May be 10-20 microns thick. Protects plant from desiccation because keeps root in contact with soil rather than shrinking. Rhizosphere may extend beyond. More motile, fast growing bacteria like Pseudomonas. Some anaerobes because there is temporary anaerobiosis caused by C sources being used.
Root exudates a.
Amino acids iii. Distribution of microorganisms in rhizosphere. Distribution of microorganisms is non-homogeneous and bacteria are aggregated in colonies. So space does not appear to be limiting. Root cap usually devoid of microorganisms.