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eighteen days, the plants seemed to have consumed all the combined Nitrogen supplied 

 to them — or rather all of ii thai had not become inaccessible to them in the 

 Tin \ then began to manifest the same indications of defective supply as before. Plants 

 so circumstanced must therefore, at b more advanced stage of growth than before they 

 had been supplied with ammonia, have passed from a point at which thej had an < 

 of combined Nitrogen, to that in which they had an insufficiency. They must 1, 

 again, have been subjected 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 plant 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 XVI. 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. 



