dirigent

The monolignol substrates are held in position by proteins called dirigents (guide proteins). […] Lignan dirigent proteins are found in the cytoplasm, but similar dirigents have been localized in the cell walls of some plants.

The straight line in which the vertical polar plane cuts the plane of the circle of contact is called the dirigent. As there are in general two circles of contact, there are two dirigents, and they are parallel to the diretrices.