A Mouth Full of Stones

Pear stone cells, x400, polarised light
Pear stone cells, x400, interference contrast optics, showing the plasmodesmatal channels that connect one cell to the next. Notice the enormous thickness of the cell walls and the tiny internal cavity (lumen) that housed the cytoplasm of the cell when it was alive
A cluster of pear stone cells - all dead - surrounded by thin-walled living cells of the succulent pear flesh, x100
Clusters of stone cells in a thin section of the pear I ate for lunch. The green layer on the left is the skin of the fruit. x40. Dark field illumination.

A nice pair of pears

There are two distinctive features of pear fruit that makes it instantly recognisable from the first mouthful – a wonderful flavour and those gritty little particles in the pear flesh that get stuck between your teeth. The particles are stone cells, technically known as sclereids, which are clusters of dead cells with amazingly thick walls. The second photograph from the bottom is a very thin section through a piece of the pear that I had for my lunch today – those bright clusters are the stone cells. Next up, a cluster of these cells at higher magnification, viewed with polarised light and revealing the interference colours created by their thick, birefringent walls. The top two photograpsh show the individual cells at much higher magnification – with polarised light (top) and using interference contrast optics (below). In each you can see the enormous thickness of the walls, perforated by the channels called plasmodesmata that connected the cytoplasm of one cell with that of the next when they were alive. The function of these stone cells is uncertain, but they might have something to do with water supply to the rosettes of thin-walled living cells around them that inflate as the fruit grows. Generally speaking, consumers prefer pears that aren’t gritty, so pear breeders try to produce varieties without too many stone cells.