What is a key characteristic of Sieve Tube Members in phloem?

Sieve tube members are a specialized type of cell found in the phloem tissue of vascular plants, and their primary role is to transport sugars and other organic nutrients throughout the plant. A key characteristic of sieve tube members is that they lack nuclei and secondary cell walls. This adaptation allows sieve tube members to maximize the flow of nutrients by keeping the cells more flexible and ensuring that there are fewer obstructions to the movement of sap through the phloem. The absence of a nucleus means that these cells require companion cells, which are nucleated and help with the metabolic functions needed for the sieve tube members to operate effectively.

In contrast to this, while sieve tube members are often described as having tapered ends and being composed of living cells, these features do not define their primary role or function in nutrient transport as clearly as the lack of nuclei and secondary walls. Therefore, the understanding of their structure emphasizes the unique adaptation that facilitates efficient transport in the plant's vascular system.