MORPHOLOGICAL DEVELOPMENT OF NODULES ON CARAGANA ARBORESCENS LAM.

Developmental stages of nodules on Caragana arborescens, a woody perennial leguminous species, were studied. Root hair invasion and early cortical infection conformed to the well-known description of herbaceous leguminous nodules having apical meristems, except that a relatively smaller number of cells was infected. In the region of maturation behind the nodule meristem the infection threads remained active throughout the growth of the nodule and served to spread the infection to newly-formed cells. In mature nodules the distribution of red pigment, leghemoglobin, corresponded exactly with the location of the active bacteroidal tissue in the apical zones. Endodermal thickening of the vascular sheath and suberization of the nodule endodermis apparently restricted nutrient diffusion and aeration within the proximal areas. Senescence commenced relatively early, presumably as a result of these factors. However, the thickening of cell walls in these areas delayed, or prevented, necrosis; and a protective periderm which supplanted the outer cortex enhanced longevity of the nodule. Nodule rootlets projecting from six- to eight-month-old nodules were observed. The development of these rootlets from lateral branches of vascular strands and from terminal extensions of the apical tips of vascular bundles within the nodule is described.