Fig. 5.1-4a and b. Transverse section of stem of flax (Linum). Although these cells have thick secondary walls, they are such broad cells that the walls appear comparatively thin. Two features indicate they are sclerenchyma, however: 1) they are stained red, and 2) the presence of pits, the fine white gaps in the secondary walls (arrows in lower micrograph). In almost all cases here, you can see that the pits occur as pit-pairs: a pit in the secondary wall of one cell faces a corresponding pit in the secondary wall of the adjacent cell. Look carefully and you will see that there is always a pit-membrane between the two pits of a pit-pair: pits are not holes all the way from one cell to another, they are just areas where there is no secondary wall (but the two primary walls and middle lamella are still present; see Fig. 5.5 in Plant Anatomy.).

Pits are typically extremely narrow, usually only 1 to 2mm in diameter; even though the lower micrograph was taken at high magnification, the details of the pits and membranes are difficult to see. However, you can see that the pits are basically narrow holes with straight sides – they do not curve or have one part wider than the other. Consequently, these are simple pits.