520 ME. J. n. LA.WE8, dr. GILBERT, wd DR. PTOH OB 



be used, in the growing process, is seen in the results of the experiment in regard to the 

 question of the decomposition of Nitrogenous matter during growth, as given in Table 

 XI. (p. 513); and the extent to which it can carry the growth of the plant is illustrated 

 in that experiment. :is well as in others, to which we shall presently refer, relating to the 

 question of assimilation itself. It is obvious that, during a part of the time at the end 

 of which the plant has reached the limit of its supply of combined Nitrogen, it has had 

 at its disposal an excess of combined Nitrogen for its immediate wants. It has then 

 passed through a stage in which the particular relation of combined to free Nitrogen 

 implied in another of our assumed conditions must have existed. It musl finally ha 1 

 reached a point at which only free Nitrogen was presented to it. 



If an analysis of the plant at the termination of the last-mentioned period showed no 

 increase of Nitrogen, the result would afford conclusive evidence against the possibility 

 of the assimilation of free Nitrogen under a wide range of conditions. If, on the con- 

 trary, a gain of Nitrogen were indicated, the question would still be open, to which 

 of the several conditions to which the plant had been subjected it owed the increase 

 found. But this question we need not discuss until we have recorded the results of our 

 experiments on the point. 



B. — Direct experiments on the question of the assimilation of free Nitrogen by plants. 

 We have thus far discussed the methods of experimenting to be adopted, the results 

 of certain collateral inquiries, and the several conditions under which the assimilation of 

 free Nitrogen by plants may be the more or the less likely to take place. We have thus 

 endeavoured to eliminate all known sources of error, and to acquire the means of form- 

 ing an estimate of the possible influence of certain unknown quantities, and so, as far as 

 practicable, to reduce the solution of our question to that of a single point to be tested 

 by direct experiment. It remains to consider the experimental evidence relating to 

 this last and final point. 



An investigation requiring several hundred analyses, and a series of observations made 

 at intervals of a few days, through periods of several months, involves an amount of 

 recorded detail much too voluminous for full publication. An abstract of the most 

 important portions of the records will, however, be given for reference in the Appendix. 

 A statement of the methods of analysis adopted, with illustrations of the limits of accu- 

 racy reached, together with a condensed summary of the details of growth of the plants, 

 will there be given. 



In the selection of the plants to submit to our adopted conditions of experiment, we 

 have been guided by several considerations : — 



1. To have such as would be adapted to the conditions of temperature, moisture, &c, 

 to which they were to be subjected. 



2. To have such as were of importance in an agricultural point of view. 



3. To acquire the means of studying any difference, in reference to the point in 

 question, between plants which belong respectively to the two great Natural Orders 



