536 MR. J. B. LAWES, DR. GILBERT, AND DR. PUGH ON © 
eighteen days, the plants seemed to have consumed all the combined Nitrogen supplied 
to them—or rather all of it that had not become inaccessible to them in the soil: 
They then began to manifest the same indications of defective supply as before. Plants 
so circumstanced must therefore, at a more advanced stage of growth than before they 
had been supplied with ammonia, have passed from a point at which they had an excess 
of combined Nitrogen, to that in which they had an insufficiency. They must: hence, 
again, have been subjectéd to those conditions which we have assumed to be probably 
very favourable to the assimilation of free Nitrogen. 
Reference to the details of growth given in the Appendix will show that several 
times during the progress of the plants the above phenomena were manifested. A new 
increment of combined Nitrogen caused a new increment of growth, a greener colour; 
and a more vigorous appearance generally. This was soon followed by the recurrence 
of the pale colour. In some instances, more ammonia was not supplied until the plants 
seemed almost past recovery: in a few cases they were quite so. The addition of 
ammonia now (excepting in the few cases just referred to) produced a revivification, to 
be followed in a short time by the indications of some want, and so on. 
A considerable range of conditions of growth was thus provided. Just after each 
addition of combined Nitrogen the plants must have been supplied with an excess of 
this element in an available form. The evidence of this was afforded in the obviously 
increased means of consumption, evinced in the formation of new shoots from the base 
of the plants, or from their nodes. But these new shoots were too vigorous to allow the 
plants to go on long without suffering for want of a new supply of combined Nitrogen. 
In passing to this point, the newly-formed and vigorously-growing portion of the 
vegetable matter would be in the condition we have assumed to be the most favourable 
for assimilating free Nitrogen. Instead of doing this, however, it soon began to suffer, 
and continued to do so until a new supply of combined Nitrogen was added, when new 
vigour succeeded, to be followed again shortly by a cessation of growth. This cycle 
of conditions, repeated several times during the growth of the same plant, and the 
experiment similarly conducted with a number of pots of plants of different kinds, with 
like results in all the cases, affords a wide range of circumstances such as we have 
assumed to be favourable to the assimilation of free Nitrogen; but such an assimilation 
has not taken place. 
Without the physiological details, it might not have been clear that the ‘pions had 
not an excess of combined Nitrogen at its disposal during the greater period of its 
growth after the addition of the artificial supplies of it, since a considerable proportion 
of that added remained in the soil at the termination of the experiments, as Tables XIV. 
and XVL. show. But it is not difficult to imagine that a few milligrammes of ammonia 
intermingled with 1500 or 1600 grammes of soil (and pot), might become distributed 
over such an extent of surface, and be so completely absorbed, as that a considerable 
‘proportion should remain inaccessible to the plant. The physiological evidence leaves no 
doubt this was the case. 
