SUMMARY : Root-nodules of Elaeagnus spp., Hippophae rhamnoides, Alnus glutinosa and Myrica gale are modified lateral roots. The enlarged cortical cells of an actively growing nodule contain the endophyte in several stages of development. The apical meristem is free from infection but the cells just behind it show a plasmodia1 stage. In older parts of the nodules, spherical vesicles are formed in species of Elaeagnus, Hippophae and Alnus and club-shaped bodies in Mym'ca and from these small, bacteroid-like granules are produced. There is some evidence that granules formed in different host cells fuse in pairs and possess some ability to migrate within the nodule and into the soil. No definite hyphae can be seen in any of the nodules, but well-marked protoplasmic strands or lines of flow are present, particularly in the endophyte of Myrica.The controversial views of earlier writers are discussed, and i t is concluded that the nodule organisms are members of the Plasmodiophorales.The conflicting opinions of earlier workers on the nature of the endophytes in the root-nodules of certain non-leguminous plants made a re-investigation desirable. Accordingly, nodules from roots of Elaeagnus macrophylla Thbg., E. angustifolia L. (E. hortensis M. Bieb), Hippophue rhamnoides L., Alnus glutinosa (L.) Gaertn. ( A . rotundifolia Stokes) and Myricagale L. were examined throughout the year by means of squash preparations, hand sections, and freezing-microtome sections of fresh material; and variously stained microtome sections of material fixed in Craf I1 (see Sass, 1940, p. 19) and embedded in paraffin wax. The most useful general stain was carbol fuchsin counterstained with light green. Aniline blue in lactic acid was also used. For cytological details gentian violet, alone or counterstained with light green, or iron alum haematoxylin, alone or similarly counterstained, were used. Other stain combinations were tried but gave no further information. Gram-staining gave variable results.The observations of earlier workers (Servettaz, 1909) on the morphology of the nodules were confirmed. In all species examined the nodules are modified lateral roots. The swelling begins a t the base of the young lateral and the nodule afterwards forks and may eventually produce a large compound gall. The young nodules have no root-hairs and are enclosed in a cork coat of two to three layers of cells. In Myrica the tips of the roots remain free from infection; they become cut off by the cork layer and are eventually sloughed off. When soil conditions are suitable the tips of the modified roots of the other species resume growth and break through the outer cork layer. They can then be seen as white tips to the branches of the nodules.
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