FIXATION OF FEEE NITROGEN. 157 



11 to 17 kilograms of soil were employed ; in which seldom 

 less than 10, and frequently nearer 25 grams of combined 

 nitrogen were involved; in which, with these quantities, the 

 soils and plants were exposed to free air and rain ; and in 

 which, under such conditions, there was, with the same de- 

 scription of plant, sometimes loss and sometimes consider- 

 able gain of nitrogen indicated. In the case of Papilionaceae 

 growing in sand, without or with only comparatively small 

 additions of combined nitrogen, but with due microbe-infec- 

 tion, inducing root-nodule-formation, the gains are propor- 

 tionally so great as to render immaterial the usual sources of 

 error incident to experiments in the open air, and to leave no 

 doubt whatever whether there had been fixation or not. At Fixation 

 present, therefore, it must be considered that the fixation of °ff ree 

 free nitrogen by the higher chlorophyllous plants themselves T/L™i#Ae/ 

 still requires confirmation. It may be added, that what is chiorophyi- 

 known of the nitrogen statistics of the growth in agriculture requires 1 & 

 of other cruciferous plants is adverse to the supposition that coiifirma- 

 they avail themselves of the free nitrogen of the atmosphere. 



But to return to the question of the explanation of the 

 undoubted fixation of free nitrogen in the growth of legu- 

 minous crops under the influence of suitable microbe-in- 

 fection, and of the development of nodules on the roots of 

 the plants. 



As in the exact quantitative series of experiments made at 

 Rothamsted in 1888 and since, some of the results of which 

 have been briefly described, the plants were not taken up 

 until they were nearly ripe, it is obvious that the roots and 

 their nodules could not be examined during growth, but only 

 at the conclusion ; when, if the gain of nitrogen be connected 

 with their development, it would be supposed that they 

 would be to a great extent exhausted of their nitrogenous 

 contents. Another series was therefore commenced in 1890, A recent 

 and is still in progress, in which the same four annuals — experiment. 

 peas, beans, vetches, and yellow lupins, and the same four 

 plants of longer life — white clover, red clover, sainfoin, and 

 lucerne — were grown in specially made pits, so arranged that 

 some of the plants of each description could be taken up, and 

 their roots and nodules studied, at successive periods of 

 growth : the annuals at three periods — namely, first when 

 active vegetation was well established ; secondly when it was 

 supposed that the point of maximum accumulation had been 

 approximately reached ; and thirdly when nearly ripe : and 

 the plants of longer life at four periods — namely, at the end 

 of the first year, and in the second year when active vegeta- 

 tion was re-established, when the point of maximum accumu- 

 lation had been reached, and lastly when the seed was nearly 



