The fine structure of the non-infected (outer) region of a soybean root nodule is described. Apart from vascular traces, the cells are mainly vacuolate, with active cytoplasm. They are connected with each other and to cells of the infected region by numerous plasmodesmata. A network of air spaces runs throughout the nodule. Atmospherically applied water stress affects the outer cells of the nodule more quickly and more severely than the inner cells. In any region, vacuolate cells are more susceptible to stress than nonvacuolate cells. Loss of about 30% of their fresh weight results in breakdown of the cytoplasm into approximately spherical sub-units, some of which are coated with ribosomes. Organelles such as nuclei and mitochondria retain their structure longer than the rest of the cytoplasm. More severe stress is needed to affect infected and pericycle cells, which are nonvacuolate.
It is concluded from the fine structure of stressed and unstressed nodules that the vacuolate cells of the cortex play an integral part in the nitrogen-fixing activities of the whole nodule.