Hollow stem.

Xylem and phloem are vascular tissues in plants that transport water and nutrients. While phloem is made of living cells, xylem is composed of dead cells that form hollow tubes, known as vessels, with a central hollow space called the lumen. This hollow structure allows for efficient transport of water and dissolved minerals from the roots to the rest of the plant. Xylem primarily transports water and minerals upwards from the roots. Phloem transports sugars (produced during photosynthesis) and other nutrients throughout the plant. Interestingly the hollow space can act as insulation, helping to regulate the plant's internal temperature. Air temperature changes faster than water, and a hollow stem can help the plant respond more quickly to temperature fluctuations, preventing overheating or chilling. Generally faster she grows the bigger the hollow.