Sclerenchyma fibres and xylem vessels are made of long, hollow plant cells
These cells are hollow due to the fact that their cell contents died (often due to the lignification of the cell wall, which makes it almost impossible for substances to enter or leave the cell)
These fibres have great strength, which is a property that is often exploited by humans
They are useful in making rope and certain fabrics are made from plant fibres (e.g. cotton and hemp)
Their strength is due to certain factors affecting the cell wall:
The arrangement of microfibrils in a mesh-like pattern
Secondary thickening, which results in a thick secondary cell wall developing which often contains lignin
The arrangement of cellulose microfibrils, along with secondary thickening, are the main reasons why plant fibres are very strong