In plant tissue culture, the undifferentiated mass from which plantlets later grow is called what?

1. Micropropagation, also called plant tissue culture, raises many plants from a single plant under laboratory conditions. 2. Sterilised plant material is placed on a medium containing all the minerals, vitamins and hormones the plant needs. 3. The plant part often gives rise to an undifferentiated mass known as callus. 4. Individual plantlets begin to grow from that callus and are later separated and hardened in a greenhouse. 5. This method suits rare or endangered species and plants in demand as disease free stock. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 32 "Plant Reproduction", section 32.3 Asexual Reproduction_