Complex sentence: It has hollow, flower stems that grow up to 10--70 cm (4-27 inches) long and 4-6 mm wide, which are unbranched and have 1-2 flowers (at the terminal ends) per stem.
Split into simpler sentences: It has hollow, flower stems that grow up to 10--70 cm (4-27 inches) long and 4-6 mm wide. Which are unbranched and have 1-2 flowers (at the terminal ends) per stem.