Fig. 3.1-3. Longitudinal section of stem of milkweed (Asclepias). This micrograph shows three columns of parenchyma cells; those on the left and right have been cut down the center, so their front and back walls are missing, just as in Fig. 3.1-1. But the middle row of cells has been cut just where those cells are pressed up against and contact another row of cells close to us. The vertically elongate ovals, outlined in material stained brown, are the contact faces: the point where two cells press against each other is the contact face. If both cells are spheres, the contact face is round, but if the two cells are columnar, the contact face is oval, as are these in the micrograph. If the cells barely touch each other, the contact faces are small, but if the two cells are forced firmly against each other, the contact face is larger. Within each contact face are numerous smaller oval areas (arrows), so pale they appear white. They look like holes in the wall but are actually primary pit fields -- areas where the two primary walls of the contact face are unusually thin and plasmodesmata occur in high density (the plasmodesmata cannot be seen by ordinary light microscopy). Examine Fig. 3.1-1: those cells fit together so tightly that all the walls are contact faces except the small regions that face the intercellular spaces. There are almost certainly many primary pit fields present in the walls of Fig. 3.1-1, but they cannot be seen in the micrograph. Fig. 3.1-2 is somewhat surprising in having such a large expanse of primary wall without having any primary pit fields, but it might be that all of that wall faces an intercellular space -- such wall do not have primary pit fields.

Related pages in Plant Anatomy (Mauseth): 38-40.