The hollow parts are not actually a problem. Think of a bicycle. It is made to be LIGHT, and STRONG. Tubular steel uses less materials and gives good rigidity. Many fast-growing herbaceous plants have hollow centers. They use less carbon, and therefor less of the plant's energy, to put on structural mass, leaving MORE energy to go into vertical growth and eventually sexual reproduction. Think of bamboo too- it's a classic example of a plant that just SOARS upwards, is VERY VERY strong, and is hollow!