216 Significance of the Root-nodules o/" Myrica gale. 



zooglea threads of bacteria are seen passing from cell to cell, comparable with 

 the " infection threads " first seen by Marshall Ward in leguminous nodules. 



The nodules arise as modifications of normal lateral roots. The cortical 

 cells of a young root, before its emergence from the main root, become 

 infected by bacteria. The normal growth of the root is thereby checked, but 

 by division and growth of the cells containing the bacteria, the characteristic 

 nodule with its tetrarch stele is formed. When the nodule has reached its 

 full size, the end of the stele, surrounded by a few cortical cells, grows out 

 from the apex of the nodule and forms a thin rootlet. Around this three 

 branches or nodules (occasionally only two) arise endogenously as outgrowths 

 from the cells surrounding the stele, repeating exactly the growth and 

 structure of the primary nodule. By repeated branching in this manner the 

 peculiar " cluster " nodules are formed. 



No fungal hyphse were observed in any of the young nodules examined, 

 but " infection threads " containing bacteria were numerous, and it was 

 evident that the formation of the nodules is caused by the action of the 

 infecting bacteria. 



Pure cultures of the bacteria from the cortical cells of the nodule were 

 made. These on examination were found to be identical in structure and 

 growth with the organism Pscudomonas radicicola found in leguminous 

 nodules. They gave the characteristic staining reaction with aniline gentian 

 violet and amyl alcohol, and formed colonies of oval shape on maltose agar. 



Cultures of the bacteria were made in flasks with a solution containing 

 1 grin, maltose, - 5 grm. potassium phosphate, O02 grm. magnesium sulphate 

 in 100 c.c. water. After incubation for seven days at 25° C. nitrogen 

 determinations of the culture solution gave the following results : — 



Control flask 0*53 mgrm. N per 100 c.c. 



Inoculated flask 2'58 „ „ 



showing a fixation of nitrogen of 2'05 mgrm. 



Young Myrica plants were obtained from Heysham Moss, some having 

 nodules on their roots, others having none. Both kinds were planted out in 

 a greenhouse in pots containing soil deficient in nitrogen. The plants 

 without nodules did not thrive, and soon died, whilst those possessing nodules 

 nourished and made a good growth. 



It is evident from these experiments that the root nodules of Myrica are 

 concerned with nitrogen assimilation, and that to the four families of non- 

 leguminous plants — Alder, Elseagnus, Cycas, and Podocarpus — known to 

 possess the power of nitrogen fixation by means of root-nodules, a fifth — 

 Myrica — must now be added. 



