<Dd> Zone I--the active meristem . This is where new nodule tissue is formed which will later differentiate into the other zones of the nodule . </Dd> <Dd> Zone II--the infection zone . This zone is permeated with infection threads full of bacteria . The plant cells are larger than in the previous zone and cell division is halted . <Dl> <Dd> Interzone II--III--Here the bacteria have entered the plant cells, which contain amyloplasts . They elongate and begin terminally differentiating into symbiotic, nitrogen - fixing bacteroids . </Dd> </Dl> </Dd> <Dl> <Dd> Interzone II--III--Here the bacteria have entered the plant cells, which contain amyloplasts . They elongate and begin terminally differentiating into symbiotic, nitrogen - fixing bacteroids . </Dd> </Dl> <Dd> Interzone II--III--Here the bacteria have entered the plant cells, which contain amyloplasts . They elongate and begin terminally differentiating into symbiotic, nitrogen - fixing bacteroids . </Dd>

What is the center of a root called