Bacteroids  in  a  soybean  root
   nodule.
<P>
<Li> Root  hairs form an infection thread by invagination of the plasma
membrane.
<Li> Rhizobium  bacteria  penetrate  the  infection  thread,  and  form
bacteroids within vesicles.
<Li> Root cells grow around a bacteroid, forming a nodule.
<Li> The  nodule  develops  vascular tissue that facilitate exchange of
materials between bacteria and plant.