What are bordered pits ?

Hint: Pits are thinner areas of the cell wall through which neighbouring cells can connect or exchange fluid. Pits are found in cell walls that have additional layers. The pit membrane is the pit chamber's principal cell wall and middle lamella, or the membrane between adjacent cell walls.

Complete answer:
Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids), which are important components of higher plants' water-transport system. The pit membrane, which is located in the heart of each pit, permits water to travel between xylem conduits while preventing embolism and vascular pathogens from spreading across the xylem. Pits account for more than half of total xylem hydraulic resistance when averaged across a wide range of species, demonstrating that they play an essential role in plant overall hydraulic efficiency.
The porosity and thickness of pit membranes fluctuate substantially between species, indicating that pit structure differs greatly. Differences in pit structure are expected to correspond with trade-offs between efficiency and safety of water transport since higher porosity reduces hydraulic resistance but increases vulnerability to embolism. However, differences in pit structure (e.g. average porosity of pit membranes) and tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connect vessels influence hydraulic function trade-offs.

Note: .
The secondary cell wall extends over the pit and bordered pits, with a small opening in the secondary cell wall allowing water to enter through. When viewed under a microscope, a bordered pit appears to be shaped like a doughnut. The pore is the donut's hole, and the donut's outer ring is the pit's bordered pit's margin.