The cytology and histology of the root nodules of some leguminosae

The nodule-forming bacteria were found to enter the host plant by the aid of an infection strand, usually through the root hairs, causing a characteristic curvature of the root hair as reported by other investigators; however, this curvature was not exhibited by all infected root hairs. The bacteria-forming nodules may also enter the host through ordinary epidermal cells. Not all of the bacteria entering the host form nodules. The time at which infection can occur in the soybean is variable. Large numbers of nodule-forming bacteria may retard the growth of a young seedling. The bacteria apparently are parasitic during early stages of nodule development and thereby interfere with the normal functioning of the root hairs. The endodermis is not penetrated by nodule-forming bacteria in the leguminous plants studied and the nodules arise only in the cortical parenchyma and not in the pericycle, as do lateral roots as reported by some authors. Nodules formed on roots, which have only a few layers of cortical parenchyma cells, may appear to arise in the endodermis or pericycle. The bacteroidal tissue in the soybean and cowpea nodules increases by division of infected cells. The infection strand is broken up by cell division and loses its identity. The bacteroidal tissue of alfalfa, sweet clover and vetch is mainly increased by the infection of new tissue which is continuously laid down by a meristem. The infection strand remains unbroken in the bacteroidal cells. Variation in the rate of cell division in the meristem will result in a branched nodule. The older bacteroidal cells in the soybean and cowpea nodule lose their ability to divide and the nucleus disintegrates. Vacuoles which enlarge and crowd the bacteria and remaining contents against the inner cell are common in this tissue. Vascular bundles are formed in the nodule, surrounding the bacteroidal cells and connecting with the xylem and phloem of the root. The vascular system develops from the cortical parenchyma by the formation of procambial strands. The vascular bundles are composed of xylem surrounded by phloem. New xylem is continuously added to the vascular bundles of the soybean and cowpea. The vascular bundles of the soybean and cowpea unite at the apex, whereas in alfalfa, sweet clover, and vetch they do not unite. In the soybean nodule a layer of sclerenchyma cells surrounding the bacteroidal and vascular tissues develops which limits the growth of the nodule. This layer of sclerenchyma tissue develops from the cortical parenchyma. It is not found in the cowpea, alfalfa, sweet clover, and vetch nodules. Starch grains are present in abundance in the uninfected cells in the bacteroidal tissue and around the vascular bundles. Filamentous fungi, which appeared to be parasitic because of their invasion of living cells, were found in the nodules. These fungus hyphae resemble the infection strands and may have been mistaken as such by earlier investigators.