518 MR. J. B. LAWES, DR. GILBERT, AND DR. PUGH ON 
for such change; and if we find that in them free Nitrogen is assimilated, we should 
then trace up the question through the circumstances in which such assimilation is less 
likely to take place. 
If, on the contrary, we find that free Nitrogen is not assimilated under the circum- 
stances which appear the most favourable for such an action, we may either generalize 
for other conditions from the negative results so obtained, or we may extend our experi- 
ments in order to widen the basis of our generalizations. 
In the consideration of what are the cases in which the assimilation of free Nitrogen 
is most likely to take place, two important classes of conditions present themselves :— 
1. Those which relate to the supply of combined Nitrogen at the disposal of the 
plant. 
2. Those which relate to the activity of growth and stage of development of the 
plant. 
These two questions, though logically distinct, are physiologically blended ; for it may 
happen that a certain activity of growth, or certain stages of development, can only be 
attained by a given supply of combined Nitrogen beyond that contained in the seed. 
If we examine these conditions a little more closely, we see that they give us the 
following possible cases for the assimilation of free Nitrogen by the plant :— 
1. The plant may be able, in the process of cell-formation, to derive the whole of its 
Nitrogen from that presented to it in the free state. 
2. It may be capable of assimilating a part of its Nitrogen from that presented to it 
in the free state, provided it be supplied with only a part of its required amount in some 
form of combination. 
3. It may assimilate free Nitrogen in the presence of an excess of combined Nitrogen. 
Again :— 
1. It may be capable of assimilating free Nitrogen in the earlier stages of its develop- 
ment. 
2. It may be so at the most active period of its growth. 
3. It may when near the period of its maturity. 
Combinations of these several circumstances present at least nine special cases, in one 
of which, if at all, an assimilation of free Nitrogen might take place without its doing 
so in any of the others. The question arises, how are we so to arrange our experiments 
as to include the greatest number of these cases, and those in which the assimilation of 
free Nitrogen is the most likely to occur? 
The obviously most probable circumstances for the assimilation of free Nitrogen at 
any stage of development of the plant, are those in which it is brought to that stage in 
a healthy condition, and then deprived of all sources of combined Nitrogen. It is 
hardly to be supposed that an assimilation of free Nitrogen would take place if there 
were an excess of combined Nitrogen at the disposal of the plant; for, if we suppose 
that the molecular and vital forces are at the same time acting upon Nitrogen supplied 
by these two sources, in a manner tending to force that from both into the constitution 
