ill its Relations to Chemistry and Gcohyy. 
233 
to benefit this or that crop. How far, also, ought the physical 
or mechanical condition and chemical composition of the soil to 
modify this state? At what period of the year or of the plants' 
growth are substances best applied? Are they best laid on all at 
once, or at successive periods? On none of these points have we 
as yet any clear scientific information ; and you see at once that 
the conjoined aid of both field and laboratory will be necessary if 
we are ever to obtain it. 
d. But more important, if possible, still are the questions which 
remain unresolved in regard to the nitrogen of plants. What are 
the natural sources of all the nitrogen which plants require ? How 
much do they need ? What functions does it perform in their in- 
terior ? How much of what enters remains in them, and how 
much escapes again into the air from their leaves? These are all 
questions of an important jiraclical bearing, to the solution of which 
recent observations, both in the field* and in the laboratory, 
impart a higher degree of importance than we were previously 
prepared to attach to them. Let me explain in part how this 
arises. 
Nitrogen is acknowledged to be an important part of the food 
of plants. It contributes to the formation of those substances 
(protein compounds) upon which the production of muscle in 
animals is supposed to depend. 
The nitrogen which enters into plants has been thought to re- 
main for the greater part within them, and to be gathered in the 
crop, in the form of these muscle-producing ingredients. But 
recent experiments made in the United States of America by 
Professor Draper have shown that plants are constantly giving off 
nitrogen from their leaves in large quantities into the air, and it 
appears probable that of the nitrogen which enters their roots 
only a small proportion remains at last in the full-grown plant 
compared with what is thus discharged into the atmosphere. 
If this be so, you will first, that plants must require a great 
deal more of this elementary body in their food than has hitherto 
been supposed; and, secondly, that its combinations must perform in 
the plant very different functions, or more numerous ones, than had 
previously been ascribed to it. A thousand pounds of the re- 
sources of practical agriculture would be economically spent, could 
they be made to clear up this one important subject of inquiry 
only.f 
* See especially those of Mr. Lawes upon wheat in the eighth vokime 
of this Journal, part i., and the speculations he has hazarded in connection 
with them. 
t A practical application of the kind ofknowledgetobe sought for by this 
inquiry is seen in the experiments of Mr. Pusey, quoted in a previous note, 
in which guano and rags added largely to the crop of beet where bone and 
rape-dust failed, and in regard to which Mr, Pusey suggested that pro- 
