The root NeuroML element. Description of cells compliant to NeuroML Level1. Collection of all Level 1 cells. A single cell specified in MorphML Unit of all length measurements. Usually has the value <b>micrometer</b>. Note: length_units will be the preferred form in v2.0 Unit of all length measurements. Usually has the value <b>micrometer</b>. Note: length_units will be the preferred form in v2.0 Unit of all volume measurements.