, 
up to a certain point of course. 
If ammonia salts or other nitrogenous compounds are used 
instead of the nitrates, the above proportion does not make 
itself evident, and the author finds that a pause ensues be- 
tween the addition of these salts and their employment by 
the grasses — it is concluded that the above-named nitro- 
genous compounds have to be oxidised to nitrates before 
they can be used by the grasses. In other words, nitrification 
must be accomplished in the soil before the grass roots can 
employ the manure used. 
Summing up the above results. The Gramineae are 
entirely dependent on the soil for their nitrogen : the atmo- 
sphere cannot furnish them with nitrogenous food, except in so 
far as rain or dew carry down nitrogenous compounds to the soil. 
The most useful source of nitrogen for Gramineae is a salt 
of nitric acid, and nitrates supply them easily and completely. 
They employ the nitrates directly, and the yield of grain &c. 
