Cycas is a gymnosperm with several distinct characteristics. It has a tall, unbranched stem and is a dioecious plant, meaning male and female reproductive organs are borne on separate plants. The roots include coralloid roots, which are branched and contain symbiotic cyanobacteria for nitrogen fixation. However, its leaves are pinnately compound, not simple and needle-like, and its pollen is carried by wind, not insects. Therefore, the correct features are the unbranched stem, dioecious nature, and presence of coralloid roots.