Symbolism, Imagery, Allegory
This is an obvious one. The sterile flowers that the supervisors give Helena represent the sterility of the robots. "Of course these flowers are infertile," Galls says. "You see, they are cultivated—developed with artificial speed" (1.309). That's a description that applies to the robots as well as to the flowers; they are made quickly in an industrial process, with artificial speed, and they can't reproduce themselves.
The description doesn't just apply to robots and flowers though. Humans go sterile too in R.U.R., and babies stop being born. Technological production and artificial speed make the whole world, and human existence, sterile. Industrialization is contrary to life and love. Humans essentially turn themselves into sterile flowers by making sterile flowers. That may not exactly make perfectly logical sense—but that's why it's a happy, fruitful symbol, rather than a scientific formula for making sterile robots.