215 



The Structure and Physiological Significance of the Root-nodules 

 of Myrica gale. 



By W. B. Bottomley, M.A., Professor of Botany in King's College, London. 



(Communicated by Prof. J. Eeynolds Green, F.R.S. Received June 21, — Eead 

 June 29, 1911.) 



The peculiar nodule formations on the roots of Myrica gale were first 

 described and figured by Brunchorst* in 1886, who stated that they were 

 caused by an inhabiting fungus with septate hyphae and terminal spores. 

 Mollerf in 1890 placed this fungus in the group Frankia, naming it Frankia 

 Brunchorstii, and considered it to be closely related to a similar fungus in 

 Alder nodules. In 1902 Shibata* stated that the fungus is found exclusively 

 in a peripheral sub-cork layer of tissue, one to three cells thick, and because 

 of its peculiar ray-branching and club-shaped spores, it belongs to the group 

 Actinomyces. Peklo§ in 1910, working on greenhouse-grown plants, 

 supported Shibata's view. 



Eoots of Myrica gale were obtained for this investigation from plants 

 growing wild in Wales, Ireland, and the North of England, and from 

 cultivated plants growing in the Chelsea Physic Gardens. In all cases the 

 roots were found to possess nodules of varying size. The young nodules are 

 from 2 — 3 mm. long and - 8 — 1 mm. broad, but these by branching form 

 " clusters," sometimes as large as *a nutmeg, and surrounded by peculiar 

 rootlets which grow out through the end of each nodule or branch. The 

 branching is associated with the outgrowth of lateral roots, and is not due to 

 dichotomy of the apical meristem of the root as is the case in the nodules of 

 Cycas, Alder, and Elaeagnus. 



A transverse section of the tubercle shows a central tetrarch vascular 

 cylinder similar to that of a normal root, and indicates that the tubercle 

 itself is a modified root. The stele is surrounded by an endodermis 

 characterised by neither radial dot nor thickened walls, but by the cells being 

 filled with oil drops. Outside the endodermis are several layers of cortical 

 cells covered on the outside with a definite small-celled cork layer. In 

 mature nodules the cortical tissue is characterised by the presence of 

 (1) somewhat enlarged cells filled with bacteria ; (2) cells filled with oil 

 drops. By means of Kiskalt's amyl Gram stain the bacteria can be seen in 

 situ in the enlarged cells as small rods. Towards the apex of the nodule 



* J. Brunchorst, ' Unters. aus dem Bot. Inst., Tubingen,' 1886, vol. 2. 

 t H. Mbller, ' Ber. d. Deutsch. bot. Ges.,' 1890, vol. 8. 

 % K. Shibata, ' Jahrb. f. Wiss. Bot.,' 1902, vol. 37. 

 § J. Peklo, 'Cent. f. Bakt.,' 1910,. (2), vol. 27. 

 VOL. LXXX1V. — B. S 



