1890.] On the Question of the Fixation of Free Nitrogen. 115 



It is, it seems to ns, a point of importance that it should be 

 established, as it appears clearly to be, that in the development of the 

 parasite the cortex of the host is penetrated, and so an intimate con- 

 nexion between the two, indeed a symbiosis, is set up. Then there 

 is abundant evidence that the nodules are very rich in nitrogen. So 

 far as the facts at command go, it would seem that their dry sub- 

 stance may contain a higher percentage of nitrogen than that of any 

 other part of the still growing plant ; and, in some cases at any rate, 

 even higher than in that of the highly nitrogenous leguminous seed 

 itself. 



Whence comes this nitrogen ? The opinions of those who have 

 specially studied the histology and biology of the subject, do not seem 

 to be very clear or definite in reference to this point. According 

 to Prazmowski, as quoted by Marshall Ward, the bacteroids " can 

 only multiply in the still living protoplasm." Again, under the 

 influence of the fungus — " the young tubercle is developed in the 

 deeper parts of the cortex, and in its tissues the bacterium-like con- 

 tents of the fungus become distributed, and grow, divide, and branch 

 at the expense of the protoplasmic contents. He regarded the phe- 

 nomenon as one of symbiosis, and as benefiting the host as well as 

 the parasite." And again — " The tubercle-bacteria penetrate through 

 young (not suberised) cell membranes into the root-hairs and epi- 

 dermis cells of the root, and there multiply at the expense of the 

 protoplasmic cell-contents." 



Further, " The contents of the bacteroid cells are resorbed as the 

 bacteroids dissolve, certain substances being left behind. In other 

 words, the plant utilises the substance of the bacteria. When 

 emptying begins, and with what energy it proceeds, depend especially 

 on the quantity of nitrogenous compounds at the disposal of the roots. 

 In a soil rich in nitrogen the tubercles go on developing unhindered, 

 become large and typical, and rosy inside, and are not exhausted till 

 late ; in poorer soils they attain no great size, are soon emptied, and 

 are green-grey inside." 



Summarising the results and conclusions of Prazmowski, Marshall 

 Ward says — 



" ~No decision is arrived at as to whether the nitrogen is got from 

 nitrogen compounds or from the free nitrogen of the air, nor as to 

 what advantage accrues to the bacteria and the host-plant respec- 

 tively." And again : — 



" Prom the preceding, we see that the tubercles depend on a 

 symbiosis which is advantageous to both the plant and the bacteria. 

 The latter feed on the sap and cell-contents, and multiply through 

 innumerable generations, and, both during the life of the host and 

 afterwards, become redistributed in the soil. The plant derives 

 advantage in that it obtains nitrogen by means of the bacteria. 



VOL. XXVII. k 



